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 peri ha. 



Function. — The principal reasons for ascribing to latex vessels the 

 function of a conducting system are the abundance <>f foods in the latex, 

 and the peculiar structural relations of the latex vessels t<> 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 i- 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 mosl 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 (an 

 scarcely mow 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 pn>< e— es in plant and animal, indeed, are essentially 

 the- same; they are wroughl by the same sorts of agents, affect the same 

 orts of substances, and result in the same sorts of products. 



No special digestive organs. — Plants differ from the' larger animals 

 in having 110 pou< hed 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 trait, its parts and accompanying 

 gland , constitute the special digestive organs of the animal, though 

 mm h important digestion takes phu e elsewhere. Plants have no spe< ial 



