CHAPTER III MORPHOLOGY GENERAL ACCOUNT OF LICHEN STRUCTURE I. ORIGIN OF LICHEN STRUCTURES The two organisms, fungus and alga, that enter into the composition of the lichen plant are each characterized by the simplicity of their original structure in which there is little or no differentiation into tissues. The gonidia-forming algae are many of them unicellular, and increase mainly by division or by sporulation into daughter-cells which become rounded off and repeat the life of the mother-cell; others, belonging to different genera, are filaments, mostly of single cell-rows, with apical growth. The hyphal elements of the lichen are derived from fungi in which the vegetative body is composed of branching filaments, a character which persists in the lichen thallus. The union of the two symbionts has stimulated both, but more especially the fungus, to new developments of vegetative form, in which the fungus, as the predominant partner, provides the framework of the lichen plant-body. Varied structures have been evolved in order to secure life conditions favourable to both constituents, though more especially to the alga; and as the close association of the assimilating and growing tissues is maintained, the thallus thus formed is capable of indefinite increase. A. Forms of Cell-StructureThere is no true parenchyma or cellular structure in the lichen thallus such as forms the ground tissue of the higher plants. The fungal hyphae are persistently filamentous and either simple or branched. By frequent and regular cell-division—always at right angles to the long axis—and by coherent growth, a pseudoparenchyma may however be built up which functions either as a protective or strengthening tissue (Fig. 36). Lindau B. Types of ThallusThree factors, according to Reinke Wallroth was the first to make a comparative study of the different lichen thalli. He distinguished those lichens in which the green cells and the colourless filaments are interspersed equally through the entire thallus as “homoiomerous” (Fig. 2), and those in which there are distinct layers of cortex, gonidia, and medulla, as “heteromerous” (Fig. 1), terms which, though now considered of less importance in classification, still persist and are of service in describing the position of the alga with regard to the general structure. A less evident definition of the different types of thallus has been proposed by Zukal a. Endogenous Thallus. The term has been applied to a comparatively small number of homoiomerous lichens in which the alga predominates in the development, and determines the form of the thallus. These algae, members of the Myxophyceae, are extremely gelatinous, and the hyphae grow alongside or within the gelatinous sheath. In the simpler forms the vegetative structure is of the most primitive type: the alga retains its original character almost unchanged, and the ascomycetous fungus grows along with and beside it (Fig. 4). Such are the minutely tufted thalli of Thermutis and Spilonema and the longer strands of Ephebe, in which the associated Scytonema or Stigonema, filamentous blue-green algae, though excited to excessive growth, scarcely lose their normal appearance, making it difficult at times to recognize the lichenoid character unless the fruits also are present. Equally primitive in most cases is the structure of the thallus associated with Gloeocapsa. The resulting lichens, Pyrenopsis, Psorotichia, etc. are simply gelatinous crusts of the alga with a more or less scanty intermingling of fungal hyphae. In the Collemaceae, the gonidial cells of which are species of Nostoc (Fig. 2), there appears a more developed thallus; but in general, symbiosis in Collema has wrought the minimum of change in the habit of the alga, hence the indecision of the earlier botanists as to the identification and classification of Nostoc and Collema. Though in many of the species of the genus Collema no definite tissue is formed, yet, under the influence of symbiosis, the plants become moulded into variously shaped lobes which are specifically constant. In some species there is an advance towards more elaboration of form in the protective tissues of the apothecia, a layer of thin-walled plectenchyma being occasionally formed beneath or around the fruit as in Collema granuliferum. In all these lichens, it is only the thallus that can be considered as primitive: the fruit is a more or less open apothecium—more rarely a perithecium—with a fully developed hymenium. Frequently it is provided with a protective thalline margin. b. Exogenous Thallus. In this group, composed almost exclusively of heteromerous lichens, Zukal includes all those in which the fungus takes the lead in thalline development. He counts as such Leptogium, a genus closely allied to Collema but with more membranous lobes, in which the short terminal cells of the hyphae have united to form a continuous cortex. A higher development, therefore, becomes at once apparent, though in some genera, as in Coenogonium, the alga still predominates, while the simplest forms may be merely a scanty weft of filaments associated with groups of algal cells. Such a thallus is characteristic of the Ectolechiaceae, and some Gyalectaceae, etc., which have, indeed, been described by Zahlbruckner Heteromerous lichens have been arranged by Hue 1. Stratosae. Crustaceous, squamulose and foliose lichens with a dorsiventral thallus. 2. Radiatae. Fruticose, shrubby or filamentous lichens with a strap-shaped or cylindrical thallus of radiate structure. 3. Stratosae-Radiatae. Primary dorsiventral thallus, either crustaceous or squamulose, with a secondary upright thallus of radiate structure called the podetium (Cladoniaceae). II. STRATOSE THALLUS1. CRUSTACEOUS LICHENSA. General StructureIn the series “Stratosae,” the plant is dorsiventral, the tissues forming the thallus being arranged more or less regularly in strata one above the other (Fig. 37). On the upper surface there is a hyphal layer constituting a cortex, either rudimentary or highly elaborated; beneath the cortex is situated the gonidial zone composed of algae and hyphae in close association; and deeper down the medulla, generally a loose tissue of branching hyphae. The lower cortex which abuts on the medulla may be as fully developed as the upper or it may be absent. The growing tissue is chiefly marginal; the hyphae on the outer edge remain “meristematic” B. Saxicolous Lichensa. Epilithic Lichens. The crustaceous lichens forming this group spread over the rock surfaces. The support must be stable to allow the necessary time for the slowly developing organism, and therefore rocks that are friable or subject to continual weathering are bare of lichens. aa. Hypothallus or Prothallus. The first stage of growth in the lichen thallus can be most easily traced in epilithic crustaceous species, especially in those that inhabit a smooth rock surface. The spore, on germination, produces a delicate branching septate mycelium which radiates on all sides, as was so well observed and recorded by Tulasne Schwendener bb. Formation of crustaceous tissues. Some crustaceous lichens have a persistently scanty furfuraceous crust, the vegetative development never advancing much beyond the first rather loose association of gonidia and 1st. An upper cortical tissue of interlaced hyphae with frequent septation and with swollen gelatinous walls, closely compacted and with the lumen of the cells almost obliterated, not unfrequently a layer of mucilage serving as an outer cuticle. This type of cortex has been called by Hue 2nd. The gonidial zone—a somewhat irregular layer of algae and hyphae below the cortex—which varies in thickness according to the species. 3rd. The medullary tissue of somewhat loosely intermingled branching hyphae, with generally rather swollen walls and narrow lumen. It rests directly on the substratum and follows every inequality and crack so closely, even where it does not penetrate, that the thallus cannot be detached without breaking it away. In Verrucaria mucosa, a smooth brown maritime lichen found on rocks between tide-levels, the thallus is composed of tightly packed vertical rows of hyphae, slender, rather thin-walled, and divided into short cells. The gonidia are chiefly massed towards the upper surface, but they also occur in vertical rows in the medulla. One or two of the upper cells are brown and form an even cortex. The same formation occurs in some other sea-washed species; the arrangement of the tissue elements recalls that of crustaceous Florideae such as Hildenbrandtia, Cruoria, etc. cc. Formation of areolae. An “areolate” thallus is seamed and scored by cracks of varying width and depth which divide it into minute compartments. These cracks or fissures or chinks originate in two ways depending on the presence or absence of hypothallic hyphae. Where the hypothallus is active, new areolae arise when the filaments encounter new groups of algae. More vigorous growth starts at once and proceeds on all sides from these algal centres, until similarly formed areolae are met, a more or less pronounced fissure marking the limits of each. This primary areolation, termed rimose or rimulose, is well seen in the thin smooth thallus of Rhizocarpon geographicum (Fig. 40); but the first-formed areolae are also very frequently slightly Secondary areolation is due to unequal intercalary growth of the otherwise continuous thallus Strongly marked intersecting lines, similar to those round the margin of the thallus, are formed when hypothalli that have themselves started from different centres touch each other. A large continuous patch of crustaceous thallus may thus be composed of many individuals (Fig. 41). b. Endolithic Lichens. In many species, only the lower hyphae penetrate the substratum either of rock or soil. In a few, more especially those growing on limestone, the greater part or even the whole of the vegetative thallus and sometimes also the fruits are, to some extent, immersed Generally the embedded tissues follow the same order as in other crustaceous lichens: an upper layer of cortical hyphae, next a gonidial zone, and beneath that an interlaced tissue of medullary or rhizoidal hyphae which often form fat-cells On siliceous rocks such as granite, rhizoidal hyphae penetrate the rock chiefly between the thin separable flakes of mica. Bachmann c. Chemical Nature of the Substratum. Lichens growing on calcareous rocks or soils are more or less endolithic, those on siliceous rocks are largely epilithic, but Bachmann Stahlecker The character of the substratum also affects to a great extent the comparative development of the different thalline layers: the hyphal tissues in silicicolous lichens are much thinner than in lichens on limestone, and the gonidial zone is correspondingly wider. In a species of Staurothele on granite, Stahlecker Lang Stahlecker’s theory is that the hyphae require more energy to grow in the acid conditions that prevail in siliceous rocks, and therefore they make larger demands on the algal symbionts. It follows that the latter must be stimulated to more abundant growth than in circumstances favourable to the fungus, such as are found in basic (calcareous) rocks; he concludes that on the acid (siliceous) rocks, the epilithic or superficial condition is not only a physical but a biological necessity, to enable the algae to grow and multiply in a zone well exposed to light with full opportunity for active photosynthesis and healthy increase. C. Corticolous LichensThe crustaceous lichens occurring on bark or on dead wood, like those on rocks, are either partly or wholly immersed in the substratum (hypophloeodal), or they grow on the surface (epiphloeodal); but even those with a superficial crust are anchored by the lower hyphae which enter any crack or crevice of wood or bark and so securely attach the thallus, that it can only be removed by cutting away the underlying substance. a. Epiphloeodal Lichens. These lichens originate in the same way as