THE GEOGRAPHICAL DISTRIBUTION OF PLANTS

'No speculation is idle or fruitless that is not opposed to truth or to probability, and which, whilst it co-ordinates a body of well established facts, does so without violence to nature, and with a due regard to the possible results of future discoveries.' Sir Joseph Hooker.

In the vegetation of the British Isles the leading rÔle is played by that large group to which the term Flowering Plants is frequently applied. This group, including the two sub-divisions Dicotyledons and Monocotyledons, is known by the name Angiosperms, a designation denoting the important fact that the seeds are developed in an ovary or protective seed-case (???e???, a vessel or box). The fact that these highly elaborated products of development made their appearance, so far as we know, at a comparatively late stage in the history of the plant-world, attests their efficiency as a class and demonstrates the rapidity with which they have overspread the surface of the earth as successful competitors in the struggle for existence. As Darwin wrote in a letter to Sir Joseph Hooker in 1881, 'Nothing is more extraordinary in the history of the vegetable kingdom, as it seems to me, than the apparently very sudden or abrupt development of the higher plants(11).' In another letter (1879) to the same friend we read, 'The rapid development as far as we can judge of all the higher plants within recent geological times is an abominable mystery(12).' Making allowance for the probability, or indeed certainty, that the imperfection of the geological record tends to exaggerate the apparent suddenness of the appearance of this vigorous class, and allowing for the fact that our knowledge of the records of the rocks in which the highest plants first occur is very incomplete, we cannot escape from the conclusion that this recently evolved group spread with amazing rapidity. Various reasons may be suggested in explanation of the dominant position which the Angiosperms hold in the floras of the world. As an instance of their rapid increase during the Cretaceous epoch[1], the period which has furnished the earliest satisfactory records of Flowering Plants, the following statement by an American writer may be quoted:—'The rapidity with which it [i.e. the group of Flowering Plants] advanced, conquering or supplanting all rivals, may be better understood when we remember that it forms 85% of the flora of the Dakota group '; that is a series of sedimentary rocks in Dakota referred by geologists to the middle of the Cretaceous period(13). In the Wealden rocks of England, which are rich in the remains of Lower Cretaceous plants, no undoubted Flowering Plant has so far been found.

The more efficient protection of the ovules, the germs which, after fertilisation, become the seeds, the extraordinary variety in the floral mechanisms for assisting cross-pollination, the arrangements for nursing the embryo, and the structural features of the wood in relation both to rapid transport of water and to the storage of food, are factors which have probably contributed to the success of the Angiosperms. The degree of weight to assign to each contributing cause cannot as yet be satisfactorily determined, but the general question raised by the recent origin of these latest products of evolution offers a promising field for work. While admitting our inability at present to do more than suggest possibilities, we may encourage research by speculation.

The members of the Vegetable Kingdom placed next to the Flowering Plants are the Gymnosperms or naked-seeded (?????, naked) plants, including (i) the Conifers, e.g. pines, firs, larches, the yew, etc., (ii) a small group of plants known as the Cycads, whose existing members, now almost confined to a few tropical regions, are the descendants of a vigorous race represented by many species in the floras of the Mesozoic epoch. A third sub-division of the Gymnosperms, the Ginkgoales, is represented by a single survivor, which is described in a later chapter as one of the most remarkable links with the past in the plant kingdom.

The Gymnosperms are geologically very much older than the Angiosperms. Members of this class played a prominent part in the vegetation of the Coal age and it is certain that they existed in the still older Devonian period. The only other group to which reference is made in later chapters is that of the Ferns, one of the sub-divisions of a large class known as the Vascular Cryptogams or Pteridophyta. These plants, like the Gymnosperms, are represented in the oldest floras of which recognisable remains have been preserved. The main groups of the vegetable kingdom, founded on existing plants, are distinguished by well-defined differences; they are comparable with separate twigs of a tree springing from larger branches and these again uniting below in a common trunk. The vegetation of to-day represents only the terminal portions of the upper branches. As we descend the geological series, records of extinct types are found which enable us either to trace the separate branches to a common origin or to recognise a convergence towards a common stock. Were a botanist to find himself in a forest of the Coal age he would experience great difficulty in assigning some of the plants to their systematic position: characters now regarded as distinguishing features of distinct groups would be met with in combination in a single individual. It is by the discovery of such generalised types, which serve as finger-posts pointing the way to lines of evolution, that the student of pre-existing plants has been able to throw light on the relative antiquity of existing forms, and to trace towards a common ancestry plants which now show but little indication of consanguinity.

