8o 



Elementary Plant Physiology. 



should be filled completely with water in this test. Seal the 



lower end of the twig, and ascertain if air bubbles will issue from 



the submerged lenticels. 



77. Osmose of gases. — Gases pass through membranes 



by osmose, but are dissolved in the water in the membrane 



during their passage 

 through it. Membranes 

 impregnated with waxes 

 or oily substances, there- 

 fore, permit the diffusion 

 of gases very slowly. On 

 the other hand, the 

 thin walls of the cells 

 in the interior of the 

 body are well adapted 

 to rapid exchanges of 

 gas between neighbor- 

 ing cells. In such sat- 

 urated walls, passage of 

 the gas can take place 

 only by osmose, and not 

 by filtration pressure. 

 As soon as a mem- 

 brane becomes dry, how- 

 ever, gas may be forced 

 through it, and osmose 

 ceases, or is reduced to 

 a minimum. The con- 

 tinuous exchange of 

 gases between the in- 

 tercellular spaces in the 

 interior and living cells 

 causes great variations 

 in the composition of 



Fig;. 56.— Preparation to demonstrate osmose 

 of g-ases. O, skin of grape attached to glass 

 tube ; T, cork and sealing- wax ; /, level of 

 mercury twenty days after beginning of test. 