the corresponding epilithic series from soredia or from germinating spores, and follow the same stages of growth; first a hypothallus with subsequent colonization of gonidia, the formation of granules, areolae, etc. The small compartments are formed as primary or secondary areolae; the larger spaces are marked out by the encounter of hypothalli starting from different centres. The thickness of the thallus varies considerably according to the species. In some Pertusariae with a stoutish irregular crust there is a narrow amorphous cortical layer of almost obliterated cells, a thin gonidial zone about 35 µ in width and a massive rather dense medulla of colourless hyphae. Darbishire b. Hypophloeodal Lichens. These immersed lichens are comparable with the endolithic species of the rock formations, as their thallus is almost entirely developed under the outer bark of the tree. They are recognizable, even in the absence of any fructification, by the somewhat shining brownish, white or olive-green patches that indicate the underlying lichen. This type of thallus occurs in widely separated families and genera, Lecidea, Lecanora, etc., but it is most constant in Graphideae and in those Pyrenolichens of which the algal symbiont belongs to the genus Trentepohlia. The development of these lichens is of peculiar interest as it has been proved that though both symbionts are embedded in the corky tissues, the hyphae arrive there first, and, at some later stage, are followed by the gonidia. There is therefore no question of the alga being a “captured slave” or “unwilling mate.” Frank It is always difficult to observe the entrance of the gonidia but they seem to spread first under the second or third layers of the periderm. With care it is possible to trace a filament of Trentepohlia from the surface downwards, and to see that the foremost cell is really the growing and advancing apex of the creeping alga. Both symbionts show increased vigour when they encounter each other: the thallus at once develops in extent and in depth, and, ultimately, reproductive bodies are formed. In some species the apothecia or perithecia alone emerge above the bark, in others the outer peridermal cells are thrown off, and the thallus thus becomes superficial to some extent as a white scurfy or furfuraceous crust. The change from a hypophloeodal to a partly epiphloeodal condition depends largely on the nature of the bark. Frank Black (or occasionally white) lines intersect the thallus and mark, as in saxicolous lichens (Fig. 41), the boundary lines between different individuals or different species. The pioneer hyphae of certain lichens very frequently become dark-coloured, and Bitter Bitter’s restriction of black boundary lines to cases of encountering thalli only, would exclude the comparison one is tempted to make between the advancing hyphae of lichens and those of many woody fungi where the extreme edge of the white invaded woody tissue is marked by a dark line. In the latter case however it is the cells of the host that are stained black by the fungus pigment. 2. SQUAMULOSE LICHENSA. Development of the SquamuleThe crustaceous thallus is more or less firmly adherent to, or confused with, the substratum. Further advance to a new type of thallus is made when certain hyphal cells of soredium or granule take the lead in an ascending direction both upwards and outwards. As growth becomes definitely apical or one-sided, the structure rises free from the substratum, and small lobules or leaflet-like squamules are formed. Each squamule in this type of thallus is distinct in origin and not merely the branch of a larger whole. In a few lichens the advance from the crustaceous to the squamulose structure is very slight. The granules seem but to have been flattened out at one side, and raised into minute rounded projections such as those that compose the thallus of Lecanora badia generally described as “subsquamulose.” The squamulose formation is more pronounced in Lecidea ostreata, and in some species of Pannaria; and the whole thallus may finally consist of small separate lobes as in Lecidea lurida, Lecanora crassa, L. saxicola, A frequent type of squamulose thallus is that termed “placodioid,” or “effigurate,” in which the squamulose character is chiefly apparent at the circumference. The thallus is more or less orbicular in outline; the centre may be squamulose or granular and cracked into areolae; the outer edge is composed of radiating lobules closely appressed to the substratum (Fig. 42). All lichens with this type of thallus were at one time included in the genus Placodium, now restricted by some lichenologists to squamulose or crustaceous species with polarilocular spores. Many of them rival Xanthoria parietina in their brilliant yellow colouring. There are also greyish-white effigurate lichens such as Lecanora saxicola, Lecania candicans (Fig. 43) and Buellia canescens, well-known British species. B. Tissues of Squamulose ThallusThe anatomical structure of the squamules is in general somewhat similar to that of the crustaceous thallus: an upper cortex, a gonidial zone, and below that a medullary layer of loose hyphae with sometimes a lower cortex. 1. The upper cortex, as in crustaceous lichens, is generally of the “decomposed” 2. The gonidia are Myxophyceae or Chlorophyceae; the squamules in some instances may be homoiomerous as in Lepidocollema, but generally they belong to the heteromerous series, with the gonidia in a circumscribed zone, and either continuous or in groups. Friedrich 3. The medullary layer, as a rule, is of closely compacted hyphae which give solidity to the squamules; in those of Heppia it is almost entirely formed of plectenchyma. 4. The lower cortex is frequently little developed or absent, especially when the squamules are closely applied to the support as in some species of Dermatocarpon. In some of the squamulose Lecanorae (L. crassa and L. saxicola) the lowest hyphae are somewhat more closely interwoven; they become brown in colour, and the lichen is attached to the substratum by rhizoid-like branches. In Lecanora lentigera there is a layer of parallel hyphae along the under surface. Further development is reached when a plectenchyma of thick-walled cells is formed both above and below, as in Psoroma hypnorum, though on the under surface the continuity is often broken. The squamules of Cladoniae are described under the radiate-stratose series. 3. FOLIOSE LICHENSA. Development of foliose ThallusThe larger leafy lichens are occasionally monophyllous and attached at a central point as in Umbilicaria, but mostly they are broken up into lobes which are either imbricate and crowded, or represent the dividing and branching of the expanding thallus at the circumference. They are horizontal spreading structures, with marginal and apical growth. The several tissues of the squamule are repeated in the foliose thallus, but further provision is made to meet the requirements of the larger organism. There is the greater development of cortical tissue, especially on the lower surface, and the more abundant formation of rhizoidal organs to attach the large flat fronds to the support. There are also various adaptations to secure the aeration of the internal tissues B. Cortical TissuesSchwendener a. Types of Cortical Structure. Zukal 1. Pseudoparenchymatous (plectenchyma): by frequent septation of regularly arranged hyphae and by coalescence a kind of continuous cell-structure is formed. 2. Palisade cells: the outer elongate ends of the hyphae lie close together in a direction at right angles to the surface of the thallus and form a coherent row of parallel cells. 3. Fibrous: the cortical hyphae lie in strands of fine filaments parallel with the surface of the thallus. 4. Intricate: hyphae confusedly interwoven and becoming dark in colour form the lower cortex of some foliose lichens. These four types, Zukal finds, are practically without interstices in the tissue and form a perfect protection against excessive transpiration. He adds yet another form: 5. A cortex formed of hyphae with dark-coloured swollen cells, which is not a protection against transpiration. It occurs among lower crustaceous forms. Hue has summed up the different varieties under four types, but as he has omitted the “fibrous” cortex, we arrive again at five different kinds of cortical formation, though they do not exactly correspond to those of Zukal. A definite name is given to each type: 1. Intricate: an intricate dense layer of gelatinous-walled hyphae, branching in all directions, but not coalescent (Fig. 44). This rather unusual type of cortex occurs in Sphaerophorus and Stereocaulon, both of which have an upright rigid thallus (fruticose). 2. Fastigiate: the hyphae bend outwards or upwards to form the cortex. A primary filament can be distinguished with abundant branches, all tending in the same direction; anastomosis may take place between the hyphae. The end branches are densely packed, though there are occasional interstices (Fig. 45). Such a cortex occurs in Thamnolia; in several genera of Roccellaceae—Roccellographa, Roccellina, Reinkella, Pentagenella, Combea, Schizopelte and Roccella—and also in the crustaceous genus Dirina. The fastigiate cortex corresponds with Zukal’s palisade cells. 3. Decomposed: in this, the most frequent type of cortex, the hyphae that travel up from the gonidial layer become irregularly branched and frequently septate. The cell-walls of the terminal branches become swollen into a gelatinous mass, the transformation being brought about by a change Zukal took no note of the decomposed cortex but the omission is intentional and is due to his regarding the structure of the youngest stages of the thallus near the growing point as the most typical and as giving the best indication as to the true arrangement of hyphae in the cortex. He thus describes palisade tissue as the characteristic cortex of Evernia, since the formation near the growing point of the fronds is somewhat palisade-like and he finds fibrous cortex at the tips of Usnea filaments. In both these instances Hue has described the cortex as decomposed because he takes account only of the fully formed thallus in which the tissues have reached a permanent condition. 