CHAP. II. FUNCTION OF NUTRITION. 187 



emission of some portion of the carbonic acid which 

 they generate ; but when they are exposed to the light, 

 they not only restore the oxygen which they had pre- 

 viously abstracted from the atmosphere, but also give 

 out another portion of this gas, which they set free by 

 the decomposition of the carbonic acid contained in the 

 air, as well as that which was in the water imbibed by the 

 spongioles. In animal respiration, the carbonic acid is 

 immediately expelled from the lungs as soon as it is 

 formed, and the function is then considered complete ; 

 and perhaps it would be more logical to divide the 

 function of vegetable respiration into two processes, one 

 of which should comprise the formation, and the other 

 the decomposition, of carbonic acid. 



(173.) Formation of Carbonic Acid. The formation 

 of carbonic acid takes place in the leaf, beneath the 

 epidermis ; but whether the air penetrates through the 

 stomata or not, is still uncertain. That it cannot uni- 

 versally be introduced through these organs is apparent, 

 since many leaves have no stomata; and in these cases 

 at least, the action takes place through the intervention 

 of the delicate membrane of which the vesicles of the 

 cellular tissue are composed. If a section perpendicular 

 to both surfaces of a leaf be examined under the highest 

 powers of the microscope (fig. 152.), the interior will be 

 observed to be chiefly 

 made up of cellular 

 matter, or " paren- 

 chyma," whose vesi- 

 cles are loosely ag- 

 gregated, so that large 

 intercellular passages 

 exist in communica- 

 tion with each other, 

 through its whole sub- 

 stance. That these passages are filled with air is readily 

 shown by placing a leaf under water, and beneath the re- 

 ceiver of the air-pump. Upon exhausting the receiver, the 

 air contained in the leaf will be seen to escape through the 



