331 



absorbing cells are chiefly placed, and cellular hairs or pro- 

 cesses of the epidermis which take the greatest share in the 

 action; these soon die and are replaced by a corky layer, 

 while fresh epidermis and hairs are again thrown out at the 

 apex. By these, water holding various salts in solution is 

 constantly being drawn up for the nourishment of the plant 

 and supply of the inorganic elements required, which can 

 only be assimilated when presented to the tissues in a liquid 

 state. Epiphytal plants, by their aerial roots, take up nutri- 

 ment from the atmosphere alone ; while parasites, which can 

 only live on the juices of other plants, send cellular pro- 

 longations into the tissues of the host, through which they 

 suck out their nutriment, and thus destroy their supporter. 



The leaves are arranged in such a way as to be fully 

 exposed to the influence of air and light ; and thus we find 

 that, however numerous, there is no overlapping or inter- 

 ference one with another. The fluids carried up from the 

 root are elaborated in the cells and vessels of the leaves, and 

 undergo important changes ; and again they absorb carbonic 

 acid, ammonia, and water, and give ofi* water, oxygen, and 

 other gases. Absorption of fluid by the surface of leaves 

 takes place most readily when the cuticle is thin and the 

 stomata numerous ; and this is the usual condition on the 

 under surface. Dew and rain are thus taken up, and pass 

 down the spiral vessels and parenchyma, and through the 

 intercellular spaces ; carbonic acid is also rapidly removed 

 by leaves, and this appears to go on continuously. The 

 young cuticle readily absorbs water, but this power is 

 greatly diminished by age, as it then becomes indurated and 

 impregnated with waxy deposit. 



Transpiration or exhalation of watery fluid is also carried 

 on by leaves, the amount varying with the structure of the 

 leaf and the surrounding atmospheric conditions. When 

 hard and dry, or when the cuticle is dense or supplied with 