4. Plectenchymatous: the last of Hue’s types corresponds with the first described by Zukal. It is the result of the lateral coherence and frequent septation of the hyphae into short almost square or rounded cells (Fig. 47). The simplest type of such a cortex can be studied in Leptogium, a genus of 5. The “fibrous” cortex must be added to this series, as was pointed out by Heber Howe More than one type of cortex, as already stated, may appear in a genus: a striking instance of variability occurs in Solorina where, as Hue b. Origin of Variation in Cortical Structure. The immediate causes making for differentiation in cortical development are: the prevailing direction of growth of the hyphae as they rise from the gonidial zone; the amount of branching and the crowding of the filaments; the frequency of septation; and the thickening or degeneration of the cell-walls which may c. Loss and Renewal of Cortex. Very frequently the cortex is covered over by a layer of homogeneous mucilage which forms an outer cuticle. It arises from the continual degeneration of the outer cell-walls and it is liable to friction and removal by atmospheric agency as was first described by Schwendener The same process of peeling was noted by Rosendahl The cortex is the most highly developed of all the lichen structures and is of immense importance to the plant as may be judged from the various adaptations to different needs d. Cortical Hairs or Trichomes. Though somewhat rare, cortical hairs are present on the upper surface of several foliose lichens. They take rise, in all the instances noted, as a prolongation of one of the cell-rows forming a plectenchymatous cortex. In Peltidea (Peltigera) aphthosa they are especially evident near the growing edges of the thallus; and they take part in the development of the superficial cephalodia Hairs are also present on the upper surface of some Parmeliae. Rosendahl In Nephromium tomentosum there is a scanty formation of hairs on the upper surface. They are abundant on the lower surface, and function as attaching organs. A thick tomentum of hairs is similarly present on the lower surface of many of the Stictaceae either as an almost unbroken covering or in scattered patches. In several species of Leptogium they grow out from the lower cortical cells and attach the thin horizontal fronds; and very occasionally they are present in Collema. C. Gonidial TissuesWith the exception of some species of Collema and Leptogium lichens included under the term foliose, are heteromerous in structure, and the algae that form the gonidial zone are situated below the upper cortex and, therefore, The fungal tissue of the gonidial zone is composed of hyphae which have thinner walls, and are generally somewhat loosely interlacing. In Peltigera A similar type of structure occurs in Cora Pavonia, one of the Hymenolichenes: the gonidial hyphae in that species form a cellular tissue in which are embedded the blue-green Chroococcus cells D. Medulla and Lower Cortexa. Medulla. The hyphal tissue of the dorsiventral thallus that lies between the gonidial zone and the lower cortex or base of the plant is always referred to as the medulla or pith. It is, as a rule, by far the most considerable portion of the thallus. In Parmelia caperata (Fig. 49), for instance, the lobes of which are about 300 µ thick, over 200 µ of the space is occupied by this layer. It varies however very largely in extent in different lichens according to species, and also according to the substratum. In another Parmelia with a very thin thallus, P. alpicola growing on quartzite, the medulla measures scarcely twice the width of the gonidial zone. It forms a fairly massive tissue in some of the crustaceous lichens—in some Pertusariae and Lecanorae—attaining a width of about 600 µ. Nylander (1) felted, which includes all those of a purely filamentous structure; (2) cretaceous or tartareous, more compact than the felted, and containing granular or crystalline substances as in some Pertusariae; and lastly (3) the cellular medulla in which the closely packed hyphae are divided The felted medulla is characteristic of most lichens and is formed of loose slender branching septate hyphae with thickish walls. This interwoven hyphal texture provides abundant air-spaces. Hue b. Lower Cortex. In some foliose lichens such as Peltigera there is no special tissue developed on the under surface. In Lobaria pulmonaria large patches of the under surface are bare, and the medulla is exposed to the outer atmosphere, sheltered only by its position. In some other lichens the lowermost hyphae lie closer together and a kind of felt of almost parallel filaments is formed, generally darker in colour, as in Lecanora lentigera, and in some species of Physcia. Most frequently however the tissues of the upper cortex are repeated on the lower surface, though differing somewhat in detail. In all of the brown Parmeliae, according to Rosendahl c. Hypothallic Structures. An unusual development of hyphae from the lower cortex occurs in the genera Anzia and Pannoparmelia—both closely related to Parmelia—whereby a loose sponge-like hypothallus of anastomosing reticulate strands is formed. In one of the simpler types, Anzia colpodes, a North American species, the hyphae passing out from the lower medulla become abruptly dark-brown in colour, and are divided into short thick-walled cells. Frequent branching and anastomosis of these hyphae result in the formation of a cushion-like structure about twice the bulk of the thallus. In another species from Australia (A. Japonica) there is a lower cortex, distinct from the medulla, consisting of septate colourless hyphae with thick walls. From these branch out free filaments, similar in structure but dark in colour, which branch and anastomose as in the previous species. In Pannoparmelia the lower cortex and the outgrowths from it are several cells thick; they may be thick-walled as in Anzia, or they may be thin-walled as described and figured by Darbishire This peculiar structure, regarded as a hypothallus, is probably of service in the retention of moisture. The thick cell-walls in most of the forms suggest some such function. E. Structures for Protection and AttachmentSuch structures are almost wholly confined to the larger foliose and fruticose lichens and are all of the same simple type; they are fungal in origin and very rarely are gonidia associated with them. a. Cilia. In a few widely separated lichens stoutish cilia are borne, mostly on the margins of the thallus lobes, or on the margins of the apothecia (Fig. 52). They arise from the cortical cells or hyphae, several of which grow out in a compact strand which tapers gradually to a point. Cilia vary in length up to about 1 cm. or even longer. In some lichens they Superficial cilia are more rarely formed than marginal ones, but they are characteristic of one not uncommon British species, Parmelia proboscidea (P. pilosella Hue). Scattered over the surface of that lichen are numerous crowded groups of isidia which, frequently, are prolonged upwards as dark-brown or blackish cilia. Nearly every isidium bears a small brown spot on the apex at an early stage of growth. Similar cilia are sparsely scattered over the thallus, but their base is always a rather stouter grey structure, which suggests an isidial origin. Cilia also occur on the margin of the lobes. As lichens are a favourite food of snails, insects, etc., it is considered that these structures are protective in function, and that they impede, if they do not entirely prevent, the larger marauders in their work of destruction. b. Rhizinae. Lichen rootlets are mainly for the purpose of attachment and have little significance as organs of absorption. They have been noted in only one crustaceous lichen, Varicellaria microsticta In species of Peltigera (Fig. 54) the rhizinae are confined to the veins or ridges (Fig. 55); they are thickish at the base, and are generally rather long and straggling. Meyer Lichen rhizinae, distinguished by Reinke In the genus Gyrophora, the rhizinae are simple strands of hyphae (G. polyrhiza) or they are corticate structures (G. murina, G. spodochroa and G. vellea). They are also present in species of Solorina, Ricasolia, Sticta and Physcia and very sparingly in Cetraria (Platysma). c. Haptera. Sernander The long cilia of Physcia ciliaris occasionally form haptera at their tips where the hyphae are loose and in active growing condition. Contact with some substance induces branching by which a spreading sheath arises; a plug-like process may also be developed which pierces the substance encountered—not unfrequently another lobe of its own thallus. The long flaccid fronds of Evernia furfuracea are frequently connected together by bridge-like haptera which rise at any angle of the thallus or from any part of the surface. The spinous hairs that border the thalline margins in Cetraria may also, in contact with some body—often another frond of the lichen—form a hapteron, either while the spermogonium, which occupies the tip of the spine, is still in a rudimentary stage, or after it has discharged its spermatia. The small sucker sheath may in that case arise either from the apex of the cilium, from the wall of the spermogonium or from its base. By means of these haptera, not only different individuals become united together, but instances are given by Sernander in which Cetraria islandica, normally a ground lichen, had become epiphytic by attaching itself in this way to the trunk of a tree (Pinus sylvestris). In Alectoria, haptera are formed at the tip of the thallus filament as an apical cone-like growth from which hyphae may branch out and penetrate any convenient object. A species of this genus was thus found clinging to After the haptera have become attached, they increase in size and strength and supply a strong anchorage for the plant; the point of contact frequently forms a basis for renewed growth while the part beneath the hapteron may gradually die off. Haptera are more especially characteristic of fruticose lichens, but Sernander considers that the rhizinae of foliose species may function as haptera. They are important organs of tundra and heath formations as they enable the lichens to get a foothold in well-lighted positions, and by their aid the fronds are more able to resist the extreme tearing strains to which they are subjected in high and unsheltered moorlands. F. Strengthening Tissues of Stratose LichensSquamulose and foliose lichens grow mostly in close relation with the support, and the flat expanding thallus, as in the Parmeliae, is attached at many points to the substance—tree, rock, etc.—over which the plants spread. Special provision for support is therefore not required, and the lobes remain thin and flaccid. Yet, in a number of widely different genera the attachment to the substratum is very slight, and in these we find an adaptation of existing tissues fitted to resist tearing strains, resistance being almost invariably secured by the strengthening of the cortical layers. a. By development of the Cortex. Such a transformation of tissue is well illustrated in Heppia Guepini. The thallus consists of rigid squamules which are attached at one point only; the cortex of both surfaces is plectenchymatous and very thick and even the medulla is largely cellular. The much larger but equally rigid coriaceous thallus of Dermatocarpon miniatum (Fig. 56) has also a single central attachment or umbilicus, and both cortices consist of a compact many-layered plectenchyma. The same structure occurs in Umbilicaria pustulata and in some species of Gyrophora, which, having only a single central hold-fast, gain the necessary stiffening through the increase of the cortical layers. In the Stictaceae there are a large number of widely-expanded forms, and as the attachment depends mostly on a somewhat short tomentum, strength is obtained here also by the thick plectenchymatous cortex of both surfaces. When areas denuded of tomentum and cortex occur, as in Lobaria pulmonaria, the under surface is not sensibly weakened, since the cortical tissue remains connected in a stout and firm reticulation. b. By development of Veins or Nerves. Certain ground lichens belonging to the Peltigeraceae have a wide spreading thallus often with very large lobes. The upper cortex is a many-layered plectenchyma, but the under surface is covered only by a loose felt of hyphae which branch out into a more or less dense tomentum. As the firm upper cortex continues to increase by intercalary growth from the branching upwards of hyphae from the meristematic gonidial zone, there occurs an extension of the upper The most perfect development of strengthening nerves is to be found in Hydrothyria venosa At the point where the stalk expands into the free erect frond, there arise a series of stout veins which spread upwards and outwards. They are definitely formed structures and not adaptations of pre-existing tissues: certain hyphae arise from the medulla at the contracted base of the frond, take a radial direction and, by increase, become developed into firm strands. The individual hyphae also increase in size, and the swelling of the nerve gives rise to a ridge prominent on both surfaces. They seldom anastomose at first but towards the tips they become smaller and spread out in delicate ramifications which unite at various points. There is no doubt, as Bitter III. RADIATE THALLUS1. CHARACTERS OF RADIATE THALLUSIn the stratose dorsiventral thallus, there is a widely extended growing area situated round the free margins of the thallus. In the radiate thallus of the fruticose or filamentous lichens, growth is confined to an apical region. Attachment to the substratum is at one point only—the base of the plant—thus securing the exposure of all sides equally to light. The cortex surrounds the fronds, and the gonidia (mostly Protococcaceae) lie in a zone or in groups between the cortex and the medulla. It is the highest type of vegetative development in the lichen kingdom, since it secures the widest room for the gonidial layer, and the largest opportunity for photosynthesis. Shrubby upright lichens consist mostly of strap-shaped fronds, either simple or branched, which may be broadened to thin bands (Fig. 57) or may be narrowed and thickened till they are almost cylindrical. The fronds vary in length according to the species from a few millimetres upwards: Lichens of filamentous growth are more or less cylindrical (Fig. 58). They are in some species upright and of moderate length, but in a few pendulous forms they grow to a great length: specimens of Usnea longissima have been recorded that measured 6 to 8 metres from base to tip. The radiate type of thallus occurs in most of the lichen groups but most frequently in the Gymnocarpeae. In gelatinous Discolichens it is represented in the Lichinaceae. It is rare among Pyrenocarpeae: there is one very minute British lichen in that series, Pyrenidium actinellum, and one from N. America, Pyrenothamnia, that are of fruticose habit. 