290 THE FOOD OF PLANTS 



this is a process which plays a very important part in translocation, and 

 indeed in all metabolic exchanges. 



No sharp and clear distinction can therefore be drawn between the 

 different classes of metabolic products, for the same substance may 

 frequently be employed in different ways, and indeed the same food 

 particle may serve a variety of purposes during the progress of development 

 or as the external conditions alter. Thus a cell-wall which has hitherto 

 formed part of the mechanical supporting tissue may be transformed into 

 soluble plastic material, and in many of the component parts of the 

 protoplast interchanges and exchanges of substance are probably of 

 continual occurrence. Similarly, the sugar present in the nectar as an 

 aplastic product may be drawn into metabolism again, and in a starving 

 plant the same fate often befalls substances which under normal conditions 

 would have remained intact (Sect. 93), while even such a pronouncedly 

 excretory product as carbonic acid may be reassimilated when the chloro- 

 plastids are functionally active. 



It is possible to distinguish in metabolism between the processes of 

 'assimilation' or anabolism and those of 'dissimilation' or katabolism. 

 The term ' assimilation ' is sometimes restricted to those processes of con- 

 structive metabolism which lead to the formation of organized structures, 

 but it is better to use it as corresponding to the more general term 

 anabolism, and as also including those processes which terminate in the 

 production of plastic substances. Thus the formation of carbohydrates 

 in chloroplastids is a process of assimilation, as is also the formation of 

 proteids by synthesis, and the production of various organic substances by 

 certain fungi from formic or acetic acids. Dissimilation includes all kata- 

 bolic changes taking place in the contrary direction, and it is easy to see 

 that the processes of assimilation and dissimilation are often inextricably 

 connected '. 



The term ' assimilation ' was used in practically the same sense as above by 

 Bischoff and Schleiden, and this corresponds to its usage in animal physiology. 

 The restriction of the term of assimilation by Sachs to the production of organic 

 material in chloroplastids is therefore incorrect, both from historical and physiological 

 standpoints. The latter is merely a special form of assimilation, and may be termed 

 carbon dioxide assimilation, or more shortly CO 2 -assimilation a . 



1 Still further distinctions may be made. Thus Roux distinguishes between auto-assimilation 

 and the assimilation of foreign substances (Ergeb. d. Anat. u. Entwick., herausg. v. Merkel u. Bonnet, 

 1892, Bd. n, p. 430). 



a Bischoff, Handbuch d. Bot. Term. u. Syst, 1833, Bd. I, p. 13; Schleiden, Grundziige d. wiss. 

 Bot., 1845, 2 - Aufl., Bd. I, p. 278; also Nageli, Sitzungsb. d. Munch. Akad., 1879, p. 284; Sachs, 

 Experimentalphysiol., 1865, p. 18. [The term Carbon-assimilation occurs in the original German, 

 but its use is hardly to be recommended since the corresponding term Nitrogen-assimilation has 



