258 COMPENDIUM OF GENERAL BOTANY. 



known causes inherent in the individual these two plants require 

 different amounts of silica in the building up of the body-substance 

 (deposition in the cell-walls, etc.). Due to causes inherent in the 

 processes of osmosis the nutritive cells of Arundo allow more SiO, 

 to enter, because it is continually removed and utilized elsewhere, 

 while in Nymphaea SiO 2 is not removed from the cell. As we 

 have already learned, the living primordial utricle possesses the 

 property of being impermeable to certain substances in solution (as 

 sugar, coloring-substances, etc.). This property is due to the in- 

 herent peculiarities of the plants themselves, and not to any " selec 

 tive " power. 



THE CYCLIC COURSE OF FOOD-SUBSTANCES. 



The entire chemism of plants may be diagramatically repre- 

 sented upon a circular line, dividing it into quadrants as follows : 

 1, assimilation ; 2, transformation ; 3, retrogressive changes ; 4, de- 

 composition. 1 and 2 are metabolic processes, 3 and 4 catabolic 

 (NAGELI). 



CO a , H a O, and NH 3 (or HNO 3 ) figure as raw material in the first 

 process and again appear as the final products in process 4, in decay, 

 fermentation, etc. Processes of transformation convert the carbo- 

 hydrates and amides of process 1 into more complicated chemical 

 compounds, as cellulose, albuminoids, fats, ethereal oils, etc. Retro- 

 gression (3) works in the opposite direction ; cellulose is changed 

 into sugar, fats into fatty acids and glycerine ; glucocides are also 

 split up into sugar and some other compound. The products of de- 

 composition (4) are again the simpler compounds CO,, H a O, NH 3 . 



II. THE PHYSIOLOGY OF GKOWTH. 



Scientific botany, like other special sciences, finds its greatest 

 difficulty in solving those problems which lie nearest at hand. 

 What is growth ? Why must cells grow ? These are questions 

 which the physics and chemistry of plants have failed to answer sat- 

 isfactorily. Growth, the specific manifestation of life, like all other 

 vegetable life-phenomena, can be traced only to plasm, in which it 

 is inherent. There is no mechanics of plasm which enables us to 

 deduce from the structure and peculiarities of plasm what actually 

 occurs in the growing cell. This statement is to be emphasized, be- 



