there is a small but well-marked element in the flora which has its home in the northern portion of the New World; in our islands these species live side by side with the Pyrenean and Mediterranean plants lately dealt with. Here, then, is the problem set before us. How are we to account for the presence of these unexpected strangers in a flora derived in the main from a westward migration from the adjoining parts of the Continent, from which they are absent? And especially what are their relations to the Glacial Epoch, during which the Continental flora was forced far southward by the advance of the ice, while that of our own islands was probably greatly reduced, and the balance forced into limited refuges in the south-west, if it survived at all? It should at once be pointed out that these peculiar Pyrenean and American elements in our flora are matched by similar elements in the fauna. Into the zoological evidence we cannot go here, but one well-marked species of each geographical group may be mentioned. The Spotted Slug of Kerry (Geomalacus maculosus) is elsewhere confined to Portugal; while a little fresh-water Sponge, Heteromeyenia ryderi, widely spread in Irish lakes and rivers, and occurring also in Scotland, is otherwise exclusively American. In speculating, therefore, as to the origin of the plants, we must not leave out of account the question of the corresponding animals.
First of all, is it possible that these unexpected organisms were introduced into our islands by man? In an earlier chapter it has been seen how human trade and intercourse have imported into our flora plants from the uttermost ends of the Earth. May we seek in this direction an explanation? The evidence is entirely against such a solution. These plants (and animals) are found chiefly—many of them entirely—in the wildest parts of the country, and bear fully the stamp of natives of old standing. Human foreign intercourse is not so old but that the introductions which it effected are still easily discernible to the student: the plants which have come to us thus bear the imprint of their origin; they spread outwards from centres of human activity, and are absent from undisturbed areas; they cannot in most cases compete with the indigenous vegetation, and only exist by confining their attempts at colonization to places where man has ousted the native flora—such as tilled land, roadsides, railway tracks. Even those aliens which have succeeded in winning a place among the native plants, such as the Monkey Flower (Mimulus Langsdorfii) or Michaelmas Daisies (Aster spp.) of North America, which are found sometimes in quite wild situations, the experienced field botanist detects readily enough. The introduction of the plants in question by man has never been advocated by a responsible biologist.
Assuming, then, that these groups owe their presence to natural agencies, the next question that arises is, Could they have come to our shores across the existing seas, or must we relegate their arrival to periods when different distribution of sea and land would aid their migration by allowing them to travel across a land surface, or at least to cross sea-barriers less wide than the present? This leads us to consider the means of dispersal possessed by the species in question, and to measure these against the nature of the barriers they would have been called on to cross. An investigation on these lines would be lengthy, and out of place here. The reader has already from [Chapter III.] acquired some insight into the powers as well as the limitations possessed by seeds for crossing such barriers. Summing up the evidence briefly, it may be said that the seeds of none of the southern group float in water; consequently transport by currents is ruled out. Secondly, none of them is so light (see pp. 62-69) as to render it possible for them to cross the intervening sea by wind currents; very much the lightest seeds in the group are those of the Orchid Neotinea intacta, yet even these could not on any reasonable theory have been transported by wind from the plant’s nearest station (in Southern France); the high speed of fall of the small seeds of the Pyrenean Heaths or Saxifrages renders their wind transport, even from the smaller distance which has to be reckoned with, in their case still more improbable. There is left, then, the agency of birds (see p. 70): can we look to these swift messengers for assistance? The rapid digestion of birds renders it futile to expect that even those which do not crush the seeds which they eat could bring over from the Pyrenees seeds which they have swallowed; so we are forced back on the uncertain method of ectozoic dispersal: that is, on the assumption that seeds of these plants have been imported by becoming entangled in the feathers of birds, or by adhering—possibly with the aid of mud—to their feet. That seeds are transported by these means has been shown by the observations of Darwin and other observers; but that the seeds of a number of different plants, growing in different situations, should be brought thus from the Pyrenees and Mediterranean to our western coasts is a highly speculative suggestion. If we discard it, there is left the hypothesis that the plants migrated long ago overland, at a time when the western coastline of Europe was continuous and lay farther seaward. Such conditions