106

GERMINATION.

ing matters which the soil contains, and that thehumates, or compounds of humic acid, thus formed,being absorbed by plants, might supply them with car-bon. There is, however, no proof whatever that thisreally is the case; on the contrary, as has already beenstated, there is every reason to believe that decayingvegetable matters merely yield carbon to growing plants,by generating carbonic acid gas.

325. The food of plants, then, is carbonic acid gas,water, and ammonia, partly derived from the air, andpartly from the soil ; and certain earthy and salinesubstances, derived almost wholly from the soil. Let usconsider the circumstances under which plants are ableto absorb and assimilate these substances, and whatother conditions are requisite to their growth.

326. The important influence which water has onthe changes which vegetable substances are liable toundergo has already been alluded to, when speaking ofgluten and the other similar substances which enterinto the composition of seeds. All these substances,which, under the joint action of air and moisture, areso liable to undergo various changes, become compara-tively fixed and unchangeable, when well dried andprotected from the action of water. During the germi-nation of seeds, a great change takes place in the natureof the substances composing them. Carbonic acid isformed by the combination of a portion of the carbonwhich they contain with the oxygen of the air. Theconditions requisite to the germination of seeds are,