THE plants belonging to the Class AlgÆ grow in water, either in that of the sea or in fresh water; a few of them, however, being found on damp earth, damp walls, &c. The marine AlgÆ are commonly known as sea-weeds; but the fresh-water AlgÆ generally receive but little popular notice, forming, as they do, slimy masses or strata, of a green or brownish, sometimes red, colour.

AlgÆ are of simple structure, consisting entirely of cells; in some these are single, in others, united end to end, to form threads or filaments, or grouped into a leaf-like expansion, or collected few together into a little spherical group or a flat plate. They possess none of the fibres, vessels, or ducts of the higher plants, although some long and slender cells, existing in the stalks of the fronds of the larger kinds, bear considerable resemblance to woody fibre. They exhibit no distinction of stem and leaf, but consist of fronds representing the stem and leaf combined and undistinguishable. And the term frond must be understood to signify the separate parts arising from the point of attachment when they are fixed; and in the case of those which are unattached or free, the entire plant is called a frond.

The AlgÆ are divided into three Orders, viz. the Fucoid´eÆ or olive-coloured AlgÆ, the Florid´eÆ or red, and the Confervoid´eÆ or green AlgÆ.

Fucoid´eÆ, Fucoid AlgÆ, or Melanospor´eÆ (??a?, black or dark). The plants composing this order form our largest sea-weeds, and are found everywhere in the sea and on the sea-shore. They are of an olive-green or olive-brown colour, and usually become darker on drying.

FÚcus vesiculÓsus, with its parallel-sided or linear olive-brown fronds, is known to every one as the seaweed which is hung up to act as a weather-glass. The fronds have a central stout vein, or midrib, and scattered air-bladders, mostly in pairs.

The fructification consists of yellowish oval enlargements of the ends of the fronds, called the receptacles (fig. 16); but these are somewhat variable in form, being often angular or truncate. On holding one of the receptacles to the light, it will appear to contain a number of little grains imbedded in its substance, slightly projecting above the surface, and in the centre of each is a minute dot or pore. These grains are the capsules, or conceptacles, and contain the spores. The substance of the receptacles is composed of a beautiful network of colourless, jointed, cellular fibres (figs. 17 a and 18 a), the meshes of which are filled with a transparent gelatinous substance; but immediately around the conceptacles the cells are shorter and more closely packed. The spores (fig. 19) are arranged in the conceptacles in a radiate manner; they are brown, and surrounded by a colourless sac, called the perispore (pe??, around, sp???, seed); and between them are numerous slender, colourless, jointed filaments, the paraph´yses. The spores are not, however, truly single spores, for they ultimately divide into eight segments or sporules, each of which is capable of producing a new plant.

In the conceptacles of some fronds of Fucus no spores will be found, the conceptacles (fig. 18) being filled with elegantly branched colourless filaments (fig. 20), the ends of many of them being distended into little yellowish sacs; these are the antherid´ia. The antheridia contain large numbers of exceedingly minute spermatozoa, furnished with two cilia, and very similar to those existing in the antheridia of the mosses; these, escaping through the pore of the conceptacle, fertilize the spores.

The figure (20) in the plate was drawn from a conceptacle of Fucus serrÁtus, another common species, differing from F. vesiculosus in having the margins of the frond serrate; the antheridia of the two species do not, however, differ in any important respect. To examine the conceptacles of Fucus and their contents, the receptacles should be soaked in water, if not fresh, and thin sections made with a sharp knife. They form very beautiful objects, and may be preserved in chloride of calcium or glycerine.

Florid´eÆ, or Rhodosper´meÆ (??d??, rose, sp??a, seed).—The second Order of AlgÆ, forming the FlorideÆ (flos, a flower), comprises the red sea-weeds; a few of them are purple, or greenish-red; so that by the colour alone they may be readily distinguished from the Fucoids, and from nearly all those of the next Order, the Confer´voids. A few of them are leaflike, or possess flat fronds; but most of them consist of finely divided or feathery fronds. They are often found upon the sea-shore of a dirty white colour, the colouring matter having been decomposed or washed out by rain.

We shall consider a few of the genera and species under the heads of the families to which they belong.

Corallina´ceÆ, the Corallines, or calcareous AlgÆ.—In this family we have the beautiful CorallÍna officinÁlis (Pl. IV. fig. 28), the common Coralline, which is very abundant on the sea-shore, attached to larger sea-weeds, shells, and rocks. It is hard and chalky, from the presence of a large proportion of carbonate of lime in its minute cells. The fronds are composed of jointed and branched filaments. The fructification (figs. 29 and 30) consists of ovate cellular capsules, or ceramid´ia (?e?????, earthen vessel), placed mostly at the ends of pinnate stalks, and containing a tuft of somewhat club-shaped jointed spores, springing from the base of the capsules (fig. 30). When ripe, the spores escape from a pore or hole in the end of the capsules. The spores are 4-jointed, and hence are called tet´raspores (t?t?a, four).

To observe these spores, the capsules must be soaked in strong vinegar for some hours, and then washed with water, to dissolve the calcareous matter.

JÁnia rÚbens (Pl. IV. figs. 7 and 8) is another common and very elegant little coralline, and is of a pale red colour. It differs from the last in the branches being dichot´omous (d??a, in two, t???, cutting) or forked, instead of pinnate. The capsules, or ceramidia, have also two short horn-like branchlets, placed one on each side, near the end.

