CHEMISTRY IN AGRICULTURE AND PHYSIOLOGY. 563 
in the globe. Pieces of torn leaves possess this function, as 
well as the entire leaves. 
The phenomena may, in fact, be summed up in the follow¬ 
ing statements. When plants are exposed to the influence 
of sunlight in atmospheric air, they remove the carbonic acid 
and exhale oxygen in its place. 
We know scarcely anything of the chemical processesi n 
the interior of plants, on which depends the assimilation of 
the nutrient matter taken up, and the gradual conversion of 
this into the various compounds which the plant contains. 
One of the most general phenomena, since it occurs in all 
green-coloured plants, is, as we have seen, the absorption of 
carbonic acid, and the exhalation of oxygen gas. The expe¬ 
riments of Saussure demonstrate that this process stands in 
most intimate connexion with the function of organic sub¬ 
stances ; nothing seemed easier than to explain this process. 
The neutral compounds of the plant, dextrin, gum, sugar, 
and starch, are composed of carbon and the elements of 
water ; it was only requisite to assume that the carbonic acid 
was decomposed in the leaves, its oxygen given out as gas, 
its carbon combined with water, which is never wanting in 
the plant, and the entire process was elucidated in the simplest 
Way. The theory consequently met with universal accepta¬ 
tion. The compounds containing nitrogen stand in oppo¬ 
sition to those devoid of it. Though in quantity they may 
stand far behind the latter, their importance in the vital 
phenomena of plants is not less; nitrogenous (proteine) sub¬ 
stances, as we have seen, line the cells, as the primordial 
vesicle , and, consequently, the contents of the cells are 
ordered under their immediate influence; they originate the 
development of new cells, and set in action the decompo¬ 
sition of carbonic acid. It is now as good as certain that 
ammonia furnishes the nitrogen requisite for the formation 
of the proteine substances. Of the formation of the other 
nitrogenous compounds, such as the vegetable alkaloids, 
essential oils, &c., and of their import to the plant, we know 
little or nothing. 
A portion of these substances, as the essential oils, the 
milky juices, the alkaloids, are in the highest degree poi¬ 
sonous, both to the plants which prepare them and to others, 
when they are caused to absorb them. These secretions are 
commonly separated from the other matters within the plant, 
being either enclosed in special cells, or contained in canals 
which run between the cells: this is universally the case 
with the milky juices. 
Having thus considered the vegetable cells with reference 
