LECTURE V. 



ABSORPTION (continued]. 



II. Absorption of Gases. 



AN interchange of gases is constantly taking place between 

 every living cell and the medium in which it exists : in the 

 case of terrestrial plants, between the plant and the air, in 

 the case of aquatic plants, between the plant and the water. 

 When the plant is a simple one, each cell of it is in direct 

 relation with the medium ; when it is of complex structure, 

 there is, in terrestrial plants, some means by which the more 

 internal cells are brought into relation with it. The members 

 which are especially adapted for this purpose in the higher 

 plants are the leaves ; so we may say that just as their roots 

 are the special organs for the absorption of water and sub- 

 stances in solution, so their leaves are the special organs 

 for the absorption of gases, although this is effected to some 

 extent by other members also. 



Let us briefly consider the structure of the leaf. The 

 blade or lamina, the part with which we are especially con- 

 cerned at present, consists, speaking generally, of a paren- 

 chymatous tissue, termed mesopkyll, between the cells of 

 which there are intercellular spaces more especially towards 

 the lower surface. This tissue is traversed by numerous 

 fibrovascular bundles, forming the so-called veins of the leaf, 

 and it is covered on both surfaces by a layer of cells 

 which is the epidermis. The parenchymatous cells have thin 