2. INTERMEDIATE TYPES OF THALLUSBetween the foliose and the fruticose types, there are intermediate forms that might be, and often are, classified now in one group and now in the other. These are chiefly: Physcia (Anaptychia) ciliaris, Ph. leucomelas and the species of Evernia. In the two former the habit is more or less fruticose as the plants are affixed to the substratum at a basal point, but the fronds are decumbent and the internal structure is of the dorsiventral type: there is an upper “fibrous” cortex of closely compacted parallel hyphae, a gonidial zone—the gonidia lying partly in the cortex and partly among the loose hyphae of the medulla—and a lower cortex formed of a weft of hyphae which also run somewhat parallel to the surface. Both species are distinguished by the numerous marginal cilia, either pale or dark in colour. These two lichens are greyish-coloured on the upper surface and greyish or whitish below. Evernia furfuracea with a basal attachment E. prunastri, the second species of the genus, is more distinctly upright in habit, with a penetrating basal hold-fast and upright strap-shaped branching fronds, light-greyish green on the “upper” surface and white on the other (Fig. 59). The internal structure is sub-radiate; both cortices are “decomposed”; the gonidial zone consists of somewhat loose groups of algae, very constant below the “upper” surface, with an occasional group in the pith near to the lower cortex in positions that are more exposed to light. There is also a tendency for the gonidial zone to pass round the margin and spread some way along the under side. The medulla is of loose arachnoid texture and the whole plant is very limp when moist. It grows on trees, often in dense clusters. 3. FRUTICOSE AND FILAMENTOUSA. General Structure of ThallusThe conditions of strain and tension in the upright plant are entirely different from those in the decumbent thallus, and to meet the new requirements, new adaptations of structure are provided either in the cortex or in the medulla. Cortical Structures. With the exception of the distinctly plectenchymatous cortex, all the other types already described recur in fruticose lichens; in various ways they have been modified to provide not only covering but support to the fronds. a. The fastigiate cortex. This reaches its highest development in Roccella in which the branched hyphal tips, slightly clavate and thick-walled, lie closely packed in palisade formation at right angles to the main axis (Fig. 45). They afford not only bending power, but give great consistency to the fronds. The cortex is further strengthened in R. fuciformis b. The fibrous cortex. This type is found in a number of lichens with long filamentous hanging fronds. It consists of parallel hyphae, rarely septate and rarely branched, but frequently anastomosing and with strongly thickened “sclerotic” walls. Such a cortex is the only strengthening element in Alectoria, and it affords great toughness and flexibility to the thong-like In Usnea longissima the cortex both of the fibrillose branchlets and of the main axis is fibrous, and is composed of narrow thick-walled hyphae which grow in a long spiral round the central strand. The hyphae become more frequently septate further back from the apex (Fig. 61). Such a type of cortex provides an exceedingly elastic and efficient protection for the long slender thallus. The same type of cortex forms the strengthening element in the fruticose or partly fruticose members of the family Physciaceae. One of these, Teloschistes flavicans, is a bright yellow filamentous lichen with a somewhat straggling habit. The fronds are very slender and are either cylindrical or slightly flattened. The Another still more familiar grey lichen, Physcia ciliaris, has long flat branching fronds which, though dorsiventral in structure, are partly upright in habit. Strength is secured as in Teloschistes by the fibrous upper cortex. Other species of Physciae are somewhat similar in habit and in structure. In Dendrographa leucophaea, a slender strap-shaped rock lichen, Darbishire B. Special strengthening Structuresa. Sclerotic strands. This form of strengthening tissue is characteristic of Ramalina. With the exception of R. thrausta (more truly an Alectoria) all the species have a rather weak cortical layer of branching intricate thick-walled hyphae, regarded by Brandt In R. evernioides, a plant with very wide flat almost decumbent fronds of soft texture, in R. ceruchis and in R. homalea there is a somewhat compact medulla which gives a slight stiffness to the thallus. The other species of the genus are provided with strengthening mechanical tissue within the cortex formed of closely united sclerotic hyphae that run parallel to the surface (Fig. 62). In a transverse section of the thallus, this tissue appears In the Cladoniae support along with flexibility is secured to the upright podetium by the parallel closely packed hyphae that form round the hollow cylinder a band called the “chondroid” layer from its cartilage-like consistency. b. Chondroid axis. The central medullary tissue in Ramalina is, with few exceptions, a loose arachnoid structure; often the fronds are almost hollow. In one species of Usnea, U. Taylori, found in polar regions, there is a similar loose though very circumscribed medullary and gonidial tissue in the centre of the somewhat cylindrical thallus, and a wide band of sclerotic fibres towards the cortex. In all other species of Usnea the medulla itself is transformed into a strong central strand of long-celled thick-walled hyphae closely knit together by frequent anastomoses (Fig. 63 A). This central strand of the Usneas is known as the “chondroid axis.” A narrow band of loose air-containing hyphae and a gonidial zone lie round the central axis between it and the outer cortex (Fig. 63 A, b). At the extreme apex, the external cortical hyphae grow in a direction parallel with the long axis of the plant, but further back, they branch out at right angles and become swollen and mostly “decomposed” as in the cortex of Ramalina. In Letharia (L. vulpina, etc.) the structure is midway between Ramalina and Usnea: the central axis is either a solid strand of chondroid hyphae or several separate strands. In three other genera with upright fruticose thalli, Sphaerophorus, Argopsis and Stereocaulon, rigidity is maintained by a medulla approaching the chondroid type. In Sphaerophorus the species may have either flattened or cylindrical branching stalks, but in all of them, the centre is occupied by longitudinal strands of hyphae. Argopsis, a monotypic genus from Kerguelen, has a cylindrical branching thallus with a strong solid axis; it is closely allied to Stereocaulon, a genus of familiar moorland lichens. The central tissue of the stalks in Stereocaulon is also composed of elongate, thick-walled conglutinate hyphae, formed into a strand which is, however, not entirely solid. C. Survey of Mechanical TissuesMechanical tissues scarcely appear among fungi, except perhaps as stoutish cartilaginous hyphae in the stalks of some Agarics (Collybiae, etc.), or as a ring of more compact consistency round the central hyphae of rhizomorphic strands. It is practically a new adaptation of hyphal structure confined to lichens of the fruticose group, where there is the same requirement as in the higher plants for rigidity, flexure and tenacity. Rigidity is attained as in other plants by groups or strands of mechanical tissue situated close to the periphery, as they are so arranged in Ramalina and Cladonia; or the same end is achieved by a strongly developed Tenacity and elasticity are provided for in the pendulous forms either by a fibrous cortex as in Alectoria, or by the chondroid axis in Usnea. Haberlandt D. Reticulate FrondsIn the upright radiate thallus, more especially among the Ramalinae, though also among Cladoniae A more complete type of reticulation is always present in a Californian lichen, Ramalina reticulata, in which the large flat frond is a delicate open network from tip to base (Fig. 64). It grows on the branches of deciduous trees and hangs in crowded tufts up to 30 cm. or more in length. Usually it is so torn, that the real size attainable can only be guessed at. It is attached at the base by a spreading discoid hold-fast, and, in mature plants, consists of a stoutish main axis from which side branches are irregularly given off. These latter are firm at the base like the parent stalk, but soon they broaden out into very wide fronds. Splitting begins at the tips of the branches while still young; they are then spathulate in form with a slightly narrower recurved tip, below which the first perforations are visible, small at first, but gradually enlarging with the growth of the frond. Ramalina reticulata is an extremely gelatinous lichen and the formation of the network was supposed by Lutz Peirce made a series of experiments to test the capacity of the tissues to support tensile strains. In a dry state, a piece of the lichen held a weight up to 150 grms.; when wet it broke with a weight of 30 grms. It was also observed that the thickness of the frond doubled on wetting. E. Rooting Base in Fruticose LichensFruticose and filamentous lichens are distinguished by their mode of attachment to the substratum: instead of a system of rhizinae or of hairs spread over a large area, there is usually one definite rooting base by which the plant maintains its hold on the support. Intermediate between the foliose and fruticose types of thallus are several species which are decumbent in habit, but which are attached at one (or sometimes more) definite points, with but little penetration of the underlying substance. One such lichen, Evernia furfuracea, has been classified now as foliose, and again as fruticose. The earliest stage of the thallus is in the form of a rosette-like sheath which bears rhizinae on the under surface, very numerous at the centre of the sheath, but entirely wanting towards the periphery. A secondary thallus of strap-shaped rather narrow fronds rises from the sheath and increases by irregular dichotomous branching. These branches, which are considered by Zopf Evernia furfuracea grows frequently on dead wood, palings, etc., as well as on trees. E. prunastri grows invariably on trees, and has a more constantly upright fruticose habit; in this species also, a basal sheath is present, and the attachment is secured by means of rhizoidal hyphae which penetrate deeply into the periderm of the tree, taking advantage of the openings Among Ramalinae, the development of the base was followed by Brandt A different type of attachment was found by Lilian Porter Several Ramalinae—R. siliquosa, R. Curnowii, etc.