EFFECTS OF GASES GENERALLY UPON LEAVES. 805 
the table, such as Sea-kale, Celery, &c. In these instances 
when the plants are grown freely exposed to light, as in their 
natural conditions, they form abundance of woody matter, 
which renders them tough or stringy ; and also peculiar secretions, 
which are either unpleasant to the taste or absolutely injurious. 
But the formation of these secretions, and also of the woody 
matter, is interfered with when the access of light is more or 
less prevented, and the plants then become useful vegetables. 
How such a vast variety of compound substances can be 
formed in such simply organised bodies as plants, is at present 
almost unknown. It is to the labours of the physiological 
chemist that we must look for the elucidation of this important 
matter ; but as it is not our purpose to allude to the various 
theories that have been entertained upon their formation and 
nature, we must refer the student to chemical works for full 
details upon this subject. It is, however, certain that the elimi- 
nation of oxygen and carbon dioxide, already described, are 
results of these chemical processes. The food of plants is 
highly oxygenated as compared with the important proximate 
principles tormed within their leaf-cells, and hence a disengage- 
ment of oxygen must occur during their formation. 
(5) Effects of Gases generally upon Leaves.—In the last section 
we have seen that those ordinary normal constituents of atmo- 
spheric air, namely, oxygen, nitrogen, carbon dioxide, and am- 
monia, in certain proportions, are especially necessary for the 
due elaboration of the various organic compounds of plants, and 
we have also shown that they are absorbed by the leaves or roots, 
or by both. It is by leaves especially, or perhaps entirely (see 
page 815), that carbon, which is so essential to plants, and which 
enters so largely into the composition of their various products and 
secretions, isabsorbed. Butit must be understood, at the same 
time, that plants will not live in an atmosphere composed simply 
of either carbon dioxide, oxygen, or nitrogen ; but that for 
their proper development these gases must be mixed in suit- 
able proportions, for if either of them be in great excess, the 
plants will either languish or perish, according to circumstances. 
Plants will, however, flourish in an atmosphere containing a 
moderate addition of carbon dioxide, even more vigorously than 
in ordinary atmospheric air; but if the amount be considerably 
increased, they will perish. This injurious effect of carbon 
dioxide, when in excessive amount, would seem to be owing 
to a directly poisonous influence. When plants are placed in 
pure nitrogen or oxygen, or under any other circumstances 
where they cannot obtain a suitable supply of carbon dioxide, | 
they soon decay. 
Whilst the above gases in suitable proportions are necessary 
to the due performance of the proper functions of plants, all other 
gases when mixed in the air in which they are placed, appear to 
act more or less injuriously upon them. ‘This is more parti- 
