239 
decompose no carbonic acid, and therefore may b 
presumed to assimilate no carbon, contain, neverthe- 
less, hydrogen, oxygen and carbon, as well as green 
plants in which this assimilated carbon is supposed to 
perform so important an office ; so that, in such 
plants, at least, this supposed use of carbon, in pro- 
moting the fixation of water, is not necessary to vege- 
table nutrition and growth. Carbonic acid, also, seems 
to be decomposed in the cellular structure or paren- 
chyme of the leaf, and not in its vascular system ; 
and, for reasons already stated (468.), it appears to 
contribute rather to the augmentation of its resinous 
or colourable matter, than to its nutrition and growth. 
Admitting, therefore, that carbon is retained in the 
plant, there is no evidence of its assimilation^ in the 
proper physiological sense of that term ; for the resi- 
nous matter, which it contributes to form, is not a 
part of the vegetable structure, but a chemical pro- 
duct, which, according as light is either admitted or 
excluded, may, or may not, be formed or withdrawn, 
without injury to the organic structure of the plant. 
524. In our apprehension of the term assimilation, 
it is that function or power, which living bodies pos- 
sess, of converting and applying inorganic matter to 
their own support and increase, in a manner which has 
not yet been imitated by any mechanical or chemical 
means ; and this function we believe to be perform- 
ed, in the vascular system of plants, by a slow and 
successive series of changes, which the vegetable is 
able to carry on and accomplish, only while it posses* 
ses life^ These changes, as far as regards the sub- 
stances concerned, may be considered chemical ; but, 
