134 VEGETABLE PHYSIOLOGY 
constituents which are within the reach of the roots of two 
plants growing almost side by side may naturally be 
materially different in their proportions. This considera- 
tion 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 applica- 
tion, 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 composition 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 previously 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 processes already described as 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. 
To make a destructive analysis of the plant, it must be 
