NUTRITION. 53 



and it is only in youth that the cellular envelope 

 of the branches, can receive a small quantity of 

 moisture : as soon as the action becomes har- 

 dened, further lateral growth is impossible. The 

 powerful action of the leaves, &c. as here de- 

 scribed, in determining the ascent of the sap, is 

 a much more probable account of that pheno- 

 menon than any propulsive vis d tergo like that 

 supposed in the extract from Professor Hen- 

 slow in paragraph 39, to be resident in the 

 lowest extremities of the roots. 



42. It is well known that fresh plants ex- 

 posed to the air part with a considerable por- 

 tion of their moisture. This exhalation is not 

 performed equally all over the plant, but is in 

 exact proportion to the quantity of stomata on 

 any given part, and it is curious that this fact 

 was established by the experiments of Guillard, 

 Saint-Martin, Bonnet, and Senebier, before the 

 existence of stomata was known. Light has 

 great influence in increasing the transpiration of 

 plants. This exhalation may sometimes be ob- 

 served in the form of drops of water resting on 

 the leaves, &c. when circumstances preclude 

 the possibility of their arising from rain or dew. 

 " The manner in which the stomata act is un- 

 known ; and consequently we are compelled to 



