RESPIRATION AND OXYGEN. 



55 



midst of massive tissue. In conditions especially unfavorable to 

 gas exchange, the amount of oxygen falls very low, to about 0.25 

 per cent. In all the cases observed, however, its total disappearance 

 never occurred. The diffusion of free air is vastly more rapid than 

 that of dissolved air, a fact which permits very perfect aeration of 

 porous tissues, but very imperfect in culture fluids. The central 

 cavity of a fruit of Cucurbita maxima contained air with a compo- 

 sition of 2.52 per cent C0 2 , 18.29 per cent oxygen, and 79.19 per cent 

 N. When air was forced into the fruit under water, numerous larger 

 and smaller bubbles appeared over the surface, coming from what are 

 essentially lenticels. In the fruits of cucurbits that lack lenticels 

 the air enters through the stomata. 



Aubert (1892 : 203) found the composition given in table 23 for 

 the internal air of several species of succulent plants. 



TABLE 23. 



Magness (1920 : 308) has analyzed the gas in the intercellular 

 spaces of apples, potatoes, and carrots at different storage temper- 

 atures, with the results included in table 24. 



TABLE 24. 



The removal of the peel from the ends of apples resulted in a 

 marked reduction in the amount of C0 2 and a similar increase of 

 oxygen, due to the greater ease of escape and entrance. The range 

 of variation in the amounts of these two gases at 20 C. was 12.5 