The genus MelobÉsia has the frond crustaceous, i. e. forming a hard crust or layer. M. polymor´pha (Pl. IV. figs. 5 and 6) is common on shells, stones, &c. The capsules (ceramidia) here form little blunt cones, scattered over the crusts, and containing the tufted tetraspores, as in Corallina.

Lithocys´tis Allman´ni (fig. 9) is very minute, and not uncommon upon sea-weeds, stones, &c. It consists of a single fan-shaped crustaceous layer of cells, closely investing the body to which it is attached; its fructification is unknown.

Leaving the family of crustaceous FlorideÆ, we shall now pass to those of softer consistence, although all the marine AlgÆ contain a considerable quantity of calcareous matter.

Rhodomela´ceÆ.—In this family we have the large genus PolysiphÓnia, in which the frond (Pl. IV. figs. 25 and 26) is filamentous, the filaments being apparently jointed and longitudinally striated. The filaments are composed of rings of cells (fig. 27), arranged end to end, and containing dark endochrome. The ends of the colourless cell-walls separating the endochromes of the cells of adjacent rings produce the jointed appearance; while the striated appearance is caused by the dark cells being elongate and the cell-walls thick, so as to form white interspaces.

The fructification consists of capsules (ceramidia), attached to the sides of the branches, containing pear-shaped spores, with tetraspores imbedded in swollen branches of separate plants.

PolysiphÓnia fastigiÁta (fig. 25, a small piece) is common, attached to the fronds of Fucus. Its filaments are rigid, bristle-like, of the same breadth throughout, forked, and forming globular brown or yellowish tufts, from 2 to 4 inches long. The joints are broader than long, each with 16-18 of the dark cells. In the centre of the branches of this sea-weed is a row of curious objects (fig. 26 a), consisting of a dark-coloured body surrounded with irregular spiny marginal processes, and with a colourless short process above and below. These require further investigation.

P. nigres´cens is also common among masses of seaweeds. Its filaments are brown, pinnate, the branches awl-shaped, and the joints about as long as broad.

DÁsya coccin´ea (Pl. IV. figs. 1 and 2, representing small portions of a filament) is a very common filamentous red sea-weed of the same family. The filaments are 6-8 inches long, and bipinnate,—the larger ones somewhat resembling those of Polysiphonia, in being composed of parallel longitudinal cells, arranged round the centre, but containing also smaller intermediate cells; while the smallest branches (fig. 2), which arise in tufts, consist of a single row of cells, little longer than broad. The fruit consists of ovate capsules (ceramidia), placed at the base of the branches, and containing a round mass of spores. There is also another kind of fructification, occurring on distinct plants; this is formed of one or two rows of tetraspores, immersed in pod-like capsules, called stichid´ia (st????, row).

Delesseria´ceÆ.—In this family, the typical or most highly developed genus of which, DelessÉria, has beautiful leaf-like rose-red fronds, we shall examine the common PlocÁmium coccin´eum (Pl. IV. figs. 23 and 22). This is of a fine red colour; the fronds are from 2 to 12 inches long, and consist of numerous branched and bushy filaments. These are compressed, with the branchlets arranged in alternate rows on the two margins of the stem. The end branchlets are acute and pectinate (pecten, a comb), or arranged like the teeth of a comb. The cells of which the filaments consist are small and angular, giving the surface the appearance of being elegantly netted under a high power. The fruit (fig. 22) consists of globular capsules, called coccid´ia (??????, a berry), placed in the axils or forks at which two branches separate, and containing a mass of angular spores. There are also tetraspore-pods (stichidia), as in Dasya; and tetraspores (fig. 24) in little leaf-like altered branches (fig. 21), called spor´ophylles (sp???, seed, f?????, leaf), and antheridia are present.

Rhodymenia´ceÆ.—In this family we have Hyp´nea purpuras´cens (Pl. IV. fig. 35). The filamentous pale purple frond of this sea-weed is from 6 inches to a foot or more in length, the branches being alternate and spreading. The fructification consists of capsules or coccidia (fig. 32), immersed in the branches, and containing the spores (fig. 34). Tetraspores also occur in the cells of the surface of the filaments.

Ceramia´ceÆ.—This is the last family to be noticed. CerÁmium nodÓsum (Pl. IV. figs. 10 and 11), which belongs to it, is a most delicate and elegant filamentous sea-weed, commonly found attached to other sea-weeds. The filaments are hair-like or capillary, irregularly dichot´omous; they consist of colourless cells, 3 or 4 times as long as broad, and with thick walls. The junctions of the cells are swollen (fig. 11), and covered with very minute dark red cells, giving them a knotty and jointed appearance to the naked eye or under a low power. The globular capsules, or favel’lÆ (favus, a honeycomb), containing the numerous spores, are situated at the ends of the branchlets, and the tetraspores (fig. 11) in twos or threes on the outer margins of them.

In CerÁmium rÚbrum, which is also very common, being found attached to stones, rocks, and the larger AlgÆ, the filaments (Pl. IV. fig. 12) are stouter than in C. nodosum, branched so as to form tufts from 2 to 10 inches long, and their ends forked, with the tips hooked inwards (fig. 14). The central cells of the filaments are large and rounded, and their walls are entirely covered with a layer of very small angular red cells. The globular capsules (fig. 15), or favellÆ, are situated on the suter surface of the branches, stalked, and supported by 3 or 4 short branchlets. The tetraspores (fig. 13) are imbedded in the branches, towards the ends. The capsules called favellÆ differ from the coccidia in the walls being simply membranous, while the walls of the coccidia, like those of the ceramidia, are composed of cells.

