CHAPTER II. — SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 721 



are collected into this tissue, and there undergo farther anabolism 

 into the proteids to be foand in the sieve-tubes. 



Inasmuch as plants destitute of chlorophyll absorb their nitrogenous food in 

 the form of ammonia- compounds (see p. 712) or of organic nitrogen-compounds, 

 it is intelligible that exposure to light is not an essential condition of their 

 nitrogenous assimilation. 



These various assimilatory processes are not, however, carried 

 on simultaneously with equal activity. In plants which contain 

 chlorophyll, when under conditions favourable for carbon-assimi- 

 lation, the construction of non-nitrogenous organic substance from 

 CO2 and HgO appears to be the most active process, for an accumu- 

 lation of non-nitrogenous organic substance can be detected in 

 the green parts of these plants when assimilation is being carried 

 on. Most commonly this excess of non-nitrogenous organic sub- 

 stance is accumulated in the form of starch-granules which are 

 formed in the chloroplastids ; less commonly in the form of sugar 

 which is held in solution in the cell-sap (e.g. leaves of Onion). 

 This excess of non-nitrogenous organic substance in the green 

 parts soon disappears, however, when, by withdrawal from the 

 influence of light, its further formation is arrested. For instance, 

 if a plant which has been exposed to light and whose leaves are 

 rich in starch, be placed in the dark for some hours, the starch 

 will then be found to have almost or entirely disappeared. 



The organic substance resulting from the anabolism of the 

 plant, is partly used in the growth of the plant, in forming new 

 protoplasm, cell-walls, etc., and is partly stored up, in various 

 organs, in the form of reserve materials which serve either for the 

 growth of the plant itself at a subsequent period (roots, tubers, 

 etc.), or for the nutrition of new individuals in the early stages of 

 their growth (spores, seeds, etc.). 



4. Cataholism. Under this term are included all the chemical 

 processes going on in the plant which lead to the formation of 

 simple substances from more complex ones. 



The chief physiological importance of the catabolic processes is 

 this : that, inasmuch as they consist in the decomposition of 

 relatively complex and unstable substances into others which are 

 relatively simple and stable, they necessarily involve a conversion 

 of potential into kinetic energy ; and it is by means of the kinetic 

 energy so evolved that the plant exhibits those phenomena, such as 

 growth, movement, etc., which characterise it as a liv^ing organism. 

 The degree of activity of life depends directly upon the degree of 



