PARI U0

DISTRIBUTION OF PLANTS. GEOGRAPHICAL

Lixnxus, whose singular genius foresaw most of the conquests reserved for his favourite science—the study of botany—laid the foundations of Botanical Geography. In the prolegomenia of his “ Flora Laponia,” the immortal Botanist of Upsal says, in the poetical and concise style which is peculiar to him: “The dynasty of the Palms reigns in the warm regions of the globe; the tropical zones are inhabited by whole races of trees and shrubs; a rich crown of plants surrounds the plains of southern Europe; armies of green Graminee occupy Holland and Denmark; numerous — tribes of Mosses are cantonned in Sweden; but the brownish-coloured Alge, and the white and grey Lichens, alone vegetate in cold and frozen Lapland, the most remote habitable spot of earth; the last of the vegetables alone live on the confines of the earth.” of plants which Linnzeus in the distribution The modifications Tournefort had from south to north, journeying had observed already observed during his travels in Armenia, upon the slopes of Mount Ararat. At the foot of this mountain he saw the plants of Armenia; higher up, he found the plants of Italy; higher up still, he found those of the environs of Paris; above these were of eternal snow, finally, on the borders the plants of Sweden; he found those of Lapland. mountain, near the summit of the of the laws which apply to the glimpse had also a Buffon which covers the earth,” vegetation of plants. ‘‘ The distribution to it than closely attached is still more he says, “and which

the animals which browse it, are even more interested than they in the nature of climate. Each country, each changing degree of temperature, has its particular plants. We find at the foot of the Alps the plants of France and Italy; at their summit we find the plants of the frozen North; and the same northern plants we find again at the summit of the mountains of Africa. Upon the range of the hills which separate the Mogul empire from the kingdom:of Cashmere, we find on the southern slopes many of the plants of the Indies, and it is not without surprise that we find on the north flanks many of those of Europe. It is also from the extremes of climate that we draw our drugs, perfumes, and poisons, and all the plants whose properties are in excess. Temperate climates, on the contrary, only produce temperate things; the mildest of herbs, the most wholesome of legumes, the most refreshing of fruits, the quietest of animals, the sais polished of men, are the heritage of the mildest climates.”’ . Such are the views with which men of genius and foresight sicelnsded the discoveries of our times concerning the geographical distribution of plants. At the commencement of. the sighteaath century Geographical Botany was in a manner created by Alexander von Humboldt, whose genius is so universal that his traces are found in connection with every modern science. On his return from his voyage to the equinoctial regions of America, Von Humboldt, in one of his finest memoirs, demonstrated that it is the predominance of certain forms of vegetation which enables us to recognise a country immediately. A forest of Firs and Pines transports us at once to the northern or to the high mountain ranges of Europe; the Oaks and Beeches to the temperate zone; the Olives to the south, and the Palms into intertropical regions; the Cape of Good Hope is the country of the Heaths, and Mexico is perhaps the country most typical of the Orchids. In another memoir Humboldt attempted to estimate the total number of plants diffused over the surface of the globe, and the influence of climate upon their distribution. For the first time he established clearly that localities, each equally distant from the equator, and at an equal elevation above the level of the sea, might nevertheless have climates very little resembling each

_ other, while countries situated under parallels very remote one _ from the other might have analogous climates.

The travels of naturalists of our own day in all parts of the globe have established, to the satisfaction of botanists, that certain characteristics belong to the vegetation of each climate, contrasts attending which we shall endeavour to convey to the reader some succinct idea. - The researches of travellers, combined with the labours of descriptive botanists, enable us to give some precision to the principles of Botanical Geography.

Let us establish, before going further, the approximate number of vegetable species which inhabit our globe. The appreciation of the statistics of plants is necessarily very varied in this sort of | estimate. Linneus, in 1753, was acquainted with 6,000 species.

Persoon, in 1807, reckoned 26,000. In 1824 Stendel carried the number up to 50,000; and in 1844 to 95,000. The most recent works contain about 120,000 species. From the species described botanists have been able to form some approximate estimate of the total number of existing species. By an ingenious calculation of the space occupied by an average-sized plant, Alphonse de Candolle thinks he may infer that the“number cannot be less than from 400,000 to 500,000. We have said that in 1844 we knew 95,000 species of plants. Of this number 80,000 were phamerogams, or cotyledonous plants, 15,000 were cryptogams, or acotyledons. Among the cotyledonous plants 6,500 belonged to the dicotyledons, and 15,000 to the monocotyledons. Such was the general budget of the terrestrial flora at this date.