Confining our attention to the dominant group of plants in the British flora, namely the Flowering Plants, we may profitably consider the question, though we cannot satisfactorily answer it,—which members of this group are entitled to be regarded as the most ancient inhabitants? The past history of our native plants, and their geographical range, not only in the British Isles but on the Continent of Europe, are subjects well worthy of the attention of field-botanists whose interests are apt to be confined within too narrow bounds. There are numerous problems relating to the composition of the present vegetation of Britain which might be discussed in reference to the relative antiquity of plants; but in a single chapter it is impossible to do more than call attention to certain considerations which are frequently overlooked by students of British species.

It is customary to speak of the British flora as consisting for the most part of species introduced into this country by natural means, while some plants owe their introduction to human agency or are 'escapes' from cultivation. It is by no means an easy task in some instances to decide whether a species is native or introduced, but in some cases, a few of which are mentioned, there is no doubt as to the alien nature of the plants. The term 'native' needs a word of explanation. It is not intended to convey the idea that a plant so designated came into existence on British soil and spread thence to other regions; but by native species we mean such as have reached this country by migration from other lands, or it may be in some instances have actually originated in this part of Europe. One of the best known aliens in Britain is the American water-weed, Elodea canadensis (or Anacharis alsinastrum), which was discovered about sixty years ago in a canal near Market Harborough in Leicestershire(14). In all probability this North American species was introduced into England with timber. Once established, it spread through the waterways with alarming rapidity and became a serious pest. Elodea affords an admirable instance of the serious interference with the balance of Nature by the introduction of a new competitor into an environment conducive to vigorous development. Another foreign water-plant, Naias graminea, for the importation of which Egyptian cotton may be responsible, has been recorded from the Reddish canal near Manchester(15). This African and Asiatic species occurs in Europe only as a colonist; it is said to have been introduced into Italy with East Indian rice. A more recent case of alien immigration due to unintentional human agency is that of Potamogeton pennsylvanicus, a pond-weed of Canada, the United States, Jamaica, and elsewhere. Specimens of this species were first noticed in 1907 in a canal near Halifax close to the effluent from a cotton mill. Since its discovery the plant has slightly extended its range. It is suggested by Mr Bennett, who first identified the alien, that its fruits were brought to this country in goods from the United States(16).

Of the introduction of these and other foreign plants we have satisfactory records; but there are many others which may owe their presence to man's agency, though we have no information as to their arrival.

It has long been recognised that several members of the British flora are related to Scandinavian species. The Scandinavian flora, as Sir Joseph Hooker says in his well-known paper on the 'Outlines of the Distribution of Arctic plants,' 'not only girdles the globe in the Arctic Circle, and dominates over all others in the North Temperate Zone of the Old World, but intrudes conspicuously into every other temperate flora, whether in the northern or southern hemisphere, or on the Alps of tropical countries'(17). The view generally held is that during the Glacial period this Arctic flora was driven South, and aided by land-bridges, which were afterwards submerged, many of the northern migrants found a more congenial home in Britain. It is however by no means improbable that this conclusion may have to be considerably modified. Mr and Mrs Reid, as the result of their careful analysis of the Pre-Glacial Flora of Britain, express the opinion that 'the pre-glacial plants suggest climatic conditions almost identical with those now existing, though slightly warmer' (27, 2). It is noteworthy that the list of plants given in their paper does not include any typical Arctic species. The occurrence on the mountains of Scotland and elsewhere of such plants as Silene acaulis, Dryas octopetala, Saxifraga oppositifolia and other Saxifrages, Rubus chamaemorus (the Cloudberry), and the dwarf Birch illustrate the Arctic-Alpine element in our flora.

