NUTRITION. 147 



same. In the smaller and simpler plants the protoplasm 

 absorbs oxygen directly through the cell wall. In multi- 

 cellular plants, however, especially when they become large 

 and complex, only the cells at the surface could do this. 

 The internal cells are too far from the source of supply to 

 allow an adequate amount of oxygen to reach them by travel 

 through other cells. In large plants, therefore, internal 

 spaces are provided, and through these oxygen moves readily. 

 In the land plants the internal spaces open into the air 

 through the epidermis, in which, with the guard cells, they 

 constitute the stomata (^ 137). In the absence of stomata, 

 however, the oxygen may pass through any part of the sur- 

 face of the plant. In submerged water plants, very large 

 intercellular spaces are formed (fig. 76), permitting the ex- 

 istence of an internal atmosphere of considerable amount, 

 within whose limits gaseous exchanges may occur. . Oxygen 

 may reach these intercellular spaces from the water through 

 the superficial cells. 



203. Intramolecular respiration. — While free oxygen is ordinarily 

 utilized for respiration, all plants seem to be capable of obtaining their 

 supply for a short time from the living matter of the plant itself. In 

 most plants it can exist for a few hours at most without producing disease 

 and, sooner or later, the death of the plant. It is precisely parallel to 

 the similar method of respiration possible among cold-blooded animals. 

 A few plants of the simpler sort, such as the bacteria, rely wholly upon 

 combined oxygen for their respiratory supply. Such plants have adapted 

 themselves to grow in the absence of free oxygen, which, instead of 

 facilitating their life processes, really checks them. 



204. Excretion, — The carbon dioxid produced by respira- 

 tion, when not used for food making, is gotten rid of by 

 the reverse of the methods described for the absorption of 

 oxygen. 



205. Release of energy. — The purpose of respiration is to 

 set free energy required for growth and movement. While 



