Part V. 



THE GENERAL CHEMISTRY AND 

 PHYSICS OF PLANT-LIFE. 



I. CHEMICAL PHYSIOLOGY. 



In the treatment of tissue-phjsiologj (II, B) we also took into 

 consideration some very important chemical processes, such as assimi- 

 lation and the formation of albuminous compounds. It now remains 

 for us to consider the more important features of the general cliem- 

 istrv of plants. (In the main we will follow Pfeffer and Sachs.) 



As a rule, the first step in making an analysis of a plant-substance 

 is to place the substance to be examined in a desiccator. The deter- 

 mination of the dry substance and water of different plants gives 

 widely different results, depending upon the conditions of develop- 

 ment. Ripe seeds contain comparatively little water, the dry sub- 

 stance constitutes about f of the entire weight, while in the germi- 

 nating seed, after the reserve material has been absorbed, it is scarcely 

 -^-^ ; later the weight of the dry substance may again increase from 

 -J to ^. In submerged plants there is of course but a very small 

 amount of dry substance, often less than J^. 



O/i hurdling the plant only a small percentage remains as ashes. 

 This important statement implies that almost the entire mass of 

 the dry substance must consist of combustible or volatile elements 

 or compounds; the elements are C, H, N, O. S remains in chemi- 

 cal union with the ash, forming basic oxides, similar to the readily 

 oxidizable P. 



What are the substances appropriated by the plant, and how are 



252 