The tetraspores are usually imbedded, among the cells of the superficial layer of the filaments, and are not very easily recognized by an unpractised eye; it will be observed in the figures that they are sometimes cleft horizontally, at others obliquely.

Confervoid´eÆ.—This Order consists principally of the green freshwater AlgÆ, although some of them are yellowish brown, purple, or red, and some are marine. Their general structure may be best illustrated by selecting certain common examples from the families composing the order. The families are 13 in number. The species which are figured in the plates are found in fresh water, except when otherwise stated.

Conferva´ceÆ.—On removing some of the soft green matter found adhering to the stems of water-plants in any pool or pond, one of the species of Conferva, C. flocculosa, is almost sure to be met with. On close inspection with the naked eye, the green filaments of which it consists are just visible, as extremely fine, soft, silky threads; and, under a high power of the microscope, the filaments are seen to be unbranched, and composed of a single row of cells (Pl. V. fig. 1), or joints, as they are called in technical works; these are 2 or 3 times as long as broad. In some specimens the joints are swollen, so as to present a rounded outline. In another common species, C. bombyc´ina, the filaments are somewhat more slender, and the joints are from 3 to 5 times as long as broad.

Cladoph´ora crispÁta.—This Confervoid forms large, entangled, dull-green masses, composed of branched, tufted, somewhat rigid and coarse filaments. It is often a troublesome overrunner of the fresh-water vivarium. The filaments are composed of thick-walled cells (Pl. V. fig. 6), from 4 to 6 times as long as broad, and often containing minute starch-granules.

The ConfervaceÆ have two modes of reproduction. The first of these consists in the division of the endochrome of the joints into a number of distinct segments, each of which becomes furnished at one end with two very slender cilia (Pl. V. fig. 6 a). After a time, these ciliated bodies, which are called zo´ospores (????, animal, sp???, seed) or gonid´ia (????, seed, e?d??, resemblance), escape from the cells either by their rupture or through a papillary orifice, and swim about in the water, ultimately losing their cilia and growing into cells resembling those of the parent plant. In the second method, which occurs, for instance, in Conferva bombycina, certain of the joints enlarge so as to become rounded or inflated; their endochrome then becomes coated with a new cell-wall, and so forms a spore, which subsequently escapes from the cell and germinates.

ChÆtophora´ceÆ.—Draparnal´dia glomerÁta forms small green jelly-like masses, adhering to sticks and stones in water. These consist of branched filaments (Pl. V. fig. 10), prolonged at the ends into colourless hair-like points, and composed of single rows of cells, the green endochrome forming a band across the middle of each cell, the ends being colourless.

In ColeochÆ´te scutÁta (Pl. V. fig. 14) the cells are closely united, so as to form a minute flat green disk. In the natural state, this beautiful little object adheres to the submerged leaves and stems of water-plants, and is therefore difficult to be found. But if a few healthy water-plants be kept for some time in a glass jar, the little ColeochÆte, which is about as large as a pin’s head, will often be found adhering to the side of the glass.

Bat´rachosper´meÆ.—The members of this family resemble to the naked eye the little masses of Draparnaldia, and they are found in the same localities. They are of various colours, being green, brown, purple, or red. They consist, as in Bat´rachosper´mum monilifor´me (Pl. V. fig. 8), of branched filaments, which have a knotty appearance under a low power. The larger filaments are composed of cells arranged end to end, the knots consisting of numerous smaller whorled filaments, i. e. filaments arising from around them at the same level (fig. 7). The cells composing the whorled filaments are beaded or moniliform, and are prolonged into colourless hair-like points. The globules seen among the branches (fig. 7) consist of groups of spores.

Zygnema´ceÆ.—The members of this family resemble the ConfervaceÆ in consisting of simple cellular filaments (Pl. V. figs. 11, 13), but differ from them in the elegant arrangement of the endochrome: this forms beautiful spiral bands, as in Spirog´yra quini´na (fig. 11), or star-shaped masses, as in Zygne´ma crucia´ta (fig. 13). A remarkably curious phenomenon met with in them is the manner in which the spores are formed, and which is known as conjugation. In this process the opposite cells of two distinct filaments, lying near together, push out protrusions of the cell-walls, which meet and open into each other, forming cross tubes, as in Spirog´yra nit´ida (Pl. V. fig. 12). The contents of the opposite cells of the filaments then unite, forming large spores, which remain either in the cells of one of the filaments or in the cross tubes.

The three species figured are common in clear pools.

Desmidia´ceÆ.—The DesmidiaceÆ are truly microscopic, few of them being even perceptible to the naked eye without the very closest examination. They are very beautiful, on account of their bright green colour and often elegant forms. Many of them are very common, existing in every pond or ditch; but they abound most in clear open boggy pools on heaths. On placing some water containing them in a glass jar and exposing it to the light, they will often be found adhering to the glass, or forming a layer on the surface of the muddy sediment.

The DesmidiaceÆ consist mostly of single cells (Pl. V. figs. 9, 16); and these consist of two equal halves or segments, as indicated either by a paleness of the endochrome or a deep constriction at the line of junction, which is called the suture. The cells are often elegantly lobed and cut, or spiny; and in many the surface exhibits minute markings, consisting of little protrusions of the cell-wall outwards, or inflations, as they are called.

