118 EARTHY SUBSTANCES IN PLANTS.
ammonia, and such saline matters as the soil contains.
If the leaves of the plant are exposed to the action of jlight, the carbonic acid and ammonia are decomposed ,together with a portion of the water, and organic mat- jter is formed. ,
863. During the night, or in the absence of light,this change cannot take place; and hence, then, thewater absorbed by the roots, which is still given off bythe leaves, carries with it the gaseous matter collected !from the soil. I
364. The exact office in the nutrition of plants, per-formed by the saline substances they absorb, is as yet 1but imperfectly known. Some plants appear to require ;particular substances,* and it is known that certain sub-stances cannot be formed by plants unless the soilcontains particular saline matters. Albumen and glutenare always found to contain a small quantity of certain :phosphates, hence these substances appear to be essential
to the formation of those principles in plants.
365. It is not known whether plants have any powerof selection by their roots ; that is to say, whether theyare able to absorb from the soil only those substanceswhich they require, or whether they absorb all the jsoluble matters present in it. Almost every plant isfound to contain a little lime, and the same may also besaid with regard to potash.
366. The quantity of these inorganic matters found
in plants is different at different periods of their growth, j