VEGETABLE PHYSIOLOGY. 
THE LEAVES OF PLANTS. IS 
have the same power independently .of hairs, is a disputed 
point. Some persons believe that absorption takes place from 
either surface of the leaf, but that some plants absorb more 
powerfully by one surface than by the other; others contend 
that the leaves themselves do not possess any power of attract¬ 
ing fluids. Considering the permeability of vegetable tissue, 
and the thinness of the cuticle, it seems hardly to admit of 
doubt that they do possess some power of absorption ; and this 
is apparently borne out by the effects produced in hot weather, 
either by a shower, or by the imitation of one in syringing the 
exhausted and drooping plants in a hothouse. 
The leaves have a peculiar power of producing chemical 
changes on the air. It may not be here out of place to remind 
you, that the air we breathe consists principally of two invisible 
gases, which are called oxygen and nitrogen; the former, 
though too powerful to be breathed in a pure state, is yet that 
principle by which life is supported ; the latter is in itself de¬ 
structive of life, and seems employed to moderate the effects of 
oxygen. When we breathe, the oxygen is separated from the 
nitrogen, in passing through the lungs, and then becomes united 
with another body supplied by the decomposition or combustion 
of food, called carbon ; by that union, the respired oxj^gen is 
converted into carbonic acid, which latter compound is injurious 
to animal life. Thus, we perceive that animal respiration vitiates 
the atmosphere ; but 'plants, when under the influence of light, 
decompose the carbonic acid ; they absorb the carbon, and give 
out again the oxygen, and by this means the equilibrium is 
maintained. It must be recollected, that this action is almost 
exclusively confined to the green parts of plants ; the flowers, 
and fruits, and even stems, being continually engaged in con¬ 
verting the oxygen into carbonic acid. This explains the cause 
of the unhealthiness attributed to cut flowers, when introduced 
into rooms, especially sleeping-rooms, the effects of which are 
most powerful at night, in the absence of light, when the puri¬ 
fying process carried on by plants becomes reversed. The im¬ 
portance of solar light to plants, which it is desired to maintain 
in a healthy condition, thus becomes evident, since it is by its 
influence that the decomposition of carbonic acid, and of water, 
and the extrication of the nitrogen of the atmosphere, are ef¬ 
fected. Perspiration, also, is principally excited by the action 
