306 PHYSIOLOGY 



This is well illustrated by using air as the partition. In fig. 622, suppose A to be 

 pure water and B sulfuric acid, with the impermeable glass partition reaching only a 

 little beyond the top of the two liquids, the space above them being filled with air. 

 Water (as vapor) can mingle with air, a; sulfuric a< id does not vaporize measurably; 

 i.e. the air, b, is practically impermeable to it but permeable to water. Water particles 

 therefore reach the b surface of the air partition and enter the sulfuric acid. Hence 

 the water level in A falls ; the acid level in B rises. 



Or, again: if one place carefully in a tumbler (fig. 623) chloroform, c, water, 

 W, and ether, c, the water may be considered as the partition. Ether, being freely 

 soluble in water, diffuses into it and reaches the sur- 

 face of c. Being also soluble in chloroform, it moves 

 on from this surface, diffusing in the chloroform. 

 The chloroform, being only slightly soluble in water, 

 diffuses into it but slightly. Finally, there remain 

 only two mixtures: the water saturated with ether 

 and chloroform, and the chloroform saturated with 

 water and containing the rest of the ether. This 

 experiment illustrates not only the solvent action of 

 the partition, but also the way in which the relations 

 of solubility between the partition and the liquids 

 Fig. 623. — Diagram : c, that it separates determine the dominant direction 

 chloroform; w, water ; e, ether, of diffusion. 



The cell wall membrane. — Among the plant membranes through 

 which solutes pass, the cell wall seems to exercise little selective influence. 

 It is permeable to most if not all substances presented to it in nature. 

 For, externally, these are chiefly mineral salts; and internally, the cyto- 

 plasmic membranes exclude from contact with it any substances that 

 it also might not allow to pass. 



Cytoplasmic membranes. — The protoplast behaves quite differently 

 from the cell wall. It is obvious from microscopic examination that it 

 is not uniform in structure. There is always next to the cell wall a deli- 

 cate cytoplasmic layer, the ectoplast, and each vacuole is bordered by a 

 similar layer, a tonoplast (fig. 620). 



Since a layer, apparently of the same sort, is formed at the surface of a fragment 

 of protoplasm released by violence from the cell wall, it seems probable that these 

 layers are the result of a change wrought in the physical structure of the cytoplasm 

 by contact with solutions of a certain sort, rather than that they are permanent 

 organs, as they were once held to be. They are perhaps advantageous in protect- 

 ing the cytoplasm from further change. 



However formed, they are limiting membranes not only in the sense 

 of bounding the protoplast, but also in the sense of admitting and emit- 

 ting some only of the great variety of solutes that come into contact with 



