292 



Plant Physiology 



ence, since heat is lost even 

 from the silvered flasks. It 

 is greatly to be desired that 

 an accurate calorimeter may 

 be perfected. 



169. The mechanism for 

 gas exchange. To a con- 

 siderable extent, the histo- 

 logical mechanism permit- 

 ting the diffusion of oxygen 

 through the tissues of the 

 plant is the same as for 

 carbon dioxid used in photo- 

 synthesis. Nevertheless , 

 the cells active in photo- 

 synthesis are localized, in 

 higher plants, and the 

 mechanism which permits 

 aeration, in general, is 

 more complex and may be 

 treated. 



Since respiration is pro- 

 ceeding in every living cell, 

 the possibility of a fairly 

 rapid gas diffusion must ex- 

 tend to the remotest tissues. 

 In developing or mature 

 roots, stems, leaves, and 

 fruits, there may be found 

 FIG. 73. Dewar bulb in cross-sec- considerable intercellular 



tion, showing also method of ex- 



periment, ->- heat release. Space providing f Or diffusion 



