PERSPIRATION OF PLANTS. 23 



introduced into the substance of Lichens through 

 their surfaces. 



M. Decandolle adopts it as a principle that 

 such pores are never found but on those parts 

 of vegetables, exposed to the influence of air and 

 light. He has not been able to find them on 

 those parts of the stalk of grasses which are 

 covered by the sheaths of the leaves, nor on the 

 inner surface of these sheaths. The same is the 

 case with the inner surface of the calycular scales 

 of the Composure. 



No part of such plants as live constantly in the 

 water are furnished with these pores ; neither are 

 they observed on the immersed leaves, nor the 

 lower part of the stalk in Callitriche, Sparganium, 

 Polygonum amphibium ; while on the other hand 

 all the upper parts above the water are covered 

 with them. The under surfaces of the leaves in 

 Nymphaea, in Menyanthes nymphoides, and in 

 Hydrocharis, which float upon the water, have 

 none, while the upper surface is furnished with 

 them. But plants entirely destitute of pores, while 

 living in the water, regain them when growing out 



