VEGETATION AND CLIMATE.

BY RICHMOND LEIGH, M. R. C. S. E.

GEOGRAPHICAL Botany was raised to the position of a special and important science by Humboldt. He laid the foundation upon which De Candolle, Schouw, Meyen, and others have built up the noble but unfinished structure of the present time. Meyen, more particularly, shewed that, as certain regions contained certain plants, these plants indicated the region in which they were found as regards its climate. This character of the vegetation of any climate is in all cases sufficiently well marked to shew plainly the climate of that region, and is a more certain guide than any other save that of registered facts, which it nearly equals. From our want of knowledge of the range of many plants, our deductions are necessarily very imperfect, but still most valuable. As our botanical knowledge increases, our climatological information will also grow, until, sooner or later, we shall be able to indicate the climate of any region from the examination of its flora, almost perfectly.

In most countries vegetation is met with under two conditions, natural, or in a state of nature, and cultivated. The former, being exposed to all natural influences, is the most valuable guide to the character of any regional climate, and should be chiefly relied upon in any such research. Cultivated vegetation is also of great value for this purpose, but this value varies according to the more or less artificial conditions of culture. A plant grown from seed sown by man, and otherwise untended, is equivalent, for climatic purposes, to one produced in a state of nature; but if the

plant be protected from cold, and watered in dry weather, it will give little indication of the climate of its habitat. The deserts of tropical and sub-tropical regions would be covered with luxuriant vegetation, were they supplied with abundant moisture, as seen in Egypt, where the overflow of the Nile is the determinant of the harvest. Therefore, in estimating the value of plants under cultivation, it is necessary to know the conditions of cultivation, so that, these being known, we may then be able to use the plants as climate indicators. It has been considered by some that plants under care and cultivation may be altered in habit, and made able to bear greater extremes of temperature and moisture than in a state of nature. This is true in but a limited degree, and it may be considered as an axiom, that plants are not capable of true acclimatisation. Our fruit trees, and other plants too numerous to mention, are as sensitive to frost now as they were in all probability a thousand years ago; and the instances in which imported plants are said to have been hardened and acclimatised will, on close examination, generally be found incorrect. Our worthy President has given us his experience in growing the Eucalyptus, which every care fails to make endure our climate. Taking into consideration, then, the conditions under which cultivated plants are grown, the knowledge obtained may be utilised in estimating any given climate.

There is of necessity, even with every favouring circumstance, always a limit (at least towards the poles) to the flourishing growth of plants, though their natural area may be greatly extended by cultivation.

Temperature-of the factors which go to form "climate,' is by far the most important, and has much more influence on vegetation than any other. A constant, though irregular, development in plant growth is found as we proceed from the

poles to the equator. The effect of heat is so notable, that Dr. Carpenter considers it to be the vital force, and though this may be considered doubtful or improbable, it is none the less certain that heat is an essential condition of the working of that mysterious agency-life.

The development of plants with increasing temperature is shewn by their greater size, both as a whole and in their individual parts, by their greater numerical ratio, both as regards species and individuals, and by their increased beauty or strangeness of form and colour, as well as by their more. vigorous growth. This development is especially shown in the Endogenous division of plants, none of which assume the arborescent form till we come to the warmer temperate zone, where a small palm, the Chamærops humilis, is met with. Endogenous trees or shrubs are not met with in numbers till we reach the tropics, where palms, screw pines, bananas, bamboos, and other members of this plant section form an important feature in the physiognomy of the vegetation. The development in size produced by increased temperature is well shown in the order Gramineæ, the representatives of our humble grasses, rising in the tropics to the height of fifty, sixty, and even one hundred feet (Gardner), thus surpassing our forest trees in height. The lilies of our climate are represented by the aloes and dragon trees of hot regions, and numberless similar examples may be cited.

Acrogens are also only found arborescent in warm climates, where the graceful tree ferns attain to fifty or sixty feet in height.

Exogens, though not exhibiting so marked an increase in size with increased temperature as a whole, still present in many individual instances very great advancement in this respect, as in the mallows and their allies, which may be considered to be represented by the enormous Baobab trees. of Africa.

T

The individual parts of plants also show this increased development. The largest leaf expansion, as seen in the banana, is only found in the moist torrid zone. The Rafflesia of Java bears flowers often eight or nine feet in circumference, and fourteen pounds in weight, and an Aristolochia, on the river Magdalena of South America, has flowers four feet in circumference, which are used occasionally by the natives as hats.

Nearly all orders of plants increase in numerical ratio towards the equator, there being but few exceptions, and these generally lowly in form, as the Juncacea, or rush tribe. Meyen remarks, that "the lower the plant form, the wider the range" a correct general axiom. The reverse may be stated, that "the higher and more developed the plant, the more limited its range." Many orders are limited to the torrid zone, and a considerable number are bounded by the tropics. The number of plants increasing individually as well as collectively, there is necessarily more variety towards the equator; a fact prominently shewn in the forests, which are much more diversified in character, and have not the social aspect of those of colder regions.

The coloration of plants depending on the light of the sun, it were to be expected that we should find the most vivid hues beneath a tropic sky, this vividness of coloration extending to the green parts of plants, which are of a much brighter and fresher hue. The various parts of plants also present more strangeness of form towards the equator, as when the Orchids mimic the forms of insect life so exactly as often to deceive the human eye.

Increase of temperature also shews remarkable results in the increased vigour of plants, which, in the torrid zone, grow with a rapidity unknown in colder regions. This may be considered to account in great measure for the strange