PHYSIOLOGY. 291 
The rays of the sun conjoined with the vital power 
of plants promote the decomposition of the water in 
its constituent parts, hydrogen and oxygen. The 
oxygen stimulates the air vessels, and even by sti- 
mulating the vegetable fibre in general, quickens all 
the secreting processes. It combines besides with 
caloric, and escapes in a gaseous form through 
the pores of the plant. ‘The imbibed atmospherical 
air is, through the increased stimulus of the vital 
power, freed from its carbonic acid and azotic gas. 
In the same manner the carbonic acid gas of the 
water, which was taken up by the roots, and which, 
éven perhaps itself is imbibed by them in its gaseous 
form from the ground, becomes fixed. ‘These mate 
ters now enter, according to the assimilating power 
which is inherent in each plant, and which appears 
to be a modification of the vital power itself, in dif- 
ferent new combinations and in different propor- 
tions, forming oils, resins and gums, and all the rest 
of the above enumerated (§ 230), vegetable princi- 
ples. 
In darkness, however, when the light no longer 
rouses the vital power to the decomposition of the 
water, the oxygen contained in the atmosphere 
again forms new and different combinations with 
the other principles. It cannot now stimulate the 
vessels, and therefore a small quantity of gas is 
emitted by the plant. The quantity of the carbonic 
acid gas cannot become fixed, and therefore again 
parts with the plant as such. Mi 
The light of the sun effects, even in aquatic plants, 
at the bottom of rivers and brooks, the decomposi- 
T 2 tion 
