ASSIMILATION. 195 



pense of already assimilated matter. But all green parts of plants, 

 such as the cellular outer bark of most herbs, act upon the sap in 

 the same manner as leaves, even supplying their place in plants 

 which produce few or no leaves, as in the Cactus, &c. Under the 

 influence of light, an essential preliminary step in vegetable diges- 

 tion is accomplished, namely, the concentration of the crude sap 

 by the evaporation or exhalation of the now superfluous water, the 

 mechanism and various consequences of which have already been 

 considered (267, 313). 



345. We have only to consider the further agency of light in 

 the process of vegetable digestion itself, namely, its action in the 

 leaf upon the concentrated sap. Here it accomplishes two per- 

 fectly unparalleled results, which essentially characterize vegeta- 

 tion, and upon which all organized existence absolutely depends 

 (1, 16). These are, 1st. The chemical decomposition of one or 

 more of the substances in Ihe sap which contain oxygen gas, and 

 the liberation of this oxygen at the ordinary temperature of the 

 air. The chemist can in certain cases liberate oxygen gas from 

 its compounds, but only with the aid of powerful reagents, or of a 

 heat equal to that of red-hot iron. 2d. The transformation of this 

 mineral food, this inorganic into organic matter, the organized 

 substance of living plants, and consequently of animals. These 

 two operations, although separately stated to convey a clearer idea 

 of the results, are in fact but different aspects of one great process. 

 We contemplate the first, when we consider what the plant gives 

 back to the air ; the second, when we inquire what it retains as 

 the materials of its own growth. The concentrated sap is decom- 

 posed ; the portion which is not required in the growth of the plant 

 is returned to the air ; and the remaining elements are at the same 

 time rearranged, so as to form peculiar organic products. 



346. The principal material given back to the air, in this pro- 

 cess, is oxygen gas,* that element of our atmosphere which alone 



* A small proportion of nitrogen gas is likewise almost constantly exhaled 

 from the leaves; but this appears to come from the nitrogen which the water 

 imbihed by the roots had absorbed from the air (326), and which passes off 

 unaltered from the leaves when this water is evaporated, or from the air which 

 the rootlets directly absorb to some extent. In the course of vegetation, no 

 more nitrogen is given out than what is thus taken in, and probably not so 

 much. So that the exhalation of nitrogen may be left out of the general view 

 of the changes which are brought about in vegetation. 