—grow on rocks, often in extremely exposed situations, in isolated tufts or in crowded swards (Fig. 65). The separate tufts are not unfrequently connected at the base by On a smooth rock surface such as quartzite a continuous sward of Ramalina growth is impossible. The basal hyphae being unable to penetrate the even surface of the rock, the attachment is slight and the plants are easily dislodged. They do however succeed, sometimes, in taking hold, and small groups of fronds arise from a crustaceous base which varies in depth from ·5 to 1 mm. The tissues of this base are very irregularly arranged: towards the upper surface loose hyphae with scattered groups of algae are traversed by strands of gelatinized sclerotic hyphae similar to the strengthening tissues of the upright fronds, while down below there are to be found not only slender hyphae, but a layer of gonidia visible as a white and green film on the rock when the overlying particles are scaled off. Darbishire The two British species of Roccella—R. fuciformis and R. phycopsis—grow on maritime rocks, the latter also occasionally on trees. In R. fuciformis, the attaching sheath is a flat structure which slopes up a little round the base of the upright frond. It is about 2 mm. thick, the cortex occupying about 40 µ of that space; a few scattered gonidia are present immediately below. The remaining tissue of the sheath is composed of firmly wefted slender filaments. Towards the lower surface, there is a more closely compacted dark brown layer from which pass out the hyphae that penetrate the rock. The sheath of R. phycopsis is a small structure about 3 to 4 mm. in width and 1·5 mm. thick. A few gonidia may be found below the dense cortical layer, but they tend to disappear as the upright fronds become larger and the shade, in consequence, more dense. Lower down the hyphae take an intensely yellow hue; mixed with them are also some brown filaments. A somewhat larger sheath 7 to 8 mm. wide forms the base of R. tinctoria. In structure it corresponds—as do those of the other species—with the ones already described. In purely filamentous species such as Usnea there is also primary sheath formation: the medullary hyphae spread out in radiating strands which force their way wherever possible into the underlying substance; on trees they enter into any chink or crevice of the outer bark like wedges; or they ramify between the cork cells which are split up by the mere growth pressure. By the vertical increase of the base, the fronds may be hoisted up and an intercalary basal portion may arise lacking both gonidia and cortical layer. Very frequently several bases are united and the lichen appears to be of tufted habit. A basal sheath provides a similar firm attachment for Alectoria jubata and allied species: these are slender mostly dark brown lichens which hang in tangled filaments from the branches of trees, rocks, etc. These attaching sheaths differ in function as well as in structure from the horizontal thallus of the Cladoniaceae. They may be more truly compared with the primary thallus of the red algae Dumontia and Phyllophora which are similarly affixed to the substratum, while upright fronds of subsequent formation bear the fructifications. IV. STRATOSE-RADIATE THALLUS1. STRATOSE OR PRIMARY THALLUSA. General CharacteristicsThis series includes the lichens of one family only, the Cladoniaceae, the genera of which are characterized by the twofold thallus, one portion being primary, horizontal and stratose, the other secondary and radiate, the latter an upright simple or branching structure termed a “podetium” which narrows above, or widens to form a trumpet-shaped cup or “scyphus” (Fig. 66). The apothecia are terminal on the podetium or on the margins of the scyphi; in a few species they are developed on the primary thallus. Some degree of primary thallus-formation has been demonstrated in all the genera, if not in all the species of the family. The genus Cladina was established to include those species of Cladonia in which, it was believed, only a secondary Cladonia squamules vary in size from very small scales as in Cl. uncialis to the fairly large foliose fronds of Cl. foliacea which extend to 5 cm. in length and about 1 cm. or more in width. It is interesting to note that when the primary thallus is well developed, the podetia are relatively unimportant and frequently are not formed. As a rule the squamules are rounded or somewhat elongate in form with entire or variously cut and crenate margins. They may be very insignificant and sparsely scattered over the substratum, or massed in crowded swards of leaflets which are frequently almost upright. In colour they are bluish-grey, yellowish or brownish above, and white beneath (red in Cl. miniata), frequently becoming very dark-coloured towards the rooting base. These several characteristics are specific and are often of considerable value in diagnosis. In certain conditions of shade or moisture, squamules are formed on the podetium; they repeat the characters of the basal squamules of the species. B. Tissues of the Primary ThallusThe stratose layers of tissue in the squamules of Cladonia are arranged as in other horizontal thalli. a. Cortical tissue. In nearly all these squamules the cortex is of the “decomposed” type. In a few species there is a plectenchymatous formation—in Cl. nana, a Brazilian ground species, and in two New Zealand species, Cl. enantia f. dilatata and Cl. Neo-Zelandica. The principal growing area is situated all round the margins though generally there is more activity at the apex. Frequently there is a gradual perishing of the squamule at the base which counterbalances the forward increase. The upper surface in some species is cracked into minute areolae; the cracks, seen in section, penetrate almost to the base of the decomposed gelatinous cortex. They are largely due to alternate swelling and contraction of the gelatinous surface, or to extension caused, though rarely, by intercalary growth from the hyphae below. The surface is subject to weathering and peeling as in other lichens; but the loss is constantly repaired by the upward growth of the meristematic hyphae from the gonidial zone; they push up b. Gonidial tissue. The gonidia consisting of Protococcaceous algae form a layer immediately below the cortex. Isolated green cells are not unfrequently carried up by the growing hyphae into the cortical region, but they do not long survive in this compact non-aerated tissue. Their empty membranes can however be picked out by the blue stain they take with iodine and sulphuric acid. Krabbe Where the squamules assume the upright position (as in Cladonia cervicornis), there is a tendency for the gonidia to pass round to the lower surface, and soredia are occasionally formed. c. Medullary tissue. The hyphae of the medulla are described by Wainio as having long cells with narrow lumen, and as being encrusted with granulations that may coalesce into more or less detachable granules; in colour they are mostly white, but pale-yellow in Cl. foliacea and blood-red in Cl. miniata, a subtropical species. They are connected at the base of the squamules with a filamentous hypothallus which penetrates the substratum and attaches the plant. In a few species rhizinae are formed, while in others the hyphae of the podetium grow downwards, towards and into the substratum as a short stout rhizoid. d. Soredia. Though frequent on the podetia, soredia are rare on the squamules, and, according to Wainio 2. RADIATE OR SECONDARY THALLUSA. Origin of the PodetiumThe upright podetium, as described by Wainio This initial tissue—the primordium of the podetium—continues to grow not only in width but in length: the basal portion grows downwards and at length displaces the gonidial zone, while the upper part as a compact cylinder forces its way through the cortex above, the cortical tissue, however, taking no part in its formation; as it advances, the edges of the gonidial and cortical zones bend upwards and form a sheath distinguishable for some time round the base of the emerging podetium. Even when the primary horizontal thallus is merely crustaceous, the podetia take origin similarly from a subcortical weft of hyphae in an areola or granule. B. Structure of the Podetiuma. General structure. In the early stages of development the podetium is solid throughout, two layers of tissue being discernible—the hyphae forming the centre of the cylinder being thick-walled and closely compacted, and the hyphae on the exterior loosely branching with numerous air-spaces between the filaments. In all species, with the exception of Cl. solida, which remains solid during the life of the plant, a central cavity arises while the podetium is still quite short (about 1 to 1·5 mm. in Cl. pyxidata and Cl. degenerans). The first indication of the opening is a narrow split in the internal cylinder, due to the difference in growth tension between the more free and rapid increase of the external medullary layer and the slower elongation of the chondroid tissue at the centre. The cavity gradually widens and becomes more completely tubular with the upward growth of the podetium; it is lined by the chondroid sclerotic band which supports the whole structure (Fig. 67). b. Gonidial tissue. In most species of Cladoniaceae, a layer of gonidial tissue forms a more or less continuous outer covering of the podetium, Krabbe Algal cells have been found to be common to different lichens, but in Cladoniae Chodat c. Cortical tissue. In some species a cortex of the decomposed type of thick-walled conglutinate hyphae is present, either continuous over the whole surface of the podetium, as in Cl. gracilis (Fig. 68), or in interrupted areas or granules as in Cl. pyxidata (Fig. 69) and others. In Cl. degenerans, the spaces between the corticated areolae are filled in by loose filaments without any green cells. Cl. rangiferina, Cl. sylvatica, etc. are non-corticate, being covered all over with a loose felt of intricate hyphae. In the section Clathrinae (Cl. retepora, etc.) the cortex is formed of longitudinal hyphae with thick gelatinous walls. d. Soredia. Frequently the podetium is coated in whole or in part by granules of a sorediate character—coarsely granular in Cl. pyxidata, finely pulverulent in Cl. fimbriata. Though fairly constant to type in the different species, they are subject to climatic influences, and, when there is abundant moisture, both soredia and areolae develop into squamules on the podetium. A considerable number of species have thus a more or less densely squamulose “form” or “variety.” C. Development of the ScyphusTwo types of podetia occur in Cladonia: those that end abruptly and are crowned when fertile by the apothecia or spermogonia (pycnidia), or if sterile grow indefinitely tapering gradually to a point (Fig. 70); and those that widen out into the trumpet-shaped or cup-like expansion called the scyphus (Fig. 69). Species may be constantly scyphiferous or as constantly ascyphous; in a few species, and even in individual tufts, both types of podetium may be present. Wainio a. From abortive Apothecia. In certain species the apothecium appears at a very early stage in the development of the podetium of which it occupies the apical region. Owing to the subsequent formation of the tubular cavity in the centre of the stalk, the base of the apothecium may eventually lie directly over the hollow space and, therefore, out of touch with the growing assimilating tissues; or even before the appearance of the tube, the wide separation between the primordium of the apothecium and the gonidia, entailing deficient nutrition, may have produced a similar effect. In either case central degeneration of the apothecium sets in, and the hypothecial filaments, having begun to grow radially, continue to travel in the same direction both outwards and upwards so that gradually a cup-shaped structure is evolved—the