Their reproduction is effected by division and conjugation. In the process of division the cells gradually separate at the suture, and a new half-cell is formed upon each old half, which grows until it attains the size and form of the original half of the parent-cell. The conjugation is effected by two cells approximating so that their sutures are near together, the cells then open at the sutures, and the effused contents become united to form a spore or sporange, from which one or more individuals are formed. These spores are often elegantly spinous on the surface.

Among the species selected for illustration is ClostÉrium acerÓsum (Pl. V. fig. 9), in which the cells are single, elongate, very slightly curved or lunate; the endochrome forms long bands, often containing numerous globules or transparent vesicles. At each end of the cells is a round transparent vesicle, containing exceedingly minute granules, which exhibit a trembling kind of motion. Between the cell-wall and the cell-contents very fine currents may also be detected, forming a circulation resembling that in the hairs of Tradescantia.

In MicrastÉrias rotÁta (Pl. V. fig. 16) the cells, which are single, are deeply cleft into two segments at the suture, the segments being again regularly cut into five lobes, which are toothed or dentate.

In HyalothÉca dissil´iens (Pl. V. fig. 21) the cells are united into a cylindrical filament, and are surrounded by a very delicate gelatinous sheath. In Ankistrodes´mus falcÁtus (Pl. V. fig. 19) the cells resemble those of Closterium in shape, but are aggregated into faggot-like bundles, and are very much smaller. In the beautiful little Pedias´trum boryÁnum (Pl. V. fig. 20) the cells are aggregated into a disk, the marginal cells being bidentate or having each two points, so that the whole resembles a star. The species of Pediastrum are reproduced by the contents of each cell subdividing into numerous ciliated segments or zoospores, which subsequently escape in a mass from the cell, ultimately losing their cilia, and reuniting to form a new individual.

In Scenedes´mus quadricau´da (Pl. V. fig. 25) the oblong cells are united, side by side, the outermost cells being furnished with a bristle at each end. The division of these cells takes place obliquely, so that in the divided groups the cells are situated in two alternate rows.

The spores of many of the DesmidiaceÆ are spinous, and they are often found fossil in flint (Pl. V. fig. 15). To detect them in this substance, thin slips of flint may be examined under a half-inch power; or the chips of flint may be cemented to a slide with balsam, and ground down on a hone.

The DesmidiaceÆ must be mounted in the moist state: the smaller ones will keep well in chloride of calcium; but the larger ones are injured both by that liquid and by glycerine. The remarks made upon mounting, at page 15, are especially applicable to these delicate organisms.

Diatoma´ceÆ, or Siliceous AlgÆ.—The members of this family are singly very minute; but when existing in large numbers, as they are often found at the bottom of ditches and ponds, on the submerged stems of water-plants, or upon damp ground, they form yellowish-brown evident masses or strata. They occur both in sea-and in fresh water. They usually consist, like the DesmidiaceÆ, of single cells, which are called frustules. But they are especially characterized by the cell-walls being imbued with silica or flint, so that if the frustules be heated to redness upon the point of a knife or a slip of platinum-foil, which destroys the organic part of the cells, the coat of silica remains, exhibiting the perfect form of the original cells or frustules. The form of the frustules is very different in the various genera and species, as represented in Pl. V. figs. 22, 23, 27, 30, 31, and Pl. VI. figs. 16, 17, 23; and it will be noticed that, in the figures, two views are given of each frustule, f indicating the front view, and s the side view. In all the front views, as in Pl. V. fig. 22, one or more lines will be observed running longitudinally down the middle of the frustules, and corresponding to the indications of division existing in the cells of the DesmidiaceÆ. Each half of a frustule is called a valve, and the line at which these valves meet is called the suture. That side or aspect of the frustule in which the suture lies (fig. 22 f) is the front view; and the other aspect of the frustule (fig. 22 s) is the side view. The frustules are mostly four-sided—the main breadths of the two opposite valves forming two sides, and the bent margins of the valves, with the back and front of the hoop, forming the two other sides; so that the view presented by the side of a frustule is the same as that of a single valve. The suture is the line at which the division of the frustules takes place in the formation of new individuals. In this process the cell-contents divide into two parts, as in ordinary endogenous cell-formation,—the two new surfaces thus produced becoming coated with a new portion of cell-wall or valve, so that two frustules now occupy the place of the original one. At the same time a siliceous band, encircling the frustules at the line of suture, is formed to fill up the interval between the edges of the parent valves; this is the hoop (Pl. V. fig. 22 f; Pl. VI. fig. 10 f), and beneath it lie the two newly formed valves. In many cases I believe that each half-frustule becomes coated with a new entire cell-wall, with its siliceous valves.

The frustules of the DiatomaceÆ are constantly undergoing division when in vigorous growth. After the frustules have divided, the new ones either separate entirely, as is perhaps most commonly the case; or they remain united, sometimes completely, so as to constitute a filament (Pl. V. fig. 23), while at others the frustules cohere only at the angles (Pl VI. fig. 23), so as to form a zigzag chain.

In some species, the frustules are attached to foreign bodies by means of a gelatinous cushion (Pl. V. fig. 5; Pl. VI. fig. 7); while in others they are situated upon a simple or branched gelatinous stalk (Pl. V. fig. 17) or stipes (stipes, a stem).

When the frustules are examined in the living state, the cell-contents resemble those of ordinary vegetable cells, excepting in regard to the colour, and exhibit granules and globules, and sometimes a nucleus is visible. It will also be noticed that many of the free frustules move slowly across the field of the microscope; but the cause of the motion is unknown.

