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 acid 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, 
tv, and ether, e, 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 ; iv, 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 
