GENERAL ^ 



importance to the organism, three classes of metabolic products may in 

 general be distinguished, namely (i) building or formative substances, 

 (2) plastic or trophic substances, and (3) aplastic or atrophic substances 

 (Sect. 50). No sharp line of demarcation can, however, be drawn between 

 these three classes, for substances which serve as building material may, at 

 a later period or under changed conditions, be drawn into metabolism, as may 

 also be the case with substances which normally remain permanently aplastic. 

 Similarly the substances which are essential for the maintenance of a certain 

 indispensable turgidity can take no part in metabolism, but since they are 

 essential to the living cell, they may be regarded as building material. 



Aplastic substances are purely negative in character, and hence the 

 group includes bodies of widely different importance, and not only those 

 which are occasionally or continually formed as unavoidable by-products 

 of metabolism, but also others which are produced for special purposes. In 

 the former case, unless the end-products are continually rendered available 

 again in metabolism, they must be excreted, so that an injurious accumula- 

 tion may be avoided. When exposed to light, green plants continually 

 reassimilate the carbon dioxide they exhale, and by the synthetic processes 

 thus induced a loss of nitrogen, phosphorus, sulphur, &c. is avoided, for the 

 simple compounds of these elements produced by katabolism are con- 

 tinually reassimilated (Sects. 68, 73). Similar reconstruction is possible 

 in heterotrophic plants, when they are supplied with sugar, or some other 

 organic compound. 



Those aplastic substances, which are formed only to a limited extent, 

 probably always subserve a definite function. Thus enzymes and acids 

 act as digestive and solvent agents, other substances act as stimuli to 

 irritable reactions, and many metabolic products have an attractive or pro- 

 tective importance owing to their smell, taste, or poisonous character. In 

 all such cases the substances in question must be permanently withdrawn 

 from metabolism, and hence the importance or value to the plant of any 

 product cannot entirely be measured by its plastic or aplastic character. 



If the term ' excreta ' is restricted to those substances which are of no 

 further use to the plant, it will include only certain aplastic products, for 

 secretory products such as enzymes, ethereal oils, &c. have a definite 

 function to perform, and in the translocation of plastic material particular 

 cells may excrete one substance and absorb another simultaneously. The 

 terms ' assimilation ' and ' dissimilation ' have already been defined from 

 a physiological standpoint (Sect. 50). 



A complete knowledge of the causal relationships of the different 

 metabolic processes can only be obtained by a thorough investigation of 

 the powers, properties, and organic mechanism of the living plant, and 

 although vital activity and metabolism are mutually dependent, they may 

 partly or entirely cease when resting periods intervene in the progress of 