When the frustules have been properly prepared, the surface of the valves exhibits a number of coarser or finer markings, consisting of dots, lines (striÆ), flutings, or networks, &c., arranged with great regularity and symmetry, often of extreme minuteness, and rendering them exquisite objects under the microscope. The exhibition of these markings requires not only that the valves shall be properly prepared and mounted, but that the object-glasses be of good quality, and that the management of the light be thoroughly understood; so that to a beginner, their examination is often a matter of great difficulty; for only the very coarsest or largest of these markings can be perceived in the natural frustules.

The appearance of these markings, and even their apparent absence or presence, frequently depends upon the kind of illumination used: thus, under one kind of illumination the valves may appear simply white or coloured, while under another they appear covered with lines, and under a third with dots. It will often be observed, also, that the colour of the valves varies according to the illumination and the power used—the same valve appearing white, yellow, brown, blue, &c.; and the wet or dry state of the frustules often cause a decided difference in their appearance as regards colour.

To illustrate the forms and markings of the frustules and valves, we may select the following species taking first those which occur in fresh water.

In EpithÉmia tur´gida (Pl. V. fig. 30), the side-view or valve (s) exhibits transverse or slightly radiating lines, with intermediate rows of dots—these markings being continued over the margins of the valves so as to appear also in the front view (fig. 30 f), ceasing at the hoop. The frustules are curved or arcuate (ar´cus, a bow) in the side view, oblong and narrowed at the ends in the front view.

In FragilÁria capucÍna (Pl. V. fig. 23), which is extremely common in fresh-water pools, &c., the frustules are united side by side into long filaments, which are often twisted. In the separate and prepared frustule, the front view (Pl. V. fig. 24 f) is rectangular, the valves (s) being narrowly lance-shaped or lanceolate. The valves under ordinary illumination appear colourless and without markings, but, by proper management of the light, very fine transverse striÆ are seen upon them, consisting of rows of very minute dots. Fig. 24 s* represents the valve of Fragilaria vires´cens, a nearly allied species.

Diat´oma elongÁtum (Pl. V. fig. 31) is often found with the above. Its frustules are coherent at the angles. The front view (fig. 32 f) is rectangular, often slightly narrowed in the middle; and the valves are narrowly linear, and capitate at the ends; they are also transversely striated.

In SynÉdra splen´dens (Pl. V. fig. 5) the frustules radiate from a soft gelatinous cushion. They are linear in the front view (fig. 4 f), the valves (fig. 4 s) being gradually narrowed or attenuated from the middle to the ends, and exhibit transverse striÆ interrupted opposite a middle longitudinal line.

In Campylodis´cus costÁtus (Pl. V. fig. 27) the frustules are disk-shaped and curved, so as somewhat to resemble a saddle. The markings consist of central dots, with radiating coarse flutings.

Nitzsch´ia minutis´sima (Pl. V. fig. 28) has oblique valves, i. e. the front half of the suture is not opposite the back portion; the valves (fig. 29) are constricted in the middle, and the ends narrowed and prolonged. The markings consist of a row of oblong dots or puncta (punctum, a point). This species often forms yellowish layers upon damp paths, &c.

In the next group, the valves have a longitudinal line running down the middle of the valves, with a little knob or nodule in its centre (Pl. V. fig. 22 s), both consisting of internal thickened portions of the valves.

In CocconÉis placen´tula (Pl. V. fig. 33) the valves are oval, and the markings consist of longitudinal rows of minute dots, with a marginal row of puncta; these markings are invisible under ordinary illumination.

In GomphonÉma acuminÁtum (Pl. V. fig. 17) the frustules are attached to a branched stalk (stipes); they are wedge-shaped or cÚneate (cÚneus, a wedge) and transversely striate (fig. 18), the striÆ consisting of dots.

In PinnulÁria vir´idis (Pl. V. fig. 22), which is very often seen slowly traversing the field of the microscope when a drop of pond-water is examined, the frustules in the front view are linear, the valves being elliptic oblong, and transversely striated, the striÆ consisting of furrows.

In Gyrosig´ma (Pleurosig´ma) attenuÁtum (Pl. VI. fig. 16) the valves are sigmoid, or somewhat resemble a Greek ? (sigma) in outline, and the markings consist of rectangularly crossed rows of very fine dots; in the front view, the frustules are linear-oblong with truncate ends.

TabellÁria flocculÓsa (Pl. VI. fig. 23) has the frustules adherent only at the angles, as in DiÁtoma. They are rectangular, and in the front view exhibit a row of longitudinal dark lines interrupted in the middle; these have been compared to the vittÆ of the fruit of the UmbelliferÆ, and have received the same name.

Among the marine species may be mentioned MelosÍra nummuloÍdes (Pl. VI. fig. 9), in which the frustules are united into a chain or cylindrical filament. This is very common among sea-weeds, &c.; and it illustrates well the process of division of the frustules (fig. 10 f). The valves are covered with fine dots, and near each end of the frustules is a projecting rim encircling it, and appearing as a curved line extending beyond the margin of the frustule in the front view. In Actinocy´clus undulÁtus (Pl. VI. fig. 5) the frustules are separate, disk-shaped, and the valves are divided into six equal parts by six rays, each alternate portion of the surface of the valves being situated on a lower level than those adjacent, so that an alteration in the focus is required to bring into view the dots on any two adjacent divisions of the valve. The surface of the valves is covered with easily recognized dots. The form of the surface is best seen in the front view (fig. 5 f) when the frustule is placed on its edge.

