ASSIMILATION. 235 



the aperture be very small compared with the area of the 

 absorbing surf ace. * 



In the cases of the leaves of two plants, Catalpa and Helian- 

 thus annuus, Brown and Escombe made approximate measure- 

 ments of the superficial area of the spongy absorptive surfaces 

 of the cells of the parenchyma and of the area of the stomata 

 opening into the space. They found a ratio in the case of the 

 sunflower of about 212 : 1, in the case of catalpa of 1159 : 1. 



In the case of helianthus the maximum rate of absorption of 

 carbon dioxide by direct measurement was about '134 c.c. per 

 square centimetre of leaf surface per hour. This, according 

 to Brown and Morris, would result if the partial pressure of 

 the carbon dioxide within the intercellular space were reduced 

 by only about 6 %. If the absorption of carbon dioxide were 

 perfect and able to keep the partial pressure at practically nil 

 the amount of absorption of a helianthus leaf should be about 

 2 or 2*5 c.c. carbon dioxide per square centimetre per hour if 

 the stomata be fully opened, or the area of the openings might 

 be reduced to ^ of their maximum and yet allow of the 

 maximum observed absorption. 



What has been said about absorbed carbon dioxide is equally 

 true of the evolved oxygen in assimilation or of carbon dioxide 

 in respiration. 



Diffusion is also quite capable of accounting for the transpi- 

 ration of water through the stomata, and the outward move- 

 ment of water or oxygen would thus not interfere with the 

 inward passage of the carbon dioxide. 



With reference to the chemical reactions which attend the 

 assimilation of carbon dioxide by plants much work has been 

 done. The chlorophyll granules frequently enclose starch 

 granules, and for a long time it was thought that starch was 

 the first body formed in the assimilative act, and that sugar, 

 also detected in leaves, was formed entirely from starch by 

 hydrolysis. 



It was shown in 1886 by Meyer f that leaves floated on 



* This explanation, based on the kinetic theory of gases, appears to the author to 

 be clearer and more in accordance with what he believes to be the true mechanism of 

 the phenomenon than the more elaborate and more mathematical conception described 

 by Brown and Escombe, in which the process of diffusion is pictured as analogous to a 

 flux or flow of carbon dioxide through the aperture. 



t Bot. Zeit. 1886, Nos. 5 & 6 ; J.C.S. 1886, abst. 902. 



