272 USE OP THE LEAVES. 



conditions are required which are necessary during 

 the second ; for the formation of seeds and fruit is 

 governed by very nearly the same laws which regu- 

 late the production of leaves and woody fibre. Those 

 causes which are most influential in maintaining the 

 healthy growth of the leaves, such as temperature, a 

 due supply of food, and more especially light, are 

 likewise the most important in the growth and per- 

 fection of the seed. 



713. The principal chemical use of the leaves is to 

 assist in the formation of the organic substances ex- 

 isting in plants, which they do by exposing the crude 

 juices to the action of light, and thus effecting those 

 peculiar changes dependent on the combination of 

 carbon, nitrogen, hydrogen, and oxygen, under the 

 influence of light, on which the growth and increase 

 of plants depend. 



714. The organic substances thus formed in the 

 leaves are either stored up in the stem of the plant, 

 or in other ways appropriated to its increase and 

 perfection ; and in most cases but little of them re- 

 mains in the leaves. The ofiSce of the fruit or seed- 

 vessel is different ; for though, like the leaves, it is 

 able to assist in the production of organic matter, 

 the substances so formed are retained and stored up 

 in itself, and the fruit or seed-vessel, not only returns 

 nothing to the plant on which it grows, but in addi- 

 tion attracts to itself a large portion of the vegetable 

 matter formed, by the combination of carbon, oxygen, 



