72 



CHAPTER XL 



THE PROCESS OF VEGETATION. — 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, sti- 

 mulates 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 exhatisted receiver ; and modern 

 chemists have determined oxygen gas, which is always 

 an ingredient in our atmosphere, to be absorbed by 

 seeds in vegetation. An experiment is recorded in the 

 Philosophical Transactions, No. 23, of sowing Let- 

 tuce-seed in two separate pots, one of which was placed 

 in the conimon air, the other in the vacuum of an air- 

 pump. Li the former 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 ai)- ag lin. 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 })lants, especially 



