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. 



