io8 



PHYSIOLOGY OF NUTRITION 



leaf, the amount of carbon dioxide absorbed is greater on the side where these 

 openings are most abundant. In the case of Alisma plantago, the number of 

 stomata on the upper is to the number on the lower surface as 135 is to 100. 

 While the upper surface was absorbing 0.10 or 0.15 g. of the gas the lower sur- 

 face absorbed 0.06 or o.n g. 



These experiments led Brown and Escombe 1 to carry out the following inter- 

 esting investigations. The Catalpa leaf has stomata only on the lower surface, 

 through which carbon dioxide is absorbed in the presence of light. Under the 

 most favorable conditions 700 cc. of this gas is absorbed per hour, per square 

 meter of leaf surface. If it is assumed that absorption proceeds equally over 

 the entire leaf surface, then each molecule of carbon dioxide enters the leaf 

 with an average velocity of 3.8 cm. per minute. This velocity is only half of 

 that with which carbon dioxide is absorbed by the exposed surface of a sodium 



Fig. 66. — Apparatus for the study of gaseous exchange through the upper and lower surfaces 



of leaves. (After Blackmail.) 



hydroxide solution. But since the gas is absorbed only through the stomata, 

 and since the total area of the stomatal openings is not greater than one-one- 

 hundredth of the entire leaf surface, then a surprisingly large number (380 cm.) 

 is obtained as the average velocity of absorption of carbon dioxide through the 

 stomata. This number is fifty times as great as that representing the absorp- 

 tion of CO2 by the free surface of sodium hydroxide solution. These results 

 led to the following experiment. Test-tubes were filled with aqueous solution 

 of sodium hydroxide and covered with thin, perforated plates, different plates 



1 Brown, 1899. [Brown and Escombe, 1900.] [See note 1, p. 34.] 