have not occurred since the Ice Age; so we have to assume that the plants, arriving perhaps in Pliocene times by slow terrestrial dispersal, and subsequently cut off by invasions of the sea upon their line of advance, survived the cold and ice of the Glacial Period within the limits of our islands. That appears, on consideration of the geological evidence of widespread glaciation, sufficiently improbable; but we must remember that the evidence supplied by the plants is buttressed formidably by that of the corresponding animals, some of which, such as the Kerry Slug, are far less fitted for transmarine dispersal than are the seeds of plants. Also, we are faced with the problem of the American plants, and such organisms as the American Sponge, Heteromeyenia: a direct crossing of the ocean appears for them wholly impossible. Yet if they crossed over long-gone land surfaces, their arrival on this side of the Atlantic must be very ancient, and they must certainly have weathered successfully the Great Ice Age. The problem, it is clear, is an exceedingly difficult one, upon which it would be rash to pronounce any hasty opinion. Students of the subject have come to widely difficult conclusions: some holding with Edward Forbes that these Lusitanian and American organisms represent the very oldest element in our fauna and flora, having migrated over bygone land surfaces in distant times and successfully survived the terrors of the Glacial Period; others claiming a much less remote period for their immigration. Indeed, one eminent recent writer on the subject, the late Clement Reid, considered that the Lusitanian plants are among the most recent arrivals in the country, their introduction being due mainly to birds driven by exceptional gales.
The question of the Lusitanian and American elements in our flora has been treated at some length both because it offers one of the most interesting problems in British botany, and because it affords a good illustration of the far-reaching nature of the questions which may lie behind the occurrence on our hills or in our valleys of even the humblest plant or animal. Each organism has a long record behind it, stretching far beyond the earliest periods of human history; and it is only by wide and patient study that we can hope to trace any portion of its story.
CHAPTER VIII
SOME INTERESTING BRITISH PLANT GROUPS
In the preceding chapters glimpses have been obtained of some of the wider aspects of plant life, particularly as seen on the hills and plains of our own country. The species composing our flora have been seen mostly, not as individuals, but as portions of regiments and armies, particular plants being mentioned but seldom, where required for purposes of illustration. In the final chapter it will be well to abandon this collective treatment, and glance at a few individual species or genera or small natural groups which possess features of interest of one sort or another. No systematic arrangement need be attempted: it will be pleasanter to ramble on, allowing our points of inquiry to turn up as they might on a country walk.
A consideration of abnormalities in the manner in which plants obtain their food-supply—irregular nutrition, as it has been called—will raise some interesting questions, and will bring us up against some of the most remarkable species which are found in the British flora. The outlines of the method by which plants manufacture their food are familiar to all, and have been referred to already (pp. [75], [132]). The roots absorb from the soil water containing dissolved salts, which is passed up by the stems into the leaves. The leaves extract from the air carbon dioxide. The chlorophyll, or green colouring-matter of the leaves, possesses the remarkable power in the presence of sunlight of breaking up and recombining these substances into the compounds which go to build up the plant-body. As has been pointed out, it is this power of forming organic out of inorganic matter that especially distinguishes plants from animals. But not all plants manufacture their food in this way. A large number feed like animals, finding their sustenance sometimes in living, more often in dead, organic material, either animal or vegetable. The whole enormous group of the Fungi do not possess chlorophyll, and in consequence are dependent on organic materials for their food. Some of the most familiar of the lower Fungi live on cheese, leather, bread, or any other damp animal or vegetable material. The higher forms, which decorate our woods and pastures, find their sustenance largely in leaf-mould. The groups of the Mosses, Hepatics, and Ferns, which are more highly organized than the Fungi, possess chlorophyll, and manufacture their own food; and it is with some little surprise, therefore, that when we come to the Seed Plants, the highest group of all, we find, though in relatively few cases, a reversion to the animal trait of using organic food. Some of our woodland plants have taken so entirely to a diet of leaf-mould that they have discarded the apparatus which would enable them to manufacture their own food. Chlorophyll, the magic wand by means of which the inorganic is transformed into the organic, and also leaves, the mills wherein the transformation takes place, are absent from these plants. For instance, the Bird’s-nest Orchis (Neottia Nidus-avis),