The opinion is held by many Swiss botanists that their Alpine species have in large measure been derived from non-glaciated parts of the Pyrenees, that is from a region which was presumably able to retain its flora at a time when more northern lands were exposed to extreme Arctic conditions. My friend Dr Moss believes that some of the so-called Scandinavian plants came to Britain from Central Europe after the retreat of the ice; if this view is correct it means that some at least of our Arctic-Alpine plants reached these islands by a southern rather than by a northern route.

Interesting examples of far-travelled northern plants recently described by Professor Engler of Berlin afford additional illustrations of the general principles enunciated many years ago by Sir Joseph Hooker. A species of flowering Rush, Luzula spicata var. simensis, occurs at an altitude of 3600 metres in Abyssinia and on Kilimanjaro. Luzula spicata is found in the whole of the Arctic and Subarctic belt in Scotland, Auvergne, the Jura mountains, and elsewhere. The species probably began its career in the northern hemisphere where it grew abundantly on the higher ground in the Arctic Circle: it eventually travelled along the North American Andes and appeared in Mexico under a guise sufficiently distinct to warrant the use of another name, Luzula racemosa. In an eastern direction it reached the Himalayas and is represented in Abyssinia by a closely allied form. From Abyssinia to Kilimanjaro Luzula spicata 'had to travel a long distance; but it is not impossible that it either still exists or has existed previously on a few of the high mountains between Abyssinia and Kenia, from which, having advanced to the Kilimanjaro, it again produced new forms.... At any rate, it is impossible to do without distribution of seeds of alpine plants by air-currents or by birds from one mountain to the other in explaining the history of distribution'(18).

The majority of British plants are identical with species in Central and Northern Europe: of these, some are among the most widely spread English species, e.g. the Daisy and Primrose, while others, such as the Oxlip (Primula elatior), are confined to the Eastern counties, and others, such as the Cheddar Pink (Dianthus caesius), are restricted to Western counties.

Before considering a small section of the British flora which is the most interesting from the point of view of origin, a short digression may be allowed in order to call attention to the importance of a branch of science which Darwin spoke of as 'that grand subject, that almost keystone of the laws of creation, geographical distribution,' and in 1847 referred to as 'that noble subject of which we as yet but dimly see the full bearing.' It was largely as the results of his study of distribution in the Galapagos Islands that Darwin determined to 'collect blindly every sort of fact which bears anyway on what are species.' The acceptance of the view 'that each species was first produced within a single region'(19), raises the subject of geographical distribution to a far higher plane than it occupied in pre-Darwinian days. Although most people are familiar with some of the commoner means by which plants are able to colonise new ground through the adaptation of their fruits and seeds to various methods of transport, the conception of a plant as a stationary organism tends to prevent due allowance being made for the comparative facility with which, in the course of successive generations, a species is able to wander from place to place. The individual animal is endowed with powers of locomotion enabling it to seek new feeding-grounds and to avoid enemies; but with the exception of some of the simplest forms a plant cannot move—'le matin la laisse oÙ la trouve le soir.'

The rate of travel may or may not be rapid, but in a comparatively short time, if the conditions are favourable, a tree may spread over a wide area. Mr Ridley, Director of the Botanic Gardens, Singapore, writes as follows in reference to the rate of travel of one of the common Malayan trees (Shorea leprosula), which bears winged fruits particularly well adapted to wind-transport: 'If we assume that a tree flowers and fruits at 30 years of age and the fruits are disseminated to a distance of 100 yards, that the furthest fruits always germinate and so continue in one direction, it will be seen that under such most favourable circumstances the species can only spread 800 yards in 100 years, and would take 58,666 years to migrate 100 miles'(20).

