Page:EB1911 - Volume 21.djvu/811

Rh 1. The Arctic-Alpine sub-region consists of races of plants belonging originally to the general flora, and recruited by subsequent additions, which have been specialized in low stature and great capacity of endurance to survive long dormant periods, sometimes even unbroken in successive years by the transitory activity of the brief summer. It is continuous round the pole and roughly is bounded by the arctic circle. Mature seeds are highly tolerant of cold and have been shown to be capable of withstanding the temperature even of liquid hydrogen. Arctic plants make their brief growth and flower at a temperature little above freezing-point, and are dependent for their heat on the direct rays of the sun. Characteristic representatives are Papaver nudicaule, Saxifraga oppositifolia, which forms a profuse carpet, and Dryas octopetala. Such plants perhaps extend to the most northern lands at present known. On May 30th, in Ward Hunt's Island, lat. 83° 5′, Sir George Nares found that “vegetation was fairly represented as regards quantity in the poppy, saxifrage and small tufts of grass.” We may compare this with extreme alpine conditions: on a spot above the Aletsch glacier at a height of 10,700 ft. Ball found the temperature one inch below the surface to be 83°, and he collected “over forty species in flower.” Taking the whole arctic flora at 762 species, Hooker found that 616 occurred in arctic Europe, and of these 586 are Scandinavian. Beyond the arctic circle some 200, or more than a quarter, are confined to the mountains of the northern hemisphere and of “still more southern regions.” This led Hooker to the striking observation already quoted. The arctic flora contains no genus that is peculiar to it, and only some fifty species that are so. Christ has objected to terming the arctic flora Scandinavian, but the name implies nothing more than that Scandinavia has been its chief centre of preservation.

A detailed examination of mountain floras shows that a large local element is present in each besides the arctic. The one is in fact the result of similar physical conditions to that which has produced the other. Thus Saxifraga cernua is regarded as an alpine form of the lowland S. granulata. Comparing the Alps with the Pyrenees, according to Ball, each has about half its flora common to the other: “the Alps have 172 endemic species and at least 15 genera that are not found in the Pyrenees, while the latter range counts about 100 endemic species with several (six or seven) genera not found in the Alps.” Drude has accordingly suggested the substitution of the term “High-mountain floras” for Alpine, which he regards as misleading. Its meaning has, however, become synonymous and is consecrated by usage.

The repetition of the same species in the arctic regions and in the high mountains of the North Temperate region is generally attributed to the exchange which took place during the glacial period. This was first suggested by Edward Forbes in 1846, though the idea had earlier suggested itself to Darwin (Life, i. 88). It took place southwards, for the arctic flora is remarkably uniform, and, as Chodat points out, it shows no evidence of having been recruited from the several mountain floras. That the arctic flora was driven south into Central Europe cannot be contested in the face of the evidence collected by Nathorst from deposits connected with the boulder-clay. And Reid has shown that during the glacial period the existing flora was replaced by an arctic one represent by such plants as Salix polaris, S. herbacea, S. reticulata and Betula nana. At the same time the then existing alpine floras descended to lower levels, though we may agree with all that they did not necessarily become extinct at higher ones as long as an land-surface remained uncovered by ice. At the close of the glacial period the alpine floras retreated to the mountains accompanied by an arctic contingent, though doubtless many species of the latter, such as Salix polaris, failed to establish themselves. Christ, while admitting an ancient endemic element, such as Campanula excisa in the arctic-alpine flora of Europe, objects that a Scandinavian colonization could not furnish such characteristic plants as the larch and edelweiss. He traces the oriinal home of the bulk of existing alpine plants to northern Asia, the mountains of which appear to have escaped glaciation. At the close of the glacial epoch the north Asiatic flora spread westwards into Europe and intermingled with the surviving vegetation. Some species, such as Anemone alpina, which are wanting in the Arctic flora of the Old World, he thinks must have reached Europe by way of Greenland from north-east America.

2 The Intermediate sub-region comprises the vegetation of the large area occupied by the steppes of the Old World, the prairies of the new and the forest region of both. The former support a copious herbaceous flora, the characteristics of which in the Old and New Worlds have been already briefly summarized. In the former that of Europe and of Central Asia are continuous. Of species common to the two, Maximowicz finds that Manchuria possesses 40% and scarcely 9% that are endemic. Of a collection of about 500 species made in that country by Sir Henry James nearly a third are British. This confirms the theory of Christ that Europe was restocked mainly from Asia after the close of the glacial epoch, the south being closed to it. In the new world no southern barriers existed and it is more difficult to draw the line between contiguous sub-regions.

