The Food of Plants. 341 
mineral substances in solution, now constitutes what is com- 
monly designated the crude sap, inasmuch as the substances 
held by it are not in such chemical condition as will enable 
them to directly participate in the nutrition of growing 
parts. This sap, however, passes upward through the outer 
layers of the woody tissue or sapwood, until it reaches the 
leaves, where it is distributed among the ramifications of 
the veins to the active, chlorophyll-containing cells, in 
which it becomes involved in the process of digestion. In 
the course of this process it suffers increase of density, due 
in part to the fact that a large portion of water is liberated 
as aqueous vapor into the surrounding air, while another 
volume is used up in the various chemical changes, and the 
fluid, now distributed from the leaves to the various centres 
of active growth, is said to be digested and capable of 
.directly promoting the formation of new structure. 
Although plants in general may be said to be the special 
agents whereby the crude material of the soil and air is 
converted into that which is of direct value in animal 
nutrition, yet we find the law subject to certain important 
exceptions, since in their power of appropriating and 
converting food, they exhibit a wide difference. 
We are all familiar with the fact that in the animal 
kingdom, certain forms live upon and draw their entire 
sustenance from other animals, in consequence of which 
they are termed parasites. Parasitism is also a common 
feature of plant life, and in each case the relations of 
supply and demand conform to the same general laws. 
The parasitic plant fastens itself upon its host and draws 
its nourishment from it. The latter is therefore forced to 
yield a portion of the food prepared for its own use, and 
in consequence of this unusual demand upon its resources, 
it sooner or later becomes diseased, exhibits malform- 
ations and may eventually be killed. Under these con- 
ditions of growth the parasite does not require to pro- 
duce its own food; we therefore find that it has no roots, 
its leaves are imperfectly formed, and it may contain 
no chlorophyll. Just in proportion, therefore, as the 
