Metabolism; Digestion and Translocation 251 



terials. It is difficult, of course, to draw any line between 

 these two types of substances. 



139. Metabolism. All of those chemical changes 

 which take place within the body incident to growth and 



'development are commonly included under the term " me- 

 tabolism." These changes may be constructive (or ana- 

 bolic) and destructive (or catabolic). Some brief indica- 

 tions have been given respecting the building up of a few of 

 the more important organic compounds, and it is necessary 

 to include now a somewhat comprehensive view of the 

 general relations of a few of the foods and by-products 

 and some characteristics of these materials. 



140. Temporary foods, storage products, and perma- 

 nent structures. It would seem that many substances 

 produced within the cell are temporary, that is, they may 

 be labile compounds readily used in the metabolism of 

 the active cell. If formaldehyde is a first product of photo- 

 synthesis, it is necessarily one of this nature. Naturally 

 the transient compounds are the lesser known as plant 

 constituents ; but it seems certain that many simple car- 

 bohydrates, fatty acids, amides, and the like are distinctly 

 temporary. Nevertheless, a substance which is, temporary 

 in one plant may be accumulated in another. 



Whenever the food manufactured is in excess of that 

 used, it accumulates, and may be regarded as a storage 

 product. The chlorenchyma of higher plants is a tem- 

 porary storage structure, for starch or other substances 

 may accumulate in the cells of this tissue during photo- 

 synthesis. Specialized storage structures are extremely 

 common among higher plants, and to such organs the 

 food in diffusible form is transported. 



