112

GROWTH OF PLANTS.

processes. In the commencement, water, air, a certaindegree of warmth, and the absence of light, appearnearly all that is required for the growth of the embryo,which obtains food from the matters stored up in theseed, until it has so far increased in size as to haveacquired roots and young leaves, and in other respectshas become fitted to derive nourishment from externalsources.

344. During the growth of the perfect plant, it is ob-served, that in addition to water and a certain degree ofwarmth, carbonic acid, ammonia, and certain earthy andsaline substances, are required; whilst light, which in thefirst stage of its growth appears prejudicial, is nowrequired to assist in those complicated chemical changeswhich are going on, when the compound organic sub-stances, such as woody fibre, gum, starch,gluten,&c., areformed in the organs of the plant by the combination ofthe elements of water, carbonic acid, and ammonia.

345. Hence during the day, or whilst plants are ex-posed to the influence of light, the carbonic acid absorbedby them is decomposed, the carbon alone being retainedin their structure, whilst the oxygen is given off.

346. During the night, or when light is withdrawn,this process of decomposing carbonic acid and acquiringcarbon cannot go on, and accordingly the carbonic acidthen absorbed by the roots is given off again unchanged,by other parts of the surface of the plants, in place ofundergoing decomposition. For plants are at all timesreceiving from the soil carbonic acid, which must neces-