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sarily enter their system with the water they absorbfrom the soil through their roots; but they can onlyderive nourishment from it when, in consequence of theinfluence of light, they are able to effect its decompo-sition, and so, by retaining the carbon, to increase thequantity of organic matter which they contain.
347. In the third stage of the existence of a plant, orduring the formation of seed, the same general con-ditions are required which are necessary during thesecond; for the formation of seeds and fruit is governedby very nearly the same laws which regulate the pro-duction of leaves and woody fibre. Those causes whichare most influential in maintaining the healthy growth ofthe leaves, such as temperature, a due supply of food,and more especially light, are likewise the most im-portant in the growth and perfection of the seed.
343. The principal chemical use of the leaves is toassist in the formation of the organic substances existingin plants, which they do by exposing the crude juices tothe action of light, and thus effecting those peculiarchanges dependent on the combination of carbon, nitro-gen, hydrogen, and oxygen, under the influence of light,on which the growth and increase of plants depend.
349. The organic substances thus formed in the leavesare either stored up in the stem of the plant, or in otherways appropriated to its increase and perfection; and inmost cases but little of them remains in the leavesThe office of the fruit or seed-vessel is different; forthough, like the leaves, it is able to assist in the pro-