Some allusion has already been made to the important results which arise out of the study of the connection between vegetation and climate. The quality of all vegetable productions is influenced essen tially by external causes ; intensity of light, atmospheric pressure, humidity, temperature, and seasons, are the great agents which modify the tissue, which control development, and which regulate the forma tion of sensible properties. Various combinations of these and other external causes are what constitute diversities of climate, and it is therefore obvious that the connection between the latter and vegetation is of the most intimate nature. But as this is a branch of the science of comparatively modern origin there are few instances of its appli cation : one of the most striking was the declaration of Dr. Hoyle, that cotton might be obtained in the East Indies equal to the fiuest from America—a prophecy which has already been fulfilled, in consequence of the practical adoption of plans similar to those which he theoreti cally suggested. Can tea be cultivated as advantageously elsewhere as in China ] Here is a single question of immense importance, in volving the interests of millions of human beings, and affecting the pecuniary interests of Great Britain as much as any commercial problem ever did. This question has been answered by the botanist in the affirmative, and already the natives of the East Indies are supplied with tea from the Himalaya, and Asam tea may be bought in the shops of London.
To the medical man the study of botany is of the highest interest, as the members of the vegetable kingdom yield to him the most important means of his art. It is only as the properties of plants are studied that new agents for the alleviation of disease can be expected, or that substitutes for those already in use eau be employed.
Thus far we have more especially referred to the study of vegetable physiology. Systematic Botany bears upon practice not less usefully, but in a different way. If the only advantage of classifying plants were to acquire the power of discovering their scientific names, even that would have a certain kind of interest, because it would ensure a uniformity of language in speaking of them ; if it had the additional property of demonstrating the gradual connection that is discoverable between all the beings in the organised part of the creation, of proving that there is an insensible transition from one form of living matter to another without break or interruption, and of explaining in a clear and intelligible manner the nature of that universal harmony of which Philosophers are used to talk, the interest and importance of botanical classifications would be still further enhanced ; but the practical importance of them would still be extremely limited. It is only when we look to the coincidence between botanical affinities and sensible properties, and to the external indications of internal qualities, that we perceive the great features of its utility to man. If the qualities of every plant required to be ascertained by a circuitous and tedious series of experiments, no life could be long enough for the task, nor, if it were, could any memory however powerful remember so extensive a series Of facts ; and if under such circumstances botanists whose whole life is occupied in the study should be unable to master the difficulties, systematic botany could never be applied at all to any useful purpose, because it must of necessity be far beyond the acquirement of those persons who would be most likely to have occa sion to employ it. But it was long since suspected that plants which
agree with each other in organisation also agree in the secretions which may be supposed to be the result of that organisation. Lin =WS, in his dissertation upon the properties of plants, declares that species of the same genus possess similar virtues, that those of the same natural order are near each other in properties, and that those which belong to the same natural class have also some relation to each other in their sensible properties. This doctrine is now admitted on all hands among men of science to be incontrovertible, and places the practical utility of systematic botany in the most striking light. Instead of endless experiments leading to multitudes of incongruous and isolated facts, the whole his€ory of the medicinal or economical uses of the vegetable kingdom is reduced to a comparatively small number of general laws; and a student instead of being compelled to entangle himself in a maze of specific distinctions, is only obliged in practice to make himself acquainted with the more striking groups ; and having accomplished this he is enabled to judge of the properties of a species he had never seen before, by what he knows of some other species to which it is related. Some idea of the extent to which this power of judging of plants k priori is practically useful may be formed from this—that supposing the vegetable kingdom to consist of 100,000 species arranged in 6000 or 7000 genera, the vast mass of characters required to distinguish them will be collected under about 300 heads, a knowledge of not more than two-thirds of which will be required for the purposes of the general observer. Thus the common hedge-mallow is a mucilaginous inert plant, whose woody tissue is tough enough to be manufactured into cordage; it has certain botanical characters, which are readily observed and remembered ; and it belongs to a group of plants consisting of not fewer than 700 species. It is only necessary to understand the structure of the common mallow to recognise all the remainder of the group, and to be aware of their uses and properties; so that a person in a foreign country who finds a plant agreeing with the mallow in those marks by which the 41,f alracetr are known, although he should never have seen or heard of the plant before, would immediately recognise it to be mucilaginous and inert, and would expect to find its vegetable fibre tough enough to be manufactured into cordage. It is this class of facts which alone can lead with any certainty to the discovery in one country of substitutes for the useful plants of another ; it has shown the similarity between the violet roots of Europe and one of the kinds of ipecacuanha of South America ; that the astringency of the alum-root of the United States finds a parallel iu those of the geraniums of England ; that madder has its representative in the Isle of France, cinchona in India, and that India-rubber trees exist in the east as well as in the west.