DIFFUSION OF GASES 



223 



(with the membrane) was exposed to air containing the normal 

 amount of gases. In this way, after many experiments carried 

 on with various kinds of moist and dry membranes, Molisch and 

 Wiesner came to the following conclusions : 



1. The cell walls of both living and dead cells, whether wet or 

 dry, do not permit gases to go through under ordinary pressures. 

 A piece of birch bark 0.09 mm. thick thus supported a column of 

 mercury 40 cm. high for two weeks without any change in the 

 height of the mercury column. 



2. Protoplasm and cell sap are also impermeable to the stream- 

 ing of gases, which shows the necessity for the air passages pre- 

 viously mentioned. 



After thus showing that streaming movements were impossible 

 through cell walls and protoplasm, further experiments were 

 continued to test the amount of molecular 

 diffusion through similar tissues. In these 

 experiments the tubes were partly filled 

 with the gas to be tested and the rate of 

 outward diffusion was measured by the 

 rate of the rise of the mercury column in 

 the tube (Fig. 10). From these experi- 

 ments, they reached the following con- 

 clusions: 



1. Gases diffuse through cell walls only 

 in solution in the water which the wall 

 has imbibed. If intercellular spaces are 

 present, they facilitate the movement up 

 to the cell wall, but the actual diffusion 

 through the wall is in solution in the 

 water of the wall. 



2. The more water a wall contains, con- Fig. 10.— Illustrating the 



sequently, the more rapid is the passage method of the Molisch and 



c ,i v -, x j.t_ -j Wiesner experiments, 



ol gases through it. In the potato periderm 



studied, the carbon dioxide diffused through the dry membrane 



only enough to cause a rise of 5 mm. of mercury, while in the wet 



periderm, the mercury rose 40 mm. during the same period of 



thirty days. This explains why water plants like Elodea, algse, etc., 



are so readily permeable to gases and permit of such rapid diffusion. 



3. Even dry walls if they contain cutin or suberin, i. e., the 

 fatty materials which characterize the outer layer of many plant 



