ASSIMILATING SYSTEM OF THE PLANT. 285 



APPROPRIATION OF CARBON, OR ASSIMILATION 

 PROPER. 



759. The appropriation of carbon, and its combination witii 

 the elements of water, is by far the most striking of the kinds of 

 assimilation ; and since it underlies to a certain extent the forma- 

 tion of the matter with which nitrogen and sulphur are incorpo- 

 rated to constitute the living substance, it may well lay claim to 

 be considered assimilation proper. It was employed in this sense 

 by Asa Gray in 1850, in. the second edition of the Text-book. 



For brevitj', therefore, the term assimilation in the present 

 section will be made to refer to the appropriation of carbon. 



760. With some exceptions, to be mentioned later, the follow- 

 ing statement holds good for all plants : assimilation is essen- 

 tially a process of reduction in which the inorganic matters are 

 (1) water taken from the soil, and (2) carbonic acid ^ taken from 

 tlie air; and the organic substance produced from these is some 

 carbohydrate which contains less oxjgen than the two together. 

 Hence in assimilation there is, with the evolution of oxygen, 

 a partial reduction of the inorganic matters emploj'ed in the 

 process. 



761. Assimilation takes place only under the following condi- 

 tions : (1) The assimilating organ must contain living chlorophyll 

 or its equivalent ; (2) water and carbonic acid must be furnished 

 in proper amount ; (3) rays of light of a certain character must act 

 upon the organ ; (4) it must be kept at a certain temperature, 

 there being a minimum degree of heat below which, and a maxi- 

 mum degree above which, no assimilation can occur ; (5) a minute 

 amount of certain inorganic matters other than those named, 

 notably some compound of potassium, must be within reach. 



762. The assimilating system of the plant. All cells which con- 

 tain chlorophyll or its equivalent, and which admit of exposure 

 to the sun's rays, constitute the assimilating system of the plant ; 

 but it must not be understood that they perform only assimilative 

 work. In the simplest vegetable organisms (unicellular or fila- 

 mentous algse) and even in some water plants of the higher 

 grade (Anacharis) these cells are at one and the same time 

 members of an absorbing, a storing, and an assimilative sys- 

 tem. In land plants, and in some water plants, however, certain 

 cells have the ofBce of assimilation as their special and dominant 



1 In general throughont this work, the term carJjonic acid will be employed, 

 instead of carbon dioxide, to denote COj. 



