128 VEGETABLE PHYSIOLOGY 
from uniform, and the constituents which are within 
the reach of the roots of two plants growing almost side 
by side naturally may be materially different in their 
proportions. This consideration makes it almost or quite 
impossible to ascertain, by observation of the soil and the 
plant growing in it, what are the substances which are 
entering its roots. 
The other method, which is of much more general 
application, consists in making an analysis of the whole 
body of the plant after its removal from the soil, and so 
ascertaining what chemical elements it contains. A plant 
gives off no solid excreta, and consequently whatever it 
absorbs remains in its substance. The ultimate composi- 
tion of the true nutritive matters, proteins, carbohydrates, 
fats, &c., is known. Such an analysis having shown what 
elements enter into the composition of a plant, and of the 
food which it has stored in its tissues, it becomes possible 
to inquire into the manner in which each is supplied to the 
plant under examination, and into the work which is done 
upon them in its cells. 
As already noticed, the structure of the plant demands 
that all the materials of a solid character shall be in such 
a solution that they can enter its substance by means of 
the physical process of osmosis taking place through the 
cell-wall. Similar considerations apply to gases, of which 
there is considerable absorption by all plants, whatever 
may be the nature of their habitat. 
The details of absorption vary to some extent, however, 
according to the environment of the plant. Aquatic plants 
can absorb water, and whatever is dissolved in it, whether 
of gaseous or solid character, by all parts of their surface. 
Those which grow with their roots embedded in soil, and 
their shoots exposed to the air, show a certain division of 
labour in this respect. The mineral constituents obtained 
from the soil are taken in by the root-hairs with the stream 
of water; those of a gaseous nature mainly find entry 
through the leaves and other green parts. 
