ABSORPTION 



165 



43), the leaf with its stoma side outward; fill them all with carbon 

 dioxide by dry displacement, and support them upright in a dish of 

 mercury. Keep one of the paper discs moist by wet filter-paper on 

 one side; observe and interpret the results. 

 After a pronounced effect appears, requiring 

 one or two days, place a piece of caustic 

 potash under each tube. Later the same 

 tubes may be filled with hydrogen and then 

 with oxygen. 



Or an instructive result is given by simply 

 binding a loose-fitting piece of thin rubber 

 sheeting tightly to the mouth of a test-tube 

 containing carbon dioxide. The parchment 

 paper may be tied on while wet, if dried 

 before use. 



The student should now make sure that 

 he understands the physical nature of the 

 diffusion of gases, the kinetic theory of gas- 

 pressure, the source of the energy causing 

 it, its molecular basis, the relative pressures 

 exerted by equimolecular volumes, the effect 

 of temperature upon volume and pressure, 

 and the diffusion movements of one, gas in a 

 mixture in relation to the others of that mix- 

 ture when the gas is being either absorbed or 

 released at some particular place, for such 

 conditions are actually found in the inter- 

 cellular passages of the plant. 



Returning, with our new knowledge, to 

 the structure of the plant, a first and 

 natural inquiry to arise would be this: 



Does carbon dioxide actually enter the plant through the sto- 

 mata? 



This may be determined, indirectly but conclusively, by observing 

 whether any photosynthesis can take place when the stomata are 

 blocked. 



EXPERIMENT. Select a plant of good photosynthetic power, but 

 having stomata only upon one surface, e.g., Fuchsia, Abulilon, Euphor- 

 bia, Senecio, and keep it in darkness over a day. Then cover one-half 

 of the stomatal surface with a thin layer of vaseline, or, better, with 



FIG. 43. DIFFUSION 



TUBES; XT- 



The heavy black represents 

 sealing-wax; further ex- 

 planation in text. 



