CHAP, m.] REASONS FOR COVERING. 37 



vegetates too far "below the surface, a part of 

 the stem of the plant must be buried ; and 

 this part, not being intended to remain under 

 ground, is not protected from the dangers it is 

 likely to meet with there. It is thus peculiarly 

 liable to be assailed by slugs and all kinds of 

 insects, and to become rotten by damp, or 

 withered by heat. It is also very possible to 

 bury a seed so deeply as to prevent it from ve- 

 getating at all. The ground has more of both 

 warmth and moisture near the surface than at 

 a great depth, as it is warmed by the rays of 

 the sun, and moistened by the rain ; but, besides 

 this, seeds will not vegetate, even when they 

 are amply supplied with heat and moisture, if 

 they are excluded from the influence of the air. 

 Every ripe seed in a dry state is a concentra- 

 tion of carbon, which, when dissolved by mois- 

 ture, and its particles set in motion by heat, is 

 in a fit state to combine with the oxygen in the 

 atmosphere, and thus to form the carbonic acid 

 gas which is the nourishment of the expanding 

 plant. For this reason, seeds and newly sprung 

 up plants do not want to be supplied with 

 manure, and air is much more essential to 

 them : they have enough carbon in their coty- 

 ledons, or in the albumen contained in the 

 seed, and they only want oxygen to combine 

 with it, to enable them to develope their other 

 leaves ; and this is the reason why young- 

 plants raised on a hotbed are always given air, 

 or they become yellow and withered. Darkness 

 is not essential to germination, but it is found 

 to promote it. Light absorbs the oxygen from 

 plants, and occasions a deposition of the car- 