amphithecium of the fruit without the thecium. The whole or only a part of the apothecium may be abortive, and the scyphus may therefore be entirely sterile or the fruits may survive at the edges. The apothecia may even be entirely abortive after a fertile commencement, but in that case also the primordial hyphae retain the primitive impulse not only to radial direction, but also to the more copious branching, and a scyphus is formed as in the previous case. It must also be borne in mind that the tendency in many Cladonia species to scyphiform has become hereditary. Baur Scyphi originating from an abortive apothecium are characteristic of species in which the base is closed (Wainio’s Section Clausae), the tissue in that case being continuous over the inside of the cup as in Cl. pyxidata, Cl. coccifera and many others. b. From polytomous Branching. Another method of scyphus formation occurs in Cl. amaurocrea and a few other species in which the branching is polytomous (several members rising from about the same level). Concrescence of the tissues at the base of these branches produces a scyphus; it is normally closed by a diaphragm that has spread out from the different bases, but frequently there is a perforation due to stretching. These species belong to the Section Perviae. c. From arrested Growth. In most cases however where the scyphus is open as in Cl. furcata, Cl. squamosa, etc., development of the cup d. Gonidia of the Scyphus. Gonidia are absent in the early stages of scyphus formation when it arises from degeneration of the apical tissues, either fertile or vegetative; but gradually they migrate from the podetium, from the base of young outgrowths, or by furrows at the edge, and so spread over the surface of the cup. Soredia may possibly alight, as Krabbe insists that they do, and may aid in colonizing the naked area. Their presence, however, would only be accidental; they are not essential, and scyphi are formed in many non-sorediate species such as Cl. verticillata. The cortex of the scyphus becomes in the end continuous with that of the podetium and is always similar in type. e. Species without Scyphi. In species where the whole summit of the podetium is occupied by an apothecium, as in Cl. bellidiflora, no scyphus is formed. There is also an absence of scyphi in podetia that taper to a point. In those podetia the hyphae are parallel to the long axis and remain in connection with the external gonidial layer so that they are unaffected by the central cavity. Instances of tapering growth are also to be found in species that are normally scyphiferous such as Cl. fimbriata subsp. fibula, and Cl. cornuta, as well as in species like Cl. rangiferina that are constantly ascyphous. The scyphus is considered by Wainio D. Branching of the PodetiumThough branching is a constant feature in many species, regular dichotomy is rare: more often there is an irregular form of polytomy in which one of the members grows more vigorously than the others and branches again, so that a kind of sympodium arises, as in Cl. rangiferina, Cl. sylvatica, etc. Adventitious branches may also arise from the podetium, owing to some disturbance of the normal growth, some undue exposure to wind or to too In a number of species secondary podetia arise from the centre of the scyphus—constantly in Cl. verticillata and Cl. cervicornis, etc., accidentally or rarely in Cl. foliacea, Cl. pyxidata, Cl. fimbriata, etc. Wainio The proliferations from the borders of the scyphus are in a different category. They represent the continuity of apical growth, as the edges of the scyphus are but an enlarged apex. These marginal proliferations thus correspond to polytomous branching. In many instances their advance is soon stopped by the formation of an apothecium and they figure more as fruit stalks than as podetial branches. E. Perforations and Reticulation of the PodetiumPerforations in the podetial wall at the axils of the branches are constant in certain species such as Cl. rangiferina, Cl. uncialis, etc. They are caused by the tension of the branches as they emerge from the main stalk. A tearing of the tissue may also arise in the base of the scyphus, due to its increase in size, which causes the splitting of the diaphragm at the bottom of the cup. Among the Cladoniae the reticulate condition recurs now and again. In our native Cladonia cariosa the splitting of the podetial wall is a constant character of the species, the carious condition being caused by unequal growth which tears apart the longitudinal fibres that surround the central hollow. A more advanced type of reticulation arises in the group of the Clathrinae in which there is no inner chondroid cylinder. In Cladonia aggregata, in which the perforations are somewhat irregular, two types of podetia have been described by Lindsay F. Rooting Structures of CladoniaeThe squamules of the primary thallus are attached, as are most squamules, to the supporting substance by strands of hyphae which may be combined into simple or branching rhizinae and penetrate the soil or the wood on which the lichen grows. There is frequently but one of these rooting structures and it branches repeatedly until the ultimate branchlets end in delicate mycelium. Generally they are grey or brown and are not In Cl. alpicola it has been found that the rooting structure is frequently as thick as the podetium itself. If the podetium originates from the basal portion of the squamule, the hyphae from the chondroid layer, surrounding the hollow centre, take a downward direction and become continuous with the rhizoid. Should the point of insertion be near the apex of the squamule, these hyphae form a nerve within the squamule or along the under surface, and finally also unite with the rhizoid at the base, a form of rooting characteristic of Cl. cartilaginea, Cl. digitata and several other species. Mycelium may spread from the rhizinae along the surface of the substratum and give rise to new squamules and new tufts of podetia, a method of reproduction that is of considerable importance in species that are generally sterile and that form no soredia. Many species, especially those of the section Cladina, soon lose all connection with the substratum, there being a continual decay of the lower part of the podetia. As apical growth may continue for centuries, the perishing of the base is not to be wondered at. G. HapteraThe presence of haptera in Cladoniae has already been alluded to. They occur usually in the form of cilia or rhizinae H. Morphology of the PodetiumIn the above account, the podetia have been treated as part of the vegetative thallus, seeing that, partly or entirely, they are assimilative and absorptive organs. This view does not, however, take into account their origin and development, in consideration of which Wainio Later lichenologists, such as Wallroth Reinke and others sought for a solution of the problem in Baeomyces, one of the more primitive genera of the Cladoniaceae. The thallus, except in a few mostly exotic species, scarcely advances beyond the crustaceous condition; the podetia are short and so varied in character that species have been assigned by systematists to several different genera. In one of them, Baeomyces roseus, the podetium or stalk originates according to Nienburg In the genus Cladonia, differentiation of the generative hyphae may take place at a very early stage. Wainio It seems probable that the podetium—as Wainio and Baur both have stated—is homologous with the apothecial stalk, though in most cases it is completely transformed into a vertical thallus. If the view of their formation from the gonidial zone is accepted, then they differ widely in origin from normal branches in which the tissues of the main axis are repeated in the secondary structures, whereas in this vertical thallus, hyphae from the gonidial zone alone take part in the development. It must be admitted that Baur’s view of the podetium as essentially thalline seems to be strengthened by the formation of podetia at the centre of the scyphus, as in Cl. verticillata, which are new structures and are not an elongation of the original conceptacular tissue. It can however equally be argued that the acquired thalline character is complete and, therefore, includes the possibility of giving rise to new podetia. The relegation of the carpogonium to a position far removed from the base or primordium of the apothecium need not necessarily interfere with the conception of the primordial tissue as homologous with the conceptacle; but more research is needed, as Baur dealt only with one species, Cl. pyxidata, and Gertrude Wolff confined her attention to the carpogonial stages at the edge of the scyphus. The Cladoniae require light, and inhabit by preference open moorlands, naked clay walls, borders of ditches, exposed sand-dunes, etc. Those with large and persistent squamules can live in arid situations, probably because I. Pilophorus and StereocaulonThese two genera are usually included in Cladoniaceae on account of their twofold thallus and their somewhat similar fruit formation. They differ from Cladonia in the development of the podetia which are not endogenous in origin as in that genus, but are formed by the growth upwards of a primary granule or squamule and correspond more nearly to Tulasne’s conception of the podetium as a process from the horizontal thallus. In Pilophorus the primary granular thallus persists during the life of the plants; the short podetium is unbranched, and consists of a somewhat compact medulla of parallel hyphae surrounded by a looser cortical tissue, such as that of the basal granule, in which are embedded the algal cells. The black colour of the apothecium is due to the thick dark hypothecium. Stereocaulon is also a direct growth from a short-lived primary squamule Stereocaulon cannot depend on the evanescent primary thallus for attachment to the soil. The podetia of the different species have developed various rooting bases: in St. ramulosum there is a basal sheath formed, in St. coralloides a well-developed system of rhizoids V. STRUCTURES PECULIAR TO LICHENS1. AERATION STRUCTURESA. Cyphellae and PseudocyphellaeThe thallus of Stictaceae has been regarded by Nylander a. Historical. Cyphellae were first pointed out by the Swiss botanist, Haller In urceolate or proper cyphellae, the base of the depression rests on the medulla; the margin is formed from the ruptured cortex and projects slightly inwards over the edge of the cup. Contrasted with these are the pseudocyphellae, somewhat roundish openings of a simpler structure which replace the others in many of the species. They have no definite margin; the internal hyphae have forced their way to the exterior and form a protruding tuft slightly above the surface. Meyer Acharius had limited the name “cyphella” to the hollow urceolate bodies that had a well-defined margin. Nylander b. Development of Cyphellae. The cortex of both surfaces in the thallus of Sticta is a several-layered plectenchyma of thick-walled closely packed cells, the outer layer growing out into hairs on the under surface of most of the species. Where either cyphellae or pseudocyphellae occur, a more or less open channel is formed between the exterior and the internal tissues of the lichen. In the case of the cyphellae, the medullary hyphae which line the cup are divided into short roundish cells with comparatively thin walls (Fig. 73). They form a tissue sharply differentiated from the loose hyphae that occupy the medulla. The rounded cells tend to lie in vertical rows, though the arrangement in fully formed cyphellae is generally According to Schwendener c. Pseudocyphellae. In these no margin is formed, the cortex is simply burst by the protruding filaments which are of the same colour—yellow or white—as the medullary hyphae. They vary in size, from a minute point up to 4 mm. in diameter. d. Occurrence and Distribution. The genus Sticta is divided into two sections: (1) Eusticta in which the