97 
of supporting vegetation. If, therefore, we pronounce 
this operation of the leaves to be a vegetative func- 
tion, we must maintain that vegetation takes place 
not merely without the presence of oxygen gas, which 
is necessary to vegetation, but that it actually goes on 
most rapidly when a gas is present, which is absolute- 
ly destructive to that process. 
336. In like manner it has been shewn (24.), that 
heat is necessary to vegetation, and that this process 
does not go on if the heat be> to a certain extent, 
withdrawn ; yet M. Senebier found, that all vegeta- 
bles, which continued green, furnished oxygen gas 
in the sun, when the temperature of the air was many 
degrees below zero *. Thus, we see that the pre- 
sence of light enables the plant to furnish oxygen 
gas, when the heat, necessary to vegetation, is with- 
drawn ; and, on the contrary, it has before been 
shewn (25.), that all plants will vegetate in circum- 
stances where heat is present, but where light is ei- 
ther partially, or totally excluded. 
337. But not only without pure air and heat do 
plants, when exposed to the sun, seem capable of 
yielding oxygen ; but they afford it also in such a 
state of mutilation, as must be completely destructive 
to their vegetative power. There is a great difference 
between the mere possession of life, and the perfor- 
mance of living action. Neither the stem alone, nor 
the leaf alone, can execute the proper vegetative 
function of the plant ; for the stem must possess an 
embryo plant or bud to enable it to grow, and, though 
* Physiologic Veg. torn. iii. p. 2 
& 
