CHAPTER X. 



ASSIMILATION IN ITS WIDEST SENSE, APPROPEIATION OF 

 CARBON, NITROGEN, SULPHUR, AND ORGANIC MATTERS. 



757. The teraa assimilation., as generall3' understood in Vege- 

 table Phjsiologj-, means the conversion by the plant, through 

 the agency of chlorophjll, of certain inorganic matters iuto 

 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 ' 

 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 b}- the plant, there is alwaj-s associated the appropriation 

 of tlie elements of water, namel}-, hydrogen and oxygen ; but the 

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

 undergo no chemical change jn the organism, have already been 

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

 the process of assimilation. There are sundry mineral matters 

 which, though absolutel}- essential to the well-being of the plant, 

 are eonvenientlj' examined witiiout special reference to assimi- 

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

 potassium, are indispensable to the process of assimilation; but 

 they do not become at an^- period an indispensable part of tiie 

 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 bj- their temporary presence in the plant contribute to 

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

 bricants without which machinerj' cannot run easily or perhaps 

 at all. 



1 See Pfefier's Pflanjenpliysiologie, i. 186. 



