NUTRITION 397 
resin, and caoutchouc. Some latex is translucent, but usually it is an 
opaque, white, yellow, or orange liquid, familiar to many as the milky 
" juice " of dandelion, poppy, milkweed, or the orange " blood " of the 
bloodroot. Latex is commercially important as the source of opium 
and its alkaloids, of India rubber, and of gutta percha. 
Function. The principal reasons for ascribing to latex vessels the 
function of a conducting system are the abundance of foods in the latex, 
and the peculiar structural relations of the latex vessels to the nutritive 
cells of the leaves. The carbohydrate and nitrogenous foods of the latex 
run as high as 30 per cent of the dry matter therein; they are most abun- 
dant when active growth and development are beginning, and least 
so when growth is checked and a resting period is at hand. In some 
leaves the latex vessels look as though they were favorably arranged to 
receive materials collected from the nutritive cells. Yet for the conduc- 
tive function the evidence is rather presumptive than convincing. It 
may be that the latex has to do rather with storage and protection. 
For further details on latex and accumulation of foods, see Part III. 
6. DIGESTION 
Nature of digestion. Whenever foods are insoluble in water (as are 
some of the most valuable ones), they cannot be used by plants until 
transformed into a soluble substance. Whenever soluble foods are ur- 
able to diffuse readily through protoplasmic membranes, they can 
scarcely move from one point to another, and are available, if at all, 
chiefly in the cell where they happen to be. Every transformation of 
food by the agency of a third body from an insoluble to a soluble and 
from an indiffusible to a diffusible condition, whatever the precise 
chemical nature of the change, is summed up in the term digestion. 
This use of the term is in exact accord with its long use in animal 
physiology. The processes in plant and animal, indeed, are essentially 
the same; they are wrought by the same sorts of agents, affect the same 
sorts of substances, and result in the same sorts of products. 
No special digestive organs. Plants differ from the larger animals 
in having no pouched tube wherein food is lodged, and in which some of 
the more striking digestive processes take place, before the food truly 
enters the body. This digestive tract, its parts and accompanying 
glands, constitute the special digestive organs of the animal, though 
much important digestion takes place elsewhere. Plants have no special 
