196 PART III. PHYSIOLOGY. [ 45 



processes in the two cases are, as far as is known, identical. 

 From the simpler plastic substances containing C, H, and 0, 

 whether they have been formed from CO., and H 2 in the one case, 

 or have been absorbed as organic food from without in the other, 

 other more complex substances such as sugar, etc., are formed, 

 probably by the polymerisation or condensation of the simpler 

 molecules. Further, the nitrogen of the food, absorbed either as 

 nitrates or salts of ammonia, is worked into the anabolic processes, 

 so that nitrogenous organic substance is produced. Probably the 

 first formed nitrogenous substances are comparatively simple crys- 

 tallisable substances, such as asparagin and leucin, which belong 

 chemically to the amides (see p. 186). The next step is doubtless 

 the formation of those more complex nitrogenous substances, 

 the proteids, and here sulphur, and phosphorus in some cases, is 

 introduced into the molecule ; and finally the series of assimila- 

 tory processes concludes with the formation of molecules of 

 protoplasm. 



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 

 C0;j and H 2 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 ba 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.). 