RhabdonÉma arcuÁtum (Pl. VI. fig. 7), which is very commonly found attached to sea-weeds, resembles TabellÁria in the frustules having the vittÆ (Pl. VI. fig. 8 f). The frustules form short filaments, attached by a little gelatinous cushion. The valves have transverse striÆ, interrupted in the middle (fig. 8 s).

Gyrosig´ma (Pleurosig´ma) angulÁtum (Pl. VI. fig. 17) resembles G. attenuÁtum in the sigmoid form; but the markings consist of lines crossing each other obliquely; and these are resolvable into rows of dots (fig. 17 a) under suitable illumination.

In Coscinodis´cus radiÁtus (Pl. VI. fig. 3) the frustules are disk-shaped, the valves being elegantly sculptured with easily recognized cell-like markings or dots, so as to resemble a piece of vegetable cellular tissue. But in some other species the dots are very minute, and difficult to be shown satisfactorily. These markings consist of depressions or pits in the surface of the valves. That this is the case may easily be seen by examining a fragment of the valve, when the shadings of the broken ends of the netted thicker portions, which project like teeth, strongly contrast with the difficultly distinguishable portions of the thin interspaces. The fossil forms from the Bermuda deposit are best for the investigation of this structure; many of these are extremely beautiful microscopic objects, their markings resembling those on the engine-turned back of a watch.

The detection of the finer markings of the DiatomaceÆ, which, according to my view, consist of depressions like those upon the valves of Coscinodiscus, is a matter of great difficulty to those who are unaccustomed to the use of the microscope, and who have not a complete set of apparatus. The main point to be attended to in bringing them into view, is to use one-sided oblique light, i. e. to turn the mirror by its stem as much as possible to one side, and then to incline it so as to throw the light upon the object. In this way the valves of the species of Gyrosigma, for instance, appear covered with lines (Pl. VI. figs. 16 and 17); but the lines are spurious, i. e. they are the optical expression of rows of minute dots (figs. 16 a, 17 a); and when oblique light is thrown upon the valves from all sides, by means of a special achromatic condenser, in which the central rays are excluded, the dots become distinct, and the markings resemble those on the valves of Coscinodiscus. To show the finer dots clearly, a valve should be crushed, so as to obtain a fragment as flat as possible; for the surface of the valves is curved more or less in all the species. The valves of G. angulatum are generally used to test the quality of the object-glasses of the microscope, and also for practice in “making out” the lines and dots; there are, however, many DiatomaceÆ more difficult.

As the nature of these markings is a disputed point, the discussion of which is not adapted for an elementary work, I must refer for further details to the ‘Micrographic Dictionary;’ it may be remarked, however, that some observers have regarded them as cells, and others as elevations or tubercles on the surface of the valves.

The preparation of the valves for showing the markings should be effected by burning the frustules, or the mass containing them, on a strip of platinum-foil over a spirit-lamp. The incinerated mass should then be transferred to a slide, and the valves separated with the greatest care by a bristle mounted in a hair-pencil stick under a low power of the microscope.

This is, however, a substitute for the proper method, which is dangerous in the hands of one unpractised in chemical manipulation. It is this:—The mass of DiatomaceÆ (the water containing it having been carefully poured off as far as possible) is put into a Florence oil-flask, and strong nitric acid (aquafortis) gently added, more than sufficient to cover it. The mixture is then carefully boiled over a spirit-lamp for some time. When it is cold, distilled water is added, the whole shaken, and allowed to settle. The watery part is then gently poured off, more water added, and this poured off after settling, and the process repeated until a drop of the water evaporated to dryness on a slide leaves no residue. The DiatomaceÆ then form a white sediment at the bottom of the water, and can be transferred to a slide with a dipping-tube. The drop is then dried with a gentle heat, and the valves mounted as dry transparent objects (p. 12).

If the valves have coarse markings, they may be mounted in balsam; but if the markings are fine, balsam makes them much more difficult of detection.

Many of the most beautiful DiatomaceÆ are found in the fossil state; and specimens of these are sold already mounted. I would advise those unacquainted with them to purchase a slide of the “Bermuda” or “Richmond” earth, which abounds in the species of Coscinodiscus; and of the “San Fiore deposit,” which contains many species of Epithemia, Navicula, Pinnularia, &c. These may be procured from Mr. Norman, 178 City Road, or from the microscope-makers.

Volvocin´eÆ.—The VolvocineÆ are inhabitants of clear fresh-water pools, on heaths and bogs. They are very minute, of a rounded or plate-like (tabular) form, of a green colour, and are pretty readily distinguished from most of the other AlgÆ by their free motion; for they swim about in the water like animals, as which they were formerly considered. They consist usually of groups of thick-walled soft cells, each being furnished with one or two cilia, by means of which the movement of the compound bodies is produced.

In the beautiful Volvox globÁtor (which is not uncommon) the cells form a hollow sphere (Pl. VI. fig. 18), studded with exceedingly minute green spots or zoospore-like bodies, representing the green endochrome of the component cells, and from each of which very fine radiating lines extend, so as to give the surface a netted appearance; the lines consisting of delicate processes of the endochrome, which may be compared with those existing in the cells of the hairs of Tradescantia. In the interior of the parent globes are often seen several young organisms, usually eight, of a deep green colour; these escape by the rupture of the parent, so as to form independent beings. Sometimes they are found of a yellow colour, and furnished with a thick transparent coat; these are called “resting spores,” as they remain for some time before undergoing their full development.

