[ 87 1 



CHAPTER XT. 



THE PROCESS OF VEGETATIOX. USE OF THE COTYLEDONS. 



When a seed is committed to the ground, it swells by 

 the moisture which its vessels soon absorb, and which, 

 in conjunction with some degree of heat, stimulates its 

 vital principle. Atmospherical air is also necessary to 

 incipient vegetation, for seeds in general will not grow 

 under water, except those of aquatic plants, nor under 

 an exhausted receiver ; and modern chemists have de- 

 termined oxygen gas, which is always an ingredient in 

 our atmosphere, to be absorbed by seeds in vegetation. 

 An experiment is recorded in the Philosophical Trans- 

 actions, No. 23, of sowing Lettuce-seed in two separ- 

 ate pots, one of which was placed in the common air, 

 the other in the vacuum of an air-pump. In the form- 

 er the young plants rose to the height of two inches, or 

 more, in a week's time ; in the other none appeared, 

 till after the pot had been removed for a similar period 

 into the air again. Seeds buried in the ground to a 

 greater depth than is natural to them do not vegetate, 

 but they often retain their power of vegetation for an 

 unlimited period. Earth taken from a considerable 

 depth will, when exposed to the air, be soon covered 

 with young plants, especially of Thistles, or of the Cress 

 or Mustard kind, though no seeds have been allowed 

 to liave access to it. If the ground in old established 

 boianic gardens be dug much deeper than ordinary, it 

 liequently happens that species which have been long- 



