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study the structure and functions of plants; nor have we to note the first appearances of life in conjunction with the simplest states of organised cellular matter, to its further development in Endogenous life; where vascular bundles, though performing the same functions, are only bound together, but have not attained that higher degree of centralisation in which the different systems unite into different organs; as the cellular into pith, bark, and medullary rays, and the vascular into rings of wood. Neither have we to watch the local circulation in the cells, nor to trace the more general progression of the sap from the roots to the leaves, and from these downwards into the bark. But we are interested in the laws of vegetable physiology, that we may be able to weigh the influence of the various stimulants of light, heat, air, and moisture; the effects of soils and aspects; that we may understand something of their operation, in modifying the products of plants, and be able to select our barks, woods, and roots, bulbs, leaves, flowers, and fruits, at the age and season when they contain the principles which render them useful as medicines in their most abundant and efficient state; whether these be gums, fecula, or saccharine principle; milky juice or resin; fixed or volatile oil; or any of those numerous alcaloids which are so completely altering the forms of medicines, since the subtleties of modern chemistry discovered them to be secreted in nature's nicer laboratory.
There are, however, two branches of this extensive science, respecting which I am desirous of making a few observations; one is the connexion between the Structure and Natural affinities of plants, and their Physical and Medical properties; and the other is their Geographical distribution, especially as connected with Climate. Both are important subjects, whether we consider them in a scientific or a practical point of view. The one teaches us the laws which influence the distribution of plants;
