CHAPTER XI 



PRODUCTS OF METABOLISM; DIGESTION 

 AND T RAN SLOG AT ION 



FROM the discussion of the general relations of the green 

 plant to carbon and nitrogen it has been developed that 

 a variety of organic products are characteristic of the plant 

 cell and plant body. Beginning, in the typical case, with 

 those which may be regarded as the products of photo- 

 synthesis (photosynthates) , on the one hand, and with 

 the elementary organic substances containing nitrogen, 

 on the other, there may be built up in various ways diverse 

 series of organic compounds, constituting the plant body 

 and including, of course, the protoplasm and the cell 

 walls. This shall not be taken, however, to indicate that 

 there is a normal sequence of products, or a continuous 

 building up, for, as will be shown subsequently, the build- 

 ing up may be interrupted at any point and breaking down 

 may occur.' The food products are then utilized in a 

 manner dependent upon the specific nature of the sub- 

 stances and upon the chemical requirements of the cell. 

 Pfeffer has distinguished plastic and aplastic substances. 

 The former term is used by him to include substances 

 which may be used as food, which may be mobilized and 

 utilized in metabolism ; while aplastic substances include 

 the solid and permanent constituents of the cell, such 

 as the cell wall, and certain by-products or waste ma- 



250 



