* = 
FOOD OF PLANTS AND ITS SOURCES. 813 
and the formation of other organic compounds, which are com- 
monly termed secretions. 
Section 1. Foop or PLANTS AND ITS SouRCEs. 
The various substances required as food can be only ascer- 
tained by determining the elementary composition of the parts 
and products of plants ; for as plants have no power of forming 
these elements for themselves, they must have derived them from 
external sources. 
As plants are incapable of locomotion, being fixed to the soil 
or to the substance upon which they grow, or floating or sus- 
pended in water, they must obtain their food from the media by 
which they are surrounded, that is, as a general rule, from the 
soi], or from the air, or from both; but no plants can take up 
their food except in the state of gas or vapour, or in a fluidstate. 
In by far the majority of cases plants take up their food, both 
from the air by their leaves in a gaseous or vaporous state, and 
from the earth dissolved in water by their roots. But plants 
which are termed Epiphytes or Air Plants, as many Orchids 
(fig. 256), derive their food entirely from the air by which they 
are surrounded (see page 131); while Parasites (figs. 257 and 
258) and Saprophytes essentially differ from both Epiphytes 
and ordinary plants in the fact that their food, instead of being 
derived entirely from inorganic materials, which are afterwards 
assimilated in their tissues, is obtained entirely or partially from 
the plants upon which they grow, that is, in an already assimilated 
condition, or, as in Saprophytes, from organic matter in a state 
of decay (see page 133). 
The materials of which plants are composed, and which, as 
stated above, are either derived from the air, or the earth, or 
more commonly from both, and which consequently constitute 
their food, form respectively their organic and inorganic com- 
pounds ; and in all plants there is also a varying proportion of 
water. The process of burning enables us conveniently to 
distinguish, to a great extent at least, the comparative propor- 
tion of these organic and inorganic compounds, and acquaints 
us with one of their distinctive peculiarities. Thus, if we take 
a piece of wood, or a leaf, or any other part of a plant, and 
burn it as perfectly as we are able, we find that the greater 
portion disappears in the form of gas and vapour, but a small 
portion of the original substance remains in the form of ash or 
-incombustible material. The former or combustible portion is 
made up of organic compounds or volatile constituents, that is to 
say, of combinations of carbon with other elements, and the 
latter portion of inorganic compounds. The relative proportion 
of the organic and inorganic constituents varies in different 
plants; but, as a general rule, the former constitute from 92 to 
99 parts, while the latter form from 1 to about 8 parts in every 
100. 