There is, however, one type of distribution—what is called discontinuous distribution—to which special attention should be directed on account of its intimate association with questions relating to the past history of living organisms. Many examples might be quoted from both the animal and plant kingdoms in support of the view that discontinuous distribution is a criterion of antiquity. When identical or very nearly identical plants occur in regions separated from one another by areas in which the particular species is unknown, the inference is either that the surviving individuals are remnants of a large number formerly distributed over a wider continuous area, or that in the course of evolution similar conditions in widely separated areas led to the production of identical types. The former view is much the more probable: it is consistent with the conclusions arrived at on other grounds as to the connexion between discontinuous distribution and ancient lineage. The explanations of the widespread occurrence among different races of similar objects or legends afford an analogous case. As Dr Andrew Lang points out in Custom and Myth, it is held by some students that the use of the bull roarer—to cite a specific instance—by different peoples denotes descent from a common stock, though he considers the more probable explanation to be that similar minds, working with simple means towards similar ends, might evolve the bull roarer and its mystic uses anywhere.

The Cedars of Lebanon afford an interesting example of discontinuous distribution. They illustrate how a species, which may be assumed to have originated in one region, in the course of its wanderings may undergo slight changes until, at a later stage when the plants have disappeared from parts of the once-continuous area, the remaining outlying groups of individuals are spoken of under different specific names. The cedars of Lebanon, known as Cedrus libani, occur as isolated groups on the Lebanon hills as outliers of the larger forests of the Taunus 250 miles distant. The African cedar, Cedrus atlantica, is separated from the Lebanon cedar by a distance of 1400 miles. Approximately the same distance divides the Lebanon cedar from the deodar, Cedrus deodara, which extends from Afghanistan along the Himalayas almost to the confines of Nepal. Sir Joseph Hooker regards the three cedars as varieties of one species which once formed a continuous forest: he attributes the present discontinuous distribution, in part at least, to the effects of a warmer succeeding a colder climate. The less favourable conditions drove the vegetation of the lowlands to seek more congenial habitats at higher altitudes. In this connexion it is interesting to find that in Algeria the cedar is confined to the higher ground where the snow lies long in the spring(21).

The Tulip Tree of North America and Central China affords one of many examples of existing flowering plants which illustrate the close connexion between present distribution and past history. The genus Liriodendron, often cultivated in the south of England, is now represented by two species, the best known of which—the Tulip Tree, Liriodendron tulipera—extends from Vermont to Florida and westwards to Lake Michigan and Arkansas. The leaves bear a superficial resemblance to those of the Sycamore, but are as a rule easily distinguished by the truncated form of the apex; the specific name was suggested by the tulip-like form of the flowers. Fossil leaves of Liriodendron are not uncommon in the Cretaceous rocks of Disco Island in latitude 70° N., where they occur with other flowering plants which bear striking testimony to the mildness of the Cretaceous climate in high northern latitudes. One of the associated flowering plants is a species of Artocarpus, described by Dr Nathorst as Artocarpus Dicksoni which bears a close resemblance to A. incisa the bread-fruit tree of the southern tropics of the Old World. Without attempting to deal fully with the past history of Liriodendron, it may be confidently stated that the records of the rocks are consistent with the idea of antiquity suggested by the present distribution of the two surviving species.

Islands such as Great Britain and Ireland, situated a short distance from a continent, contain many plants which are widely spread in different parts of the world, together with a very small number peculiar to the British Isles though closely allied to species on the neighbouring continent or to plants farther afield. The occasional recognition of species previously believed to be confined to Britain tends to reduce the short list of our endemic types.

An enquiry into the origin of an island flora involves a consideration of the data in regard to changes in level and relative distribution of land and water in the course of geological evolution. It is generally agreed that at no distant date, in a geological sense. Great Britain and Ireland were united to the continent. There is, however, another fact to reckon with, namely the prevalence of Arctic conditions in northern Europe when a thick sheet of ice spread over the greater part of the British Isles. There can be no doubt that the severity of the climate during the Glacial period was such as to destroy a large proportion of the vegetation. The question is, were all the flowering plants destroyed or were some of the hardier species able to survive, either on the higher peaks which kept their heads above the level of the ice or on the southern fringe of England beyond the ice-covered region? It is impossible to give a definite answer: the probability is that nearly all the pre-Glacial species were destroyed, but it is not impossible that some Alpine-Arctic plants escaped extinction, while others retreated to more southern and less Arctic areas by means of a land-connexion with France or crossed the intervening sea by ocean-currents, by animal agency, or by wind.

