CHAPTER X. 



ASSIMILATION IN ITS WIDEST SENSE, APPROPRIATION OF 

 CARBON, NITROGEN, SULPHUR, AND ORGANIC MATTERS. 



757. THE term assimilation, as generally understood in Vege- 

 table Physiology, means the conversion by the plant, through 

 the agency of chlorophyll, of certain inorganic matters into 

 organic substance. 



Some authors, however, give to the word assimilation a wider 

 signification, namely, the conversion into utilizable substance 

 of all matters whatsoever brought into the organism. Such 1 

 regard chlorophyll assimilation as only a special case under a 

 general class which comprises the appropriation of (1) carbon, 

 (2) nitrogen, (3) sulphur, so far as this is a constituent of 

 protoplasm, (4) certain organic matters. 



758. It will presently be seen that with the appropriation of 

 carbon by the plant, there is always associated the appropriation 

 of the elements of water, namely, hydrogen and oxygen ; but the 

 mere entrance, transfer, and exit of water, which is known to 

 undergo no chemical change in the organism, have already been 

 examined in Chapters VII. and IX., and do not strictly belong to 

 the process of assimilation. There are sundry mineral matters 

 which, though absolutely essential to the well-being of the plant, 

 are conveniently examined without special reference to assimi- 

 lation, even in its widest sense. Some of them, like the salts of 

 potassium, are indispensable to the process of assimilation; but 

 they do not become at any period an indispensable part of the 

 substance of the plant. In the case of sulphur, however, a small 

 amount of the element is appropriated by the plant and consti- 

 tutes a component part of its protoplasmic matter. The matters 

 which by their temporary presence in the plant contribute to 

 its activities, have been likened to the absolutely necessaiy lu- 

 bricants without which machinery cannot run easily or perhaps 

 at all. 



1 See Pfeffer's Pflanzenphysiologie, i. 186. 



