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 un- 

 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 