Although we possess but imperfect information as to the extent and duration of land-bridges between Britain and the continent, there are no special difficulties in the way of accounting for the presence of Scandinavian, Germanic, and other elements in the British flora. There are, however, other and more difficult problems to consider in reference to a small group of flowering plants which are met with in the west and south of Ireland, also, to a less extent, in Cornwall and in a few other localities in the south-west of England. In Connemara in the west of Ireland, where hard frosts are unknown and winter snows are rare, there are three kinds of Heath, St Dabeoc's Heath (Daboecia polifolia), the Mediterranean Heath (Erica mediterranea) and Erica Mackaii which are not found elsewhere in the British Isles or in the whole of northern Europe, but reappear in the Pyrenees. The London Pride (Saxifraga umbrosa), another Pyrenean plant, grows on the south and west coast of Ireland from Waterford to Donegal. Arbutus Unedo, the Strawberry tree, which flourishes in the Killarney district of County Kerry and occurs in neighbouring localities, has a wide distribution in the Mediterranean region. Devonshire and Cornwall possess two other Heaths, Erica ciliaris, which extends into Dorsetshire and occurs in north Brittany, and Erica vagans, both Pyrenean species, while a Mediterranean plant. Gladiolus illyricus, grows in the New Forest.

In 1846 Edward Forbes dealt with the problems presented by the distribution of British plants in an essay which has exercised a far-reaching influence. When Forbes published his work, comparatively little was known as to the possibilities of transport of seeds and fruits across barriers of water(22). His conviction that the known means of dispersal were insufficient to account for the presence of Mediterranean or Lusitanian plants in Ireland led him to turn to geology for a solution of the problem. He was thus led to put forward the view that in the course of the Tertiary period when, as we know from palaeontological evidence, the climate of north and west Europe was much warmer than it is now, and long-before the beginning of the climatic changes which culminated in the Glacial period, there was a land-connexion between the west of Ireland and the south-west of the continent. Mr Praeger, whose work on the Irish flora is well known to systematic botanists, agrees with the conclusions of Forbes, and sees in the Portuguese and Mediterranean plants 'relics of a vegetation which once spread along a bygone European coast-line which stretched unbroken from Ireland to Spain'(23). If this explanation is correct it entitles Arbutus, St Dabeoc's heath and other members of this southern group to be regarded as a very old section of our flora. There is, however, another side to the question: granting that a certain number of Irish plants were able to withstand the rigours of an Ice age, it is hardly likely that the strawberry tree and other southern types, which it is admitted flourish in the south-west of Ireland because of the mildness of the climate, were of the number of those which endured an extreme Arctic phase. Moreover, if these Mediterranean species are survivals from the Tertiary period, if they have been isolated since pre-Glacial days as an outlier of a southern flora, we might fairly expect that during the long interval between their arrival and the present day new forms would have been produced closely related to, though not identical with, the parent types. This, however, has not been proved to be the case. Darwin in speaking of Forbes' Essay in a letter to de Candolle in 1863 says that he differs from most of his contemporaries 'in thinking that the vast continental extensions of Forbes, Heer, and others are not only advanced without sufficient evidence, but are opposed to much weighty evidence'(12). The alternative view is to regard Arbutus and its compatriots as post-Glacial arrivals and not as survivals from a widely spread Tertiary flora.

A recently published account of the New Flora of the volcanic island of Krakatau furnishes an instructive and remarkable demonstration of the facility with which a completely sterilised island, separated by several miles of ocean from neighbouring lands, may be restocked with vegetation(24). In 1883 the island of Krakatau, then densely covered with a luxuriant tropical vegetation, was partially destroyed by a series of exceptionally violent volcanic explosions. After this catastrophe only a third of the island was left: the surface was deeply covered by pumice and volcanic ash and no vestige of life remained. In 1906 a party of botanists who spent a few hours on Krakatau collected 137 species of plants: the vegetation was in places so dense that it was with the greatest difficulty they penetrated beyond the coastal belt, and some of the trees had reached a height of 50 feet. The seeds and fruits of this new flora have been carried by ocean-currents, by wind, and by the agency of birds from other islands in the Malay Archipelago. The nearest islands, except the small island of Sebesi, about 12 miles distant, are Java and Sumatra, separated from Krakatau by a stretch of water about 25 miles in breadth. It is reasonable to wonder whether, had Forbes known of this and similar modern instances of the capabilities of plants as travellers, he would have adopted the view he did. In this connexion it may be added that in recent years the glaciation of Ireland has been shown to be more extensive than it was believed to be when Forbes wrote his essay.