The numerical proportion of species belonging to the phanerogams or cryptogams, varies according to the latitudes of the globe. As we advance towards the north, the number of cryptogams increases; the number of phanerogams, on the other hand, increases as we approach the equator. In the frozen or temperate zones the cryptogamia are humble vegetables which scarcely raise themselves above the surface of the soil; but in the burning regions of the tropics, the elegant arborescent ferns rise to the height of the loftiest Palm-trees. . The vegetation of each species corresponds with a determinate thermometer, and this interval is not interval in the scale of the

the same for all plants. Meleze and Dwarf Birches resist cold of 40° Cent. below zero. While many of the Palms, the Orchids, and Tree Ferns die when the thermometer descends to 10°. While Alpine or northern plants subjected to the same temperature of 10°, fade and shrivel after a few days’ exposure, other plants accommodate themselves to the burning sands of Africa, whose temperature may be from 60° to 72° Cent.

Another point of some importance to consider is the thermometrical degree at which each species begins to vegetate; thus the charming Soldanella of the mountains germinates and prospers at zero, while the Cocoa-nut-tree and other vegetables of the torrid zone remain unmoyed until the temperature of 15° or 22° is attained. Again, when te has commenced, what is the temperature necessary to develop the flowers and ripen the fruits?

The Barley plant (Hordeum), the cereal which extends farthest towards the north, begins to vegetate when the thermometer ranges at 5° C. If, then, we would determine with precision the amount of heat which a plant must accumulate in order to accomplish its various processes of germination—budding, blossoming and fruiting, and maturing its fruit—it is not necessary to take account of the lower temperature, but adding the mean temperature of each day on which the thermometer has exceeded that degree, we find that in high latitudes the barley plant ripens when it has received an aggregate amount of heat equal to 1,500°. In order to produce death in wheat, an accumulation of heat equal to 2,000° is required. The vine, in order to produce a — drinkable wine, requires the accumulated heat of 2,900° Cent., the — point of departure being a mean heat of 10°.

We can now comprehend why certain vegetables live in some countries without flowering and others without bearing fruit. The short summers and short days in such countries fail to yield the amount of heat, and that supplied is just sufficient to—aggregate develop their leaves, but not enough to expand their flowers; and their fruits are abortive. The influence of heat on vegetation is so marked that we can scarcely name a single species which is truly cosmopolitan. Most vegetables occupy a determinate zone ~ of their own, which they rarely is The cold prevents them

from passing its limits towards the north; the heat exercises the same influence towards the south. All laity have their polar and their tropical limits.

Humidity of,the atmosphere and the solar influence have, on the other hand, a notable influence on the geographical distribution of plants. It is still more necessary to consider the influence of elevation. In proportion as we rise in the atmosphere the temperature decreases, and this lowering of the temperature is so sudden, that in ascending a mountain we pass through many degrees of decreasing temperature in the course of a few hours. From this it follows that a high mountain under the equator may be clothed at its base in the richest vegetation, while its summit is covered with eternal snow, and the space. between is clothed with all the diversity of vegetation (on a limited scale) which the traveller meets with in his journey from the equator to the pole.

With these general remarks upon the principal causes which influence the distribution of plants, we shall proceed to consider the botanical circumstances, namely, the zones of vegetation which result from them. For botanical purposes we may divide the surface of the globe into three great zones:—1. The Torrid zone, which comprises the tropics between 24° north and south latitude. 2. The Temperate zone, which in each hemisphere extends from the tropics to the polar circle. 8. The Polar zone, which in the northern hemisphere includes all beyond the arctic, and in the southern all ~ beyond the antarctic circle.

The Tropical zone, which receives the direct rays of the sun all but perpendicularly, is almost entirely exempt from winter. It includes the warmest regions of the globe. The year is there divided into two seasons—the one dry and burning, during which vegetation is sensibly suspended; the other the rainy season, during which vegetation revives. This large zone, which erhbraces in its circle continents, seas, and islands of all sizes, and which bristles with immense chains of the loftiest mountains in the world, presents climatal changes equally diversified, yielding productions which are very far from resembling each other. To render the subject clearer, it will be necessary to heap this zone into middle, — and tropical zones.

The Middle, Tropical, or Equatorial zone, extends from the fifteenth north parallel to the fifteenth degree south latitude. The two others are the Tropical zones, properly so called, ranging on each side the Equatorial zone up to the twenty-fourth parallel north and south. The two Temperate zones are contiguous on the one side to the Torrid zone, and on the other to the frozen regions of the Pole, extending over a space of forty-two degrees of latitude. Like the Tropical zones, they present great varieties of climate and of vegetable products. ‘These zones we also subdivide in a botanical point of view into four secondary zones, namely, the Juxta-tropical, the Warm Temperate, the Cold Temperate, and the Arctic zone. The — Polar zone comprehends the polar regions, which extend from 60° to 80° north latitude. We shall not follow in our few remarks the order of these natural regions, and we feel that the motives for this determination are justified by the following remarks of Alphonse de Candolle. “TI hold,” says the learned Botanist of Geneva, “ I hold the divisions of the globe by regions first proposed as artificial systems in great part. The laws respecting them are much too arbitrary, and the regions indicated are neither like each other in the majority of books, nor are recognised by the greater number es botanists.” We think it simpler, in place of dwelling upon these

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