GROWTH AND NUTRITION 25' 



destination the materials in solution are formed into 

 complex proteins, starch, and cellulose, as above stated, 

 and become a part of the plant structure. 



26. Course of the foods. The water and salts from 

 the soil pass simply upward into the stem and to all 

 branches. The elaborated foods in the leaf must pass 

 both downward and upward to reach all portions of the 

 plant. Evidently, in order to supply other sides of the 

 culm not in line with leaf bearing branches, the food must 

 also be able to pass across to other bundles. It is espe- 

 cially necessary in cereal plants where the leaves are two- 

 ranked ; that is, occur only on two sides of the culm. 

 However, in these plants, which usually have hollow 

 internodes, there is nevertheless pith present in all nodes, 

 through which the fibrovascular bundles from all sides 

 cross and anastomose with each other, so that food in 

 any one bundle may pass into many others, and thus to 

 other sides of the stem. 



27. Transpiration. Water not only carries food" in 

 solution, but may itself become an important constitu- 

 ent of foods. Its escape from the plant, therefore, by 

 simple surface evaporation or through the stomata, called 

 in either case transpiration, often proves to be a serious 

 loss. There is evidence, also, that transpiration is often 

 beneficial. By means of the transpiration current dis- 

 solved foods are largely enabled to pass quickly and over 

 comparatively long distances in the plant. It appears 

 to aid in the translocation of ash constituents, which are 

 of much importance in certain cereals. It may aid in 

 gaseous exchanges within the plant and may have a cool- 

 ing effect upon plants that would otherwise be much in- 

 jured by exposure to great heat. As suggested by Pfeffer 

 (1900, p. 235), there must be an optimum rate of trans- 



