270 GROWTH OF PLANTS. 



706. When plants are shaded from the light, or 

 covered up, either artificially or by the leaves of 

 surrounding plants, the action of light is impeded, and 

 they are unable to effect the proper decomposition of 

 carbonic acid. Every one knows that, under these 

 circumstances, plants become unhealthy and send up 

 long weak shoots, endeavoring to reach the light, and 

 to place their leaves in a situation where they will 

 be exposed to its influence. 



707. The existence of most plants may be divided 

 into four periods : firstly, germination, or the deve- 

 lopment of the young plant from the embryo; second- 

 ly, the growth of the plant to maturity; thirdly, 

 blossoming, and the formation of seed or reproductive 

 parts ; and, fourthly, decay, or a cessation of vital- 

 ity, and consequent decomposition of the organic 

 structure. 



708. In each of these four periods different che- 

 mical changes are going on, and therefore different 

 conditions are requisite for the perfection of those 

 various processes. In the commencement, water, air, 

 a certain degree of warmth, and the absence of light, 

 appear nearly all that is required for the growth of 

 the embryo, which obtains food from the matters 

 stored up in the seed, until it has so far increased in 

 size as to have acquired roots and young leaves, and 

 in other respects has become fitted to derive nourish- 

 ment from external sources. 



709. During the growth of the perfect plant, it is 

 observed that, in addition to water and a certain 



