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A STUDY OF THE PROBLEM OF WATER ABSORPTION. 31 
permeable membrane or a membrane that is only partially 
permeable to dissolved substances surrounds living cells. 
The literature shows that there are numerous facts that do 
not accord with the assumption that such a membrane reg- 
ulates absorption and secretion. But it is certainly not 
possible to prove that a membrane possessing some degree of 
semi-permeability does not surround certain cells. The best 
we can do is to determine the value of the assumption that 
such a membrane does exist. It was with these facts in mind 
that a study of this problem was undertaken. 
III. EXPERIMENTAL STUDY. 
1. Mechanical Injury to the Membrane. 
Since the permeability of living protoