ASSIMILATION. 191 



matter of plants, upon which, or in connection with which, the hght 

 exerts its wonderful action, is first developed. When plants are 

 made to grow in insufficient light, as when potatoes throAV out shoots 

 in cellars, this green matter is not formed. When light is with- 

 drawn, it is soon decomposed ; as we see when Celery is blanched 

 by heaping the soil around its stems. So, also, the naturally green- 

 less leaves of plants parasitic upon the roots or stems of other species 

 (152) have no direct power of assimilation, but feed upon and grow 

 at the expense of already assimilated matter. But all green parts, 

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

 the same manner as leaves, even supplying theu- places in plants 

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

 influence of light, an essential preliminary step in vegetable digestion 

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

 evaporation or exhalation of the now superfluous water, the mechan- 

 ism and consequences of which have already been considered (313). 



345. We have now to consider the further agency of light in vege- 

 table digestion itself, namely, its action in the leaf upon the concen- 

 trated sap. Here it accomplishes two unparalleled results, which es- 

 sentially characterize vegetation, and upon wliich all organized exist- 

 ence absolutely depends (1, IG). These are, — 1st. The chemical 

 decomposition of one or more of the substances in the sap which 

 contain oxygen gas, and the liberation of this oxygen at the ordi- 

 nary temperature of the air. The chemist can liberate oxygen 

 gas from its compounds only by powerful reagents, or by great 

 heat. 2d. The transformation of this mineral, inorganic food 

 into organic matter, — the organized substance of livinfg plants, 

 and consequently of animals. These two operations, although 

 separately stated, 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 decomposed ; the portion not required in the growth of the plant 

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

 time rearrimged, so as to form peculiiu* 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 