The dominant characteristics of the arboreous vegetation are, besides deciduous and amentiferous trees, Coniferae, especially the more recent tribe of Abietineae—pines, silver-firs, hemlocks, spruces and larches, of which, unlike the older types, no representative

crosses the tropic. The prominent deciduous trees of Europe appear to be of eastern origin, and the progress of western migration has continued to historic times. The evidence of the peat bogs shows that the Scots fir, which is now extinct, was abundant in Denmark in the Roman period. It was succeeded by the sessile-fruited oak, which was in turn supplanted by the pedunculate form of the same tree. Quercus Robur has left no trace in the Tertiary deposits of Europe and it is most nearly allied to east Asiatic species. The oak in turn has been almost superseded in Denmark by the beech, which, if we may trust Julius Caesar, had not reached Britain in his time, though it existed there in the pre-glacial period, but is not native in either Scotland or Ireland. Its eastern limit in Europe is a line from Königsberg to the Caucasus; thence through China it is continued by varietal forms to Japan. It has a solitary representative in North America.

Broadly speaking, the American portion of the sub-region consists of an Atlantic and Pacific forest area and an intervening non-forest one, partly occupied by the Rocky Mountains, partly by intervening plains. Its arboreal vegetation is richer both in genera and species than that corresponding to it in the Old World. Glacial elimination has been less severe, or rather there has been, at any rate on the Atlantic side, an unimpeded return of Miocene types. The Atlantic area has five magnolias, a tulip tree, an Anonacea (Asimina), two Ternstroemiaceae (Stuartia and Gordonia), Liquidambar, Vitis (the fox-grape, V. Labrusca, reappears in Japan), and others; an assemblage, as long ago pointed out by Asa Gray, which can only be paralleled in the Chino-Japanese region, another centre of preservation of Miocene types. All these are wanting in the Pacific area, though there are indications in its gold-bearing gravels that it once possessed them. On the other hand, the latter “is rich in coniferous types beyond any country except Japan” (A. Gray), but till we reach California these are boreal in type. The Atlantic flora has also numerous oaks and maples, signalized by their autumnal coloration. These were abundant in Tertiary Europe, as they are now in Japan, and represent perhaps a cooler element in the flora than that indicated above. The highlands of Central America and the West Indies have preserved a number of Chino-Japanese types—Bocconia, Deutzia, Abelia, &c.—not met with elsewhere in the New World.

3. The Medrterraneo-Oriental sub-region contrasts no less vividly with the Intermediate than the Arctic-Alpine. It includes the Azores and Canaries, the Mediterranean basin, northern Africa as far as the Atlas and Sahara, Asia Minor, Persia and the countries eastward as far as Sind, being bounded to the north by the mountains which run from the Caucasus to the Hindu-Kush. Its extreme richness in number of species (it comprises six-sevenths of the European flora) and the extremely restricted areas of many of them point to a great antiquity. The Mediterranean basin has been a centre of preservation of Miocene vegetation: the oleander is said to have been found in local deposits of even earlier age, and the holm oak (Quercus Ilex) is the living representative of a Miocene ancestor. Extensions of the flora occur southwards of the high mountains of tropical Africa; Adenocarpus, a characteristic Mediterranean genus, has been found on Kilimanjaro and 2000 m. distant on the Cameroons. Two British plants may be added which both reach North Africa: Sanicula europaea, extends from Abyssinia to the Cameroons and southwards to Cape Colony and Madagascar; Sambucus Ebulus reaches Uganda. The Mediterranean, however, has apparently been a barrier to the southward passage of the arcto-alpine flora which is totally wanting on the Atlas. The vegetation of the sub-region is rich in shrubs: myrtle, bay, Cistus, Pistacia, Arbutus, heaths in its western portion, and the ground-palm (Chamaerops). It is even richer in more herbaceous plants tolerant of a hot summer; giant Umbelliferae (such as Ferula) are especially characteristic and yield gum-resins which have long been reckoned valuable. Of the 1000 known species of Astragalus it possesses 800. Evergreen oaks and Conifers form the forests. Asia Minor has a Liquidambar. The Argan tree (Argania Sideroxylon), which forms forests in Morocco, is a remarkable survivor of a tropical family (Sapotaceae). Amongst Conifers Cedrus is especially noteworthy; it is represented by geographical races in the north-west Himalaya, in Syria, Cyprus and North Africa.

This well-marked sub-region has a deeper interest than the botanical. It has been the cradle of civilization, and to it is due the majority of cultivated plants. Such are the date in Mesopotamia (a second species of Phoenix occurs in the Canaries); most European fruits, e.g. the vine, fig, mulberry, cherry, apricot, walnut; pulses, e.g. the bean, pea and lentil; pot-herbs, e.g. lettuce, endive, beet, radish, cress; cereals; and fodder plants such as lucerne and carob.

4. The Chino-Japanese sub-region.—Of the vegetation of China till recently very little has been known. In 1873 Elwes pointed out that the Himalayan avifauna extended into north-west China and established the Himalo-Chinese sub-region. Shortly afterwards the collections of Prejewalsky confirmed it for the flora. And we now know that, excluding the southern tropical area, it has the same character throughout the whole of China proper. We may therefore regard the Himalayan flora as a westward extension of the Chinese rather than the latter as a development of the former. Of four genera which Hooker singles out as the largest in Sikkim, in China Corydalis has 76 species, Saxifraga 58, Pedicularis 129, and Primula 77. Of Rhododendron there are 134 species. Upwards of 8000