gonidia are bright-green algae, and (2) Stictina in which they are blue-green. Cyphellae and pseudocyphellae are fairly evenly distributed between the sections; they never occur together. Stizenberger In the British Isles Sticta is rather poorly represented as follows: § Eusticta (with bright-green gonidia). Cyphellate: S. damaecornis. Pseudocyphellate: S. aurata. § Stictina (with blue-green gonidia). Cyphellate: S. fuliginosa, S. limbata, S. sylvatica, S. Dufourei. Pseudocyphellate: S. intricata var. Thouarsii, S. crocata. Structures resembling cyphellae, with an overarching rim, are sprinkled over the brown under surface of the Australian lichen, Heterodea MÜlleri; the thallus is without a lower cortex, the medulla being protected by thickly woven hyphae. Heterodea was at one time included among Stictaceae, though now it is classified under Parmeliaceae. Pseudocyphellae are also present on the non-corticate under surface of Nephromium tomentosum, where they occur as little white pustules among the brown hairs; and the white impressed spots on the under surface of Cetraria islandica and allied species, first determined as air pores by Zukal There seems no doubt that the chief function of these various structures is, as Schwendener B. Breathing-Poresa. Definite Breathing-Pores. The cyphellae and pseudocyphellae described above are confined to the under surface of the thallus in those lichens where they occur. Distinct breathing-pores of a totally different structure are present on the upper surface of the tree-lichen, Parmelia aspidota (P. exasperata), one of the brown-coloured species. They are somewhat thickly scattered as isidia- or cone-like warts over the lichen thallus (Fig. 74) and give it the characteristically rough or “exasperate” character. They are direct outgrowths from the thallus, and Zukal Zukal b. Other openings in the Thallus. Lobaria is the only genus of Stictaceae in which neither cyphellae nor pseudocyphellae are formed; but in two species, L. scrobiculata and L. pulmonaria, the lower surface is marked A somewhat similar but reversed structure characterizes Umbilicaria pustulata, which as the name implies is distinguished by the presence of pustules, ellipsoid swellings above, with a reticulation of cavities below. Bitter In some Parmeliae there are constantly formed minute round holes, either right through the apothecia (P. cetrata, etc.), or through the thallus (P. pertusa). Minute holes are also present in the under cortex of Parmelia vittata and of P. enteromorpha, species of the subgenus Hypogymnia. Nylander Still other minute openings into the thallus occur in Parmelia vittata, P. obscurata and P. farinacea var. obscurascens. In the two latter the openings like pin-holes are terminal on the lobes and are situated exactly on the apex, between the pith and the gonidial zone; sometimes several holes can be detected on the end of one lobe. Further growth in length is checked by these holes. They appear more frequently on the darker, better illuminated plants. In Parmelia vittata the terminal holes are at the end of excessively minute adventitious branches which arise below the gonidial zone on the margin of the primary lobes. All these terminal holes are directed upwards and are visible from above. Bitter does not attribute any physiological significance to these very definite openings in the thallus. It has been generally assumed that they aid in the aeration of the thallus; it is also possible that they may be of service in absorption, and they might even be regarded as open water conductors. C. General Aeration of the ThallusDefinite structures adapted to secure the aeration of the thallus in a limited number of lichens have been described above. These are the breathing-pores of Parmelia exasperata and the cyphellae and pseudocyphellae of the Stictaceae, with which also may be perhaps included the circumscribed breaks in the under cortex in some members of that family. Though lichens are composed of two actively growing organisms, the symbiotic plant increases very slowly. The absorption of water and mineral salts must in many instances be of the scantiest and the formation of carbohydrates by the deep-seated chlorophyll cells of correspondingly small amount. Active aeration seems therefore uncalled for though by no means excluded, and there are many indirect channels by which air can penetrate to the deeper tissues. In crustaceous forms, whether corticate or not, the thallus is often deeply seamed and cracked into areolae, and thus is easily pervious to water and air. The growing edges and growing points are also everywhere more or less loose and open to the atmosphere. In the larger foliose and fruticose lichens, the soredia that burst an opening in the thallus, and the cracks that are so frequent a feature of the upper cortex, all permit of gaseous interchange. The apical growing point of fruticose lichens is thin and porous, and in many of them the ribs and veins of their channelled surfaces entail a straining of the cortical tissue that results in the formation of thinner permeable areas. Zukal It is unquestionable that the interior of the thallus of most lichens contains abundant empty spaces between the loose-lying hyphae, and that these spaces are filled with air. 2. CEPHALODIAA. Historical and DescriptiveThe term “cephalodium” was first used by Acharius FlÖrke Further descriptions of cephalodia were given by Th. M. Fries As cephalodia contain rather dark-coloured, blue-green algae, they are nearly always noticeably darker than the thalli on which they grow, varying from yellowish-red or brown in those of Lecanora gelida to pale-coloured in Lecidea consentiens B. ClassificationForssell has drawn up a classification of these structures, as follows: I. Cephalodia vera.1. Cephalodia epigena (including perigena) developed on the upper outer surface of the thallus, which are tuberculose, lobulate, clavate or branched in form. These are generally corticate structures. 2. Cephalodia hypogena which are developed on the under surface of the thallus; they are termed “thalloid” if they are entirely superficial, and “immersed” when they are enclosed within the tissues. They are non-corticate though surrounded by a weft of hyphae. Forssell further includes here certain placodioid (lobate), granuliform and fruticose forms which develop on the hypothallus of the lichen, and gradually push their way up either through the host thallus, or, as in Lecidea panaeola, between the thalline granules. Nylander II. Pseudocephalodia.These are a small and doubtful group of cephalodia which are apparently in very slight connection with the host thallus, and show a tendency to independent growth. They occur as small scales on Solorina bispora Forssell and others have found and described cephalodia in the following families and genera: Sphaerophoraceae. Sphaerophorus (S. stereocauloides). Lecideaceae. Lecidea (L. panaeola, L. consentiens, L. pelobotrya, etc.). Cladoniaceae. Stereocaulon, Pilophorus and Argopsis. Pannariaceae. Psoroma (P. hypnorum). Peltigeraceae. Peltigera (Peltidea), Nephroma and Solorina. Stictaceae. Lobaria, Sticta. Lecanoraceae. Lecania (L. lecanorina), Aspicilia Physciaceae. Placodium bicolor C. Algae that form CephalodiaThe algae of the cephalodia belong mostly to genera that form the normal gonidia of other lichens. They are: Stigonema,—in Lecanora gelida, Stereocaulon, Pilophorus robustus, and Lecidea pelobotrya. Scytonema,—a rare constituent of cephalodia. Nostoc,—the most frequent gonidium of cephalodia. It occurs in those of the genera Sticta, Lobaria, Peltigera, Nephroma, Solorina and Psoroma; occasionally in Stereocaulon and in Lecidea pallida. Lyngbya and Rivularia,—rarely present, the latter in Sticta oregana Chroococcus and Gloeocapsa,—also very rare. Scytonema, Chroococcus, Gloeocapsa and Lyngbya are generally found in combination with some other cephalodia-building alga, though Nylander As a general rule only one kind of alga enters into the formation of the cephalodia of any species or genus. A form of Nostoc, for instance, is invariably the gonidial constituent of these bodies in the genera, Lobaria, Sticta, etc. In other lichens different blue-green algae, as noted above, may occupy the cephalodia even on the same specimen. Forssell finds alternative algae occurring in the cephalodia of: Lecanora gelida and Lecidea illita contain either Stigonema or Nostoc; Lecidea panaeola, with Gloeocapsa, Stigonema or Chroococcus; Lecidea pelobotrya, with Stigonema or Nostoc; Pilophorus robustus, with Gloeocapsa, Stigonema, or Nostoc. Riddle Instances are recorded of algal colonies adhering to, and even penetrating, the thallus of lichens, but as they never enter into relationship with the lichen hyphae, they are antagonistic rather than symbiotic and have no relation to cephalodia. D. Development of Cephalodiaa. Ectotrophic. Among the most familiar examples of external cephalodia are the small rather dark-coloured warts or swellings that are scattered irregularly over the surface of Peltigera (Peltidea) aphthosa. This lichen has a grey foliose thallus of rather large sparingly divided lobes; it spreads about a hand-breadth or more over the surface of the ground in moist upland localities. The specific name “aphthosa” was given by Linnaeus to Th. M. Fries b. Endotrophic. Winter Both the algal cells of internal cephalodia and the hyphae in contact with them increase vigorously, and the newly formed tissue curving upwards or downwards appears on the outside as a swelling or nodule varying in size from that of a pin-head to a pea. On the upper surface the gonidial zone partly encroaches on the nodule, but the foreign alga remains in the centre of the structure well separated from the thalline gonidia by a layer of hyphae. The group is internally divided into small nests of dark-green algae surrounded by strands of hyphae (Fig. 79). The swellings, when they Endotrophic cephalodia occur in many groups of lichens. Hue c. Pseudocephalodia. Under this section have been classified those cephalodia that are almost independent of the lichen thallus though to some extent organically connected with it, as for instance that of Lecidea panaeola which originate on the hypothallus of the lichen and maintain their position between the crustaceous granules. The cephalodia of Lecanora gelida, as described by Sernander The pseudocephalodia of Usnea species are abortive apothecia; they are surrounded at the base by the gonidial zone and cortex of the thallus, and they contain no foreign gonidia. E. Autosymbiotic CephalodiaBitter Bitter’s work has been criticized by Linkola In the earliest stages, according to Linkola, a small group of algae may be observed in the cortical tissue of the Peltigera apart from the gonidial zone and near the upper surface. Gradually a protruding head is formed which is at first covered over with a brown cortical layer one cell thick. The head increases and becomes more lobate in form, being attached to the thallus at the base by a very narrow neck and more loosely at other parts of the scale. In older scales, the gonidia are entirely separated from those of the thallus, and a dark-brown cortex several cells in thickness covers over the top and sides; there is a colourless layer of plectenchyma beneath. At this advanced stage the scales are almost completely superficial and correspond with the cephaloidal rather than with the isidial type of formation. The algae even in the very early stages are distinct from the gonidial zone and the whole development, if isidial, must be considered as somewhat abnormal. 