Oil the Food of Plants. 
509 
scription, but simply, that in vegetables no nervous centres are 
known to exist, but the power, if present at all, is diffused 
throughout the whole substance of the plant. In fact, these 
beings seem to possess a kind of " cellular life," and this uni- 
formity of structure is such, that almost any one part may be 
made, by very slight modification, capable of doing the duty of 
another. 
It is a very old idea to compare a living plant or animal to a 
chemical laboratory, in the different parts of which various jjro- 
cesses are constantly going on, partly chemical in their nature, 
but modified by the agency of that mysterious pirinciple, life. VYe 
have no reason to take a different view of the subject ; the dis- 
coveries of modern chemistry tend to confirm it ; we see inorganic 
bodies of simple constitution, such as water and carbonic acid and 
ammonia, enter the plant, and there become converted into such 
complicated substances as sugar and starch and albumen, in virtue 
of powers and agencies with which we have little or no acquaint- 
ance. To decompose carbonic acid and set free its carbon re- 
quires in our hands the exertion of the most intense chemical 
energies, such as the action of potassium at a high temperature ; 
yet the green plant, under the influence of sunshine, effects this 
with the utmost apparent ease. Nevertheless, I am very far from 
thinking that we shall never know more of these things than we 
do at present ; indeed recent discoveries respecting the phenomena 
of fermentation hold out some prospect of ultimately obtaining a 
key to a portion of this magnificent chemistry of nature. 
The Food of Plants. 
The chemical history of the most important among the proxi- 
mate vegetable principles having been thus shortly discussed, we 
are now in a condition to grapple with the far more difficult 
question of the source or sources from whence the plant has de- 
rived the elements of which these principles are composed ; in a 
word, the food upon which plants subsist. Is this food derived 
from the earth, or from the air, or from both ? And if the latter 
supposition be true, what share does each contribute towards the 
effect observed ? 
The atmosphere which surrounds the earth, and which extends 
with a rapidly diminishing density to a height of about 45 miles 
from its surface, consists of a mixture of oxygen and nitrogen 
gases in the proportion by measure of 21 volumes of oxygen to 79 
volumes of nitrogen,* and further, these proportions are quite 
" More correctly, according to the recent elaborate researches of Dumas, 
'20-8 meas. oxygen to 79'2 meas. nitrogen. (Annales de Chiraie et de 
Physique, Novembre, 1841.) In addition to the substances above enume- 
rated, it is proper to observe that an opinion is held by some that the air 
