172 PLANT LIFE. 



the gas used is the same. In the smaller and simpler plants 

 the protoplasm absorbs oxygen directly through the cell wall. 

 In multicellular plants, however, especially when these be- 

 come large and complex, only the superficial cells 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 

 osmosis through other cells. In large plants, therefore, 

 intercellular spaces are provided, communicating \yith the 

 external air, and througli these oxygen diffuses. In the 

 land plants the intercellular spaces are continued through the 

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

 the stomata (^| 166). On the older parts of woody plants 

 which have begim to form a periderm the stonata are replaced 

 by lenticels, through which the internal intercellular spaces 

 communicate with the outer air (^ 140). In the absence of 

 stomata or lenticels, however, the oxygen may pass through 

 any part of the surface of the plant. In submerged water 

 plants, very large intercellular spaces are formed (fig. 117), 

 permitting the existence of an internal atmosphere of con- 

 siderable amount, within whose limits gaseous exchanges may 

 occur. Oxygen may reach these intercellular spaces from 

 the water through the suiierficial cells. 



243. Intramolecular respiration. — While free oxygen is 

 ordinarily utilized for resiiiration, all plants seern to be 

 capable ol obtaining their supply for a short time from the 

 organic matter of the plant itself Such respiration has there- 

 fore been called intramolecular respiratinn. It can exist at 

 most for a few hours without producing" disease and, sooner 

 or later, the death of the plant. It is [irecisely parallel to 

 the similar method of respiration possible among cold-blooded 

 animals. y\. few jdants of the simpler sort, such as the 

 bacteria, rely whollv iqiou comliined ox3-gen fir their respira- 

 tory sii])ply. Such [danls ha\-e adapted themsehes to grow 

 in the absence of free oxygen, whith, instead of facilitating 



