VEGETABLE PHYSIOLOGY. — THE LEAVES OF PLANTS. 167 
chemical union with the constituents of the atmosphere, until it 
finally becomes either a secretion peculiar to the species, or is 
made supplementary to its increase. But hitherto we have 
mentioned only a stem producing branches, and these again 
bearing leaves: an abundant secretion of food by means of 
these essential organs, assisted by a peculiar and mysterious 
vital process, induces, however, a fuller degree of developement, 
by means of which they are produced in a changed form, and 
in an aggregated position — in other words, the principle of life, 
having attained to its ultimatum, becomes arrested in its course 
of extension, and is diverted into a new channel for the per¬ 
petuation of the species. In illustration of this, I may refer to 
the contraction of the internodes, and the consequent coalescence 
of a branch and its leaves, which forms a flower ; to the disin¬ 
tegration of the internal tissue of a petal, which forms an anther; 
to the folding inwards of a leaf, which constitutes apistillum; 
and to the distension of this organ by an abundant supply of 
fluid it cannot part from, which produces a fruit. The admir¬ 
able skill thus evinced in every part conveys the most forcible 
argument, that as a work of design they are perfect and unap¬ 
proachable. But does any one ask, Why, then, if such a process 
is universal, do individual flowers and fruit vary in their exterior 
configuration as well as their internal character? I answer — 
no one can tell : but let me point to the constant change of 
one part to another, which an attentive eye may discern; to the 
production of bulbs in the axils of the leaves of some plants, such 
as the lilies, and the production of bulbs in place of flowers, as 
in varieties of onions ; to the production of plants on the panicle 
of viviparous grasses — instances showing the identity of cha¬ 
racter between buds, bulbs, and seeds, and also the similarity, if 
not identity, of their originating sources : and, weighing well these 
matters without partiality and prejudice, the truth-seeking and 
unsophisticated mind will assent to the fact which has already 
been stated. 
My present object is not, however, to deal directly with this 
portion of the subject, but to endeavour to direct your attention 
to the structure and the properties of the leaves of plants. 
Leaves are produced by the extension of the woody and vas¬ 
cular tissue of the stem, and of the cellular tissue of the back of 
the plants by which they are borne ; and they occur immediately 
below where the force of the vital action has either formed a 
