s 
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 toge- 
ther, 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 pro- 
gression 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^ 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; 
b 2 
