720 PART IV. THE PHYSIOLOGY OF PLANTS. 



be supplied with them as food. From this point onwards the 

 anabolic processes in the two cases are, as far as is known, identi- 

 cal. From the simpler plastic substances containing C, H, and O, 

 whether they have been formed from C0 2 and H 3 O 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 

 crystallisable substances, such as asparagin and leucin, which 

 belong chemically to the amides (see p. 707). The next step is 

 doubtless the formation of those more complex nitrogenous sub- 

 stances, the proteids ; and here sulphur, and phosphorus in some 

 cases, is introduced into the molecule; and finally the series of 

 assimilatory processes concludes with the formation of molecules 

 of protoplasm. 



Comparatively little is known as to the details of nitrogenous 

 anabolism ; but there is evidence to show that, in green plants, the 

 assimilation of the nitrates (see p. 712), and probably also of the 

 sulphates and phosphates, of the food can only go on in the pre- 

 sence of light ; and when it is borne in mind that these salts are, 

 like carbon dioxide, highly oxidised substances, it is not surprising 

 that the conditions for the reducing process in the one case should 

 obtain also in the others. And further, there is evidence to show 

 that the next step in nitrogenous anabolism, the production of 

 proteids from amides, is also dependent not merely upon exposure 

 to light, but upon the coincidence of this with the other conditions 

 necessary to the assimilatory function. For instance, asparagin 

 (whether formed anabolically or catabolically) will accumulate in 

 a shoot kept in the dark, although carbohydrates may also be abun- 

 dantly present : it will also accumulate if the shoot be exposed to 

 light, but in an atmosphere deprived of C0 2 : whereas it will not 

 accumulate in a shoot exposed to light under conditions which 

 enable the shoot to assimilate carbon dioxide. How and where 

 this formation of proteids from amides takes place is not accu- 

 rately known. But there is some evidence that, for instance, in 

 the leaf of a vascular plant, the process is carried on in the com- 

 panion-cells of the sieve-tissue or their physiological equivalents. 

 It appears that the amides formed in the green assimilating cells 