There would seem to be no insuperable objection to the conclusion that the Mediterranean plants in Ireland and in the south of England reached their present home after the retreat of the ice at the end of the Glacial period, and after Ireland became an island. A full consideration of the problem is beyond the scope of this book, but I have briefly stated the case, not with the authority of an expert but in order to draw attention to a particularly fascinating study in plant-migration.

In a volume by W. Canton entitled A Child's book of Saints a story is told in which the presence in Ireland of Mediterranean species receives a more picturesque explanation. The Monk Bresal was sent to teach the brethren in a Spanish monastery the music of Irish choirs. In later years Bresal longed for a sight of his native land, though he loved his home and 'every rock, tree, and flower' in his adopted country. After returning to Ireland, his thoughts reverted to Spain; 'it appeared to him as though he was once again in a granite nook among the rocks beside the Priory'; he saw the ice-plant with its little stars of white flowers sprinkled with red (the London Pride) and a small evergreen tree from which he had often gathered the orange-scarlet berries (Arbutus). The Prior of the Spanish monastery 'with heavenly vision saw Bresal gazing at the evergreen tree and the ice-plant, and turning to the trees blessed them and commended them to go and make real his dream. As Bresal brushed away his tears he saw with amazement at his feet the ice-plant and hard by the evergreen tree.'

The plant represented in (Fig. 2) is another British species which tasks the ingenuity of students of plant-geography. This is the Pipe Wort (Eriocaulon septangulare), the sole representative in Europe of a certain family of Monocotyledons: it flourishes in the west of Ireland and in the western islands of Scotland but nowhere else in Europe; it is native on the other side of the Atlantic in Canada and the northern United States of America. Mr Praeger in describing the striking mixture of species in the west of Ireland writes, 'The pool from which we gather the American Pipe Wort is fringed with Pyrenean Heathers. The cracks which are filled with the delicate green foam of the maiden hair are set in Bearberry and Spring Gentian; Habenaria intacta, far from its Mediterranean home, sends up its flower-spikes through carpets of mountain Avens; and St Dabeoc's Heath and the dwarf Juniper straggle together over the rocky knolls'(25).

The presence of Eriocaulon on the western edge of Europe may be attributed to migration in pre-Glacial days from North America by way of a land-connexion, of which Greenland and Iceland represent surviving portions. The opinion held by Forbes, and advocated by some later naturalists, that the southern companions of Eriocaulon in the west of Ireland are survivors from a Tertiary flora which have lived through the Ice Age, is consistently extended to the Pipe Wort. On the other hand, before yielding to the temptation to regard these American and Mediterranean species as links with the Tertiary period, we must be convinced that the possibilities of post-Glacial introduction, even without the aid of land-bridges, have been exhausted. The Pipe Wort is a botanical puzzle which affords a good example of the accession of interest to field-botany effected by a knowledge of the distribution of the component members of the British flora. The problem of its past history suggests an experimental enquiry into the adaptability of its seeds to dispersal, and emphasises the importance of the co-operation of botanists and geologists in a common endeavour to trace the origin of British plants.

In addition to the Pipe Wort, mention may be made of three other American flowering plants recognised in the Irish flora. Sisyrinchium angustifolium recorded from the west of Ireland is native in temperate North America; the orchid, Spiranthes romanzoffiana, met with in the south and north of Ireland, is widely spread in Canada and the northern States, while Sisyrinchium californicum, a native of California and Oregon, was discovered by Mr Marshall in marshy meadow-land near Wexford(26). In the case of the more recently discovered American immigrants, the possibility of human introduction must be borne in mind, though there are no special reasons for doubting that some, as in the case of Eriocaulon, reached the Irish coast by natural agencies.