166 PLANT PHYSIOLOGY 



a mixture of three parts cocoa-butter with one part beeswax, applied 

 somewhat warm, which has the advantage of caking off readily in 

 cold water; also as a control cover half of the astomatal surface of 

 another leaf with the same mixture. Leave them for two or three 

 hours in light, then remove the vaseline or wax, blanch them, and apply 

 the iodine test. Or the leaves may be left with the coatings for two 

 days in light without the previous period in darkness. 



This method was described by STAHL in the Botanische Zeitung, 52, 

 1894, 129, where fuller particulars are given, and independently by BLACK 

 MAN in Science Progress, 4, 1895, 30. 



The ready entrance of gases through stomata in the face of 

 the very small size and relatively small number of the latter 

 (compare the Stoma Quantities later) suggests an inquiry into 

 the facts about the passage of the gases through small apertures, 

 a matter of importance involving rather unexpected phenomena, 

 which the student should study through the works on the sub- 

 ject, above all in the paper of BROWN and ESCOMBE in the Annals 

 of Botany, 14, 1900, 537. 



The mode and impelling energy of the entrance of gases into 

 the intercellular system of the plant being understood, it remains 

 to consider the absorption from the air-passages by the living 

 cells, which present moist membranes to the gases. That car- 

 bon dioxide can pass readily through moist membranes has 

 already been shown by an earlier experiment (page 164), and 

 may readily be demonstrated by binding a moist membrane 

 over a vial of lime-water which is then inserted into a vessel con- 

 taining a considerable percentage of carbon dioxide. Various 

 considerations show that the carbon dioxide here goes into solu- 

 tion, and then it passes by diffusion to the interior of the cell. 



LITERATURE OF ABSORPTION. There are, of course, the usual 

 general works of PFEFFER and of JOST, but the special papers have 

 been cited under their respective sections. 



