PHYSIOLOGY. 289 
On this account we find, that the roots of herbas 
ceous plants which we collect for medicinal pur- 
poses, are more efficacious in winter and*spring, than 
in summer, when in full leaf and flower, because 
then they have athe new sap by their young 
radicles. 
§ 278. 
That plants emit oxygen in day-light, and in the 
dark principally carbonic acid gas, has been already 
mentioned, (§ 273). The reason of this, as the 
latest discoveries in chemistry have shewn, we are 
now to explain. | | | 
Plants imbibe through the pores of the cutis, 
(§ 274), atmospheric air, which consists of azote, 
oxygen, and carbonic acid gas; the azote being the 
greatest in quantity, the carbonic acid gas the least. 
Experiments prove this phenomenon clearly. Plants 
which were put in carbonic acid gas, soon decayed, 
as well as when inclosed in azote and hydrogen gas, 
with this difference only, that in the two last gases 
they decayed slower. ‘The cause of their decay is 
| certainly no other, but that they want the necessary 
oxygen in the inclosed air, and their vessels theres 
fore become relaxed. 
From the ground, plants imbibe water and care 
bonic acid gas, (§ 274), as well as oxygen. We 
know, that the carbonic acid gas is specifically 
heavier then the other gases, that it precipitates and 
is absorbed by water, and that on this account it is 
easily taken up by the radicles of plants. For this 
very reason Sennebier alleges, that plants grow so 
very 