3. SOREDIAA. Structure and Origin of SorediaSoredia are minute separable parts of the lichen thallus, and are composed of one or more gonidia which are clasped and surrounded by the lichen hyphae (Fig. 80). They occur on the surface or margins of the thallus of a fairly large number of lichens either in a powdery excrescence or in a pustule-like body comprehensively termed a “soralium” (Fig. 81). The soralia vary in form and dimensions according to the species. Each individual soredium is capable of developing into a new plant; it is a form of vegetative reproduction characteristic of lichens. Acharius According to Schwendener a. Scattered Soredia. The simplest example of soredial formation may be seen on the bark of trees or on palings when the green coating of algal cells is gradually assuming a greyish hue caused by the invasion of hyphal lichenoid growth. This condition is generally referred to as “leprose” and has even been classified as a distinct genus, Lepra or Lepraria. Somewhat similar soredial growth is also associated with many species of Cladonia, the turfy soil in the neighbourhood of the upright podetia being often powdered with white granules. Such soredia are especially abundant in that genus, so much so, that Meyer Soredia are only occasionally present on the apothecial margins: the rather swollen rims in Lobaria scrobiculata are sometimes powdery-grey, and Bitter b. Isidial Soredia. In a few lichens soredia arise by the breaking down of the cortex at the tips of the thalline outgrowths termed “isidia.” In Parmelia verruculifera, for instance, where the coralloid isidia grow in closely packed groups or warts, the upper part of the isidium frequently becomes soredial. In that lichen the younger parts of the upper cortex bear hairs or trichomes, and the individual soredia are also adorned with hairs. The somewhat short warted isidia of P. subaurifera may become entirely sorediose, and in P. farinacea the whole thallus is covered with isidia transformed into soralia. The transformation is constant and is a distinct specific character. Bitter c. Soredia as Buds. Schwendener B. SoraliaIn lichens of foliose and fruticose structure, and in a few crustaceous forms, the soredia are massed together into the compact bodies called soralia, and thus are confined to certain areas of the plant surface. The simpler soralia arise from the gonidial zone below the cortex by the active division of some of the algal cells. The hyphae, interlaced with the green cells, are thin-walled and are, as stated by Wainio a. Form and Occurrence of Soralia. The term “soralium” was first applied only to the highly developed soredial structures considered by Acharius to be secondary apothecia; it is now employed for any circumscribed group of soredia. Soralia of definite form are of rather rare occurrence in crustaceous lichens, b. Position of soraliferous Lobes. According to observations made by Bitter That type of growth is in marked contrast with the thallus obliged to take a vertical direction as on a tree. In such a case the lobes, growing downward from the point of origin, form soralia at their tips at an early stage (Fig. 84). The lateral lobes, and especially those that lie close to the substratum, are the next to become soraliate. Similar observations have been made on the soraliferous lobes of Cetraria pinastri. The cause is probably due to the greater excess of moisture draining downwards to the lower parts of the thallus. The lobes that bear the soralia are generally Similar soralia are characteristic of Physcia hispida (Ph. stellaris subsp. tenella), the apical helmet being a specially pronounced feature of that species, though, as Lesdain Apical soralia are rare in fruticose lichens, but in an Alpine variety of c. Deep-seated Soralia. In the cases already described Schwendener A slightly different development is found in Lecanora tartarea, one of the “crottle” lichens, which has been placed by Darbishire in Pertusariaceae. The hyphae destined to form soredia also start from the weft of tissue at the base of the thallus, but they simply grow through the gonidial zone instead of pushing it aside. In his examination of Pertusariaceae Darbishire found that the apothecia also originated from a similar deeply seated blue-staining tissue, and he concluded that the soralia represented abortive apothecia and really corresponded to Acharius’s “apothecia of the second order.” His conclusion as to the homology of these two organs is disputed by Bitter C. Dispersal and Germination of SorediaSoredia become free by the breaking down of the hyphal stalks at the septa or otherwise. They are widely dispersed by wind or water and soon make their appearance on any suitable exposed soil. Krabbe Darbishire More success attended Tobler’s A suggestion has been made by Bitter D. Evolution of SorediaSoredia have been compared to the gemmae of the Bryophytes and also to the slips and cuttings of the higher plants. There is a certain analogy between all forms of vegetative reproduction, but soredia are peculiar in that they include two dissimilar organisms. In the lichen kingdom there has been evolved this new form of propagation in order to secure the continuance of the composite life, and, in a number of species, it has almost entirely superseded the somewhat uncertain method of spore germination inherited from the fungal ancestor, but which leaves more or less to chance the encounter with the algal symbiont. From a phylogenetic point of view we should regard the sorediate lichens as the more highly evolved, and those which have no soredia as phylogenetically That soredia are ontogenetic in character, and not, as Nilson Bitter 4. ISIDIAA. Form and Structure of IsidiaMany lichens are rough and scabrous on the surface, with minute simple or divided coral-like outgrowths of the same texture as the underlying thallus, though sometimes they are darker in colour as in Evernia furfuracea. They always contain gonidia and are covered by a cortex continuous with that of the thallus. This very marked condition was considered by Acharius The development of the isidial outgrowth has been described by Rosendahl In Parmelia scortea the cortex is several cells thick, and the outermost rows are compressed and dead in the older parts of the thallus; but here also the first appearance of the isidium is in the form of a minute wart. The lower layers (4 to 6) of living cortical cells divide actively; the gonidia also share in the new growth, and the protuberance thus formed pushes off the outer dead cortex and emerges as an isidium (Fig. 85). They are always rather stouter in form than those of P. papulosa and may be simple or branched. The gonidia in this case do not form a definite zone, but are scattered through the pith of the isidium. Here also should be included the coralloid branching isidia that adorn the upper surface and margins of the thallus of Umbilicaria pustulata. They begin as small tufts of somewhat cylindrical bodies, but they sometimes broaden out to almost leafy expansions with crisp edges. Most frequently they are situated on the bulging pustules where intercalary growth is active. Owing to their continued development on these areas, the tissue becomes slack, and the centre of the isidial tuft may fall out, leaving a hole in the thallus which becomes still more open by the tension of thalline expansion. New isidia sprout from the edges of the wound and the process may again be repeated. It has been asserted that these structures are only formed on injured parts of the thallus—something like gall-formations—but Bitter B. Origin and Function of IsidiaNilson This view can hardly be accepted; isidia as well as soredia are typical of certain species and are produced regularly and normally in ordinary conditions; both of them are often present on the same thallus. It is not denied, however, that their development in certain instances is furthered by increased shade or moisture. In Evernia furfuracea isidia are more freely produced on the older more shaded parts of the thallus. Zopf Bitter Isidia are primarily of service to the plant in increasing the assimilating surface. Occasionally they grow out into new thallus lobes. The more slender are easily rubbed off, and, when scattered, become efficient organs of propagation. This view of their function is emphasized by Bitter who points out that both in Evernia furfuracea and in Umbilicaria pustulata other organs of reproduction are rare or absent. Zopf VI. HYMENOLICHENSA. Supposed Affinity with other PlantsLichens in which the fungal elements belong to the Hymenomycetes are confined to three tropical genera. They are associated with blue-green algae and are most nearly related to the Thelephoraceae among fungi. The spores are borne, as in that family, on basidia. The best known Hymenolichen, Cora Pavonia (Fig. 86), was discovered by Swartz It was made the subject of more exact investigation by Mattirolo Johow found that Cora grew on the mountains usually from 1000 to 2000 ft. above sea-level. As it requires for its development a cool damp climate with strong though indirect illumination, it is found neither in sunny situations nor in the depths of dark woods. It grows most freely in diffuse light, on the lower trunks and branches of trees in open situations, but high up on the stem where the vegetation is more dense. It stands out from the tree like a small thin bracket fungus, one specimen placed above another, with a dimidiate growth similar to that of Polystictus versicolor. Both surfaces are marked by concentric zones which give it an appearance somewhat like Padina Pavonia. These zones indicate unequal intercalary growth both above and below. The whole plant is blue-green when wet, greyish-white when dry, and of a thin membranaceous consistency. B. Structure of ThallusThere is no proper cortex in any of the genera, but in Cora there is a fastigiate branching of the hyphae in parallel lines towards the upper surface; just at the outside they turn and lie in a horizontal direction, and, as the branching becomes more profuse, a rather compact cover is formed. The gonidia, which consist of blue-green Chroococcus cells, lie at the base of the upward branches and they are surrounded with thin-walled short-celled hyphae closely interwoven into a kind of cellular tissue. The medulla of loose hyphae passes over to the lower cortex, also of more or less loose filaments. The outermost cells of the latter very frequently grow out into short jagged or crenate processes (Fig. 87). In Corella, the mature lichen is squamulose or consists of small lobes; in Dictyonema there is a rather flat dimidiate expansion; in both the alga is Scytonema, the trichomes of which largely retain their form and are surrounded by parallel growths of branching hyphae. The whole tissue is loose and spongy. Corella spreads over soil on a white hypothallus without rhizinae. In the other two genera which live on soil, or more frequently on trees, there is a rather extensive formation of hold-fast tissue. When the dimidiate thallus grows on a rough bark, rhizoidal strands of hyphae travel over it and penetrate between the cracks; if the bark is smooth, there is a more continuous weft of hyphae. In both cases a spongy cushion of filamentous tissue develops at the base of the lichen between the tree and the bracket thallus. There is also in both genera an encrusting form which Johow regarded as representing a distinct genus Laudatea, but which MÖller found to be merely a growth stage. MÖller C. Sporiferous TissuesAs in Hymenomycetes, the spores of Hymenolichens are exogenous, and are borne at the tips of basidia which in these lichens are produced on the under surface of the thallus. In Cora the fertile filaments may form a continuous series of basidia over the surface, but generally they grow out in separate though crowded tufts. As these tufts broaden outwards, they tend to unite at the free edges, and may finally present a continuous hymenial layer. Each basidium bears four sterigmata and spores (Fig. 87 e); paraphyses exactly similar to the basidia are abundant in the hymenium. In Dictyonema the hymenium is less regular, but otherwise it resembles that of Cora. No hymenium has as yet been observed in Corella; it includes, so far as known, one species, C. brasiliensis, which spreads over soil or rocks. |