Links With the Past in the Plant World

by A. C. Seward

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.