The cilia of Volvox, of which there are two to each of the component cells, are difficult to detect; they are best seen when the organism is dried without a cover, or after moistening them with a little solution of iodine, which dyes them brown.

SynÚra volvox (Pl. VI. fig. 13) is a still more minute member of this family, and is often found rolling along among ConfervÆ. The greenish zoospore-like bodies of this Alga have one cilium only, and arise from a common centre by a narrowing of the base (fig. 14).

In GÓnium pectorÁle (fig. 11 a) the green bodies, which are sixteen in number, and furnished each with two cilia, are grouped into a flat square plate; and in the very minute Gonium tranquil´lum (fig. 11 c) these bodies are also sixteen in number, and arranged in a tabular form, but are without cilia.

Siphona´ceÆ.—The structure of this family may be illustrated by the genus VauchÉria, of which two or three species are common on damp ground or in freshwater pools, forming a green layer. At first sight, the filaments of which the little plants consist appear like those of a stout Conferva; but on close examination they are found to be branched, and not jointed, consisting of a single cell from end to end (Pl. VI. fig. 26). The reproduction is effected by the agency of two kinds of organs, antheridia and capsules (sporangia), situated near each other (fig. 26 a) on the walls of the filaments, of which they are protrusions or outgrowths—their cavities being separated from that of the filament by a partition or septum. The antheridia produce spermatozoa, and the sporangia each a spore, the one fertilizing the other in the ordinary manner. In addition to this method of fructification, zoospores are also produced—the ends of the filaments becoming swollen, the contents cut off by a septum, and forming single large zoospores covered with cilia, the further development of which resembles that occurring in the ConfervaceÆ.

Oscillatoria´ceÆ.—The members of this family are commonly found in stagnant water or on shaded damp ground, especially in the cold seasons of the year, forming green strata or masses.

Oscillatoria autumnÁlis (Pl. VI. fig. 1) occurs everywhere upon damp shaded banks of ditches, especially when newly made, forming a greenish-black closely adherent stratum. Under the microscope it is seen to consist of innumerable palish-green filaments; these are jointed or transversely striated, some being straight, others curved, the ends often exhibiting a writhing or worm-like movement. The appearance of these fibres is peculiar, seeming as if they were solid throughout, and so differing from that of the ConfervaceÆ, in which the cell-walls are readily distinguishable from the cell-contents. The fibres easily break across at the joints; and the last few segments are often narrowed and rounded, so as to form a blunt point. When they have been left in water, they exhibit colourless tubular sheaths surrounding and extending beyond them. These sheaths consist of the consolidated outer portions of the cell-walls; for when the cells undergo transverse division, and expand by growth in the direction of the length of the filament, the original septa or inner walls are broken through, and their remains may often be seen on the inner surface of the sheath, appearing as little teeth.

Oscillatoria nigra (Pl. VI. fig. 2) is another very similar species, forming blackish-green masses, and is common in ditches. It has longer filaments than the last, with narrowed and slightly curved ends; and the endochrome is distinctly granular.

In two other genera of this family, Vib´rio and SpirulÍna, the filaments are spiral. Vib´rio spiril´lum is excessively minute, colourless, and found in decomposing vegetable mixtures. The short filaments move rapidly through the water, with a corkscrew-like motion. In SpirulÍna oscillarioÍdes (Pl. VI. fig. 12), which is more rarely found in clear pond-waters among ConfervÆ, the filaments are greenish, and form a beautiful simple spiral, resembling that of a very slender spiral vessel.

Lyng´bya murÁlis (Pl. V. fig. 2) is very common on damp walls, gravel walks, &c. It forms a bright grass-green layer, consisting of somewhat rigid curled filaments. The endochrome is usually broader than long; and the cells of the filaments are often found empty, the endochrome having escaped in the form of gonidia.

Pl. VI. fig. 6 represents a species of BactÉrium which is not uncommon in decomposing vegetable liquids; the filaments are short, curved, pointed at the ends, and have four joints.

Fig. 26 represents a SchizogÓnium, found upon damp paths. The filaments resemble those of Lyngbya, but are united in pairs.

Fig. 3 represents a filament of a U´lothrix, which is common in freshwater pools, showing the curious manner in which the endochrome is arranged in the cells, forming bands partially lining the cell-walls.

Nostocha´ceÆ.—Two species of the typical genus Nos´toc will serve to represent this family. Nos´toc commÚne is found on damp ground or in ponds, and forms to the naked eye firmish, olive-green, skin-like, plaited masses, an inch or more in diameter. Under the microscope it is seen to consist of numerous beaded fibres, imbedded in worm-like gelatinous sheaths; these are curved and interwoven to form the compound mass. In the middle of many of the filaments is an enlarged colourless cell, called the vesicular cell, which is related to the reproduction, but in a manner not yet determined.

Nostoc minutis´simum (Pl. VI. fig. 4) forms solid gelatinous bluish-green masses, varying in size from a pin’s head to a pea; it is found upon unhealthy water-plants kept in glass vessels. The component filaments are very slender, wavy, and the sheaths often have a brownish tinge.

UlvaceÆ.—These AlgÆ are mostly marine—some, however, being found in brackish or fresh water, or on damp ground, thatch, moss, &c. They are generally of considerable size, forming flat or tubular fronds, often several inches long, a few being filamentous. They consist of one or more sheets or layers of cells, containing mostly green endochrome. This at first fills the cells, but subsequently becomes converted into single spores, or subdivided into numerous ciliated zoospores.

Ul´va latis´sima is very common on the sea-coast, being found attached to stones, shells, &c. It forms a broad, flat, green, rounded or oblong, thin frond, wavy and crumpled at the margins, and from 6 to 18 inches in length. The minute cells form two layers, adherent to each other. The zoospores formed are numerous in each cell.

Enteromor´pha compres´sa (Pl. IV. fig. 31) is also common in the sea and in brackish ditches; it is often found floating. The frond is green, tubular, flattened or compressed, and branched, the branches being usually simple and narrowed at the base. The frond consists of two layers of minute cells, separated by a space rendering it hollow. The zoospores are numerous in the cells (fig. 32).

Palmella´ceÆ.—These AlgÆ are found in fresh or salt water, or on damp earth, wood, &c. They are green or red, forming round or irregular masses or strata. They consist of loosely connected cells, imbedded in a gelatinous mass or matrix, thus forming a frond.

Chlorococ´cum vulgÁre (Pl. II. fig. 1) is very common upon the bark of elm-trees, palings, &c., forming a green granular crust. It consists of minute rounded or oval cells, mostly undergoing division into twos, fours, or eights. These cells are attached to the sides or ends of very fine colourless filaments. It is most probable that this organism, which is usually placed among the AlgÆ, consists of the gonidia of a Lichen.

Chlorococcum muror´um forms a somewhat similar but soft and thin green layer, upon damp walls or other porous bodies. It consists of very minute oval green cells, with thick walls, and imbedded in the ends of prolongations of a gelatinous matrix.

Palmel´la cruen´ta (Pl. VI. fig. 25) forms a portwine-red layer at the bottom of damp walls or on the ground. It is composed of pale red cells, imbedded in no definite order in a colourless gelatinous matrix. The cells are filled with red granules, and are often found undergoing division.

Pl. VI. fig. 19 represents a species of Gloeocap´sa, in which the cell-envelopes do not soften and unite to form a gelatinous matrix, as in Palmella and other members of the family, but are persistent. This species occurs in fresh water containing ConfervÆ.

Chara´ceÆ.—This family consists of the single genus ChÁra, the systematic position of which is not agreed upon by authors; as however its structure will be better understood after what has been gone over, it may be conveniently considered here.

There are several species of Chara, the one illustrated, Chara vulgÁris (Pl. VI. fig. 21), being commonly found in ditches and pools. It consists of long main stems, often a foot in length, which are branched, and surrounded at tolerably regular intervals by whorls of branchlets. In some species, the stems and branches consist simply of elongated cells, arranged end to end; while in others, of which Chara vulgaris is one, the central cells are surrounded by a number of narrower spirally arranged cells, forming an outer coating.

The CharÆ have long formed interesting microscopic objects, on account of the circulation of the protoplasm being visible in the cells, as in the hairs of Tradescantia. This is best seen in those species in which the outer layer of cells is absent from the stems, and which were formerly arranged in a separate genus (Nitella). But it may also be seen in the stems and especially the young branchlets of any of the other species; and as the granules of the protoplasm are large, the phenomenon is more easily witnessed than in Tradescantia.

The fructification consists of two kinds of organs, viz. red globules (Pl. VI. fig. 22) representing the anther-organ, and green capsules (fig. 22), or nucules, corresponding to the ovaries. The structure of the globules is very curious. Their transparent walls (fig. 20) consist of eight somewhat triangular plates, each of which is composed of cells radiating from a centre; and from the inside of each of these centres arises a tubular cell extending to the middle of the globule, the unattached ends giving origin to numerous colourless coiled filaments, consisting of minute cells arranged end to end, each containing a very minute coiled spiral fibre, to which are attached two exceedingly slender cilia. These ciliated fibres are the spermatozoa. The capsules or nucules (fig. 22), which are situated near the globules, are urn-shaped, coated with spiral cells, and crowned with five shorter cells. When the globules are ripe, they become ruptured by the separation of the valves; and the spermatozoa, escaping from the cells of the coiled filaments, swim about and enter a canal in the capsules to fertilize the ovule contained within.

The CharÆ grow readily in a glass jar of fresh water, with a few pebbles at the bottom; and if the plants be not overgrown with Confervoids, the fructification will continue to be produced almost throughout the year.

The circulation is best seen in the whorled branchlets, a portion of the growing ends being placed in a live-box, or simply laid upon a slide in water and covered with thin glass.

Preservation.—The AlgÆ are best preserved in two ways,—the entire fronds being dried upon paper under pressure, as directed for the Ferns; and small portions, showing the minuter structures and fructification, being mounted in chloride of calcium or glycerine. If it is required to preserve the marine AlgÆ according to the first method, they should first be immersed for a time in fresh water, to dissolve out the saline matters derived from the sea-water, which would keep them damp and ultimately spoil them. After these matters have been removed, the fresh water should be changed, and pieces of paper placed beneath them while suspended in the water; on withdrawing the paper carefully, keeping the AlgÆ at the same time spread out, they may be made to retain the required position; and when the water has drained away, and the remaining moisture has mostly evaporated, they may be submitted to pressure in a press.

The Confervoid AlgÆ may be conveniently spread out upon paper and preserved in the same manner, as some of the distinguishing characters are founded upon their appearance in the dry state, their adhesion to the paper, &c. Moreover they can then at any time be minutely examined, by the immersion of a small portion in water.