856 CHANGES OF OKGANIC MATTER IN THE PLANT. 



responds nearly to what has been called metastasis, 1 the second 

 to respiration. But it must be remembered that the distinction 

 between the two groups is not absolute. 



930. The contrast between assimilation and respiration is very 

 marked : one is substantially the opposite of the other. The 

 following tabular view displays the essential differences between 

 them. 



Assimilation proper Respiration 



Takes place only in cells containing Takes place in all active cells. 



chlorophyll. 



Requires light. Can proceed in darkness. 



Carbonic acid absorbed.oxygen set free. Oxygen absorbed, carbonic acid set free. 



Carbohydrates formed. Carbohydrates consumed. 



Energy of motion becomes energy of Energy of position becomes energy of 



position. motion. 



The plant gains in dry-weight. The plant loses dry-weight. 



Some of the changes which are grouped under transmutation, 

 or metastasis, present almost as great a contrast to assimilation 

 proper as that shown in the above table. 



931. Course of transfer of assimilated matters. In the present 

 state of knowledge it is impossible to trace all the chemical 

 changes which assimilated matters undergo in the plant, or even 

 the course which such matters take ; only a few of the more ob- 

 vious modifications have been investigated. Before proceeding 

 to describe the important forms of organic substance in the 

 plant, the following general considerations should be presented. 



The carbohydrates are believed to be transferred from one 

 part to another, in the higher plants, through the thin-walled 

 parenchyma. The reaction of these cells is almost uniformly 

 acid. The transfer takes place only when the carbohydrates 

 are in solution. 



The albuminoids are probably carried chiefly by means of 

 the soft bast of the fibro- vascular bundles ; the cells of this bast 

 have a slightly alkaline reaction. 



But that these are not the only paths of transfer, appears from 

 the frequent occurrence of minute starch-grains in the sieve- 

 cells, and, on the other hand, of dissolved albuminoids in paren- 

 chyma cells. 



1 The German word Stoffwechsel is usually translated metastasis, a word 

 long known in medicine with a totally different signification from that above. 

 Schwann's term, metabolism, much used in human physiology, expresses its 

 idea better, but for some reasons the term transmutation appears preferable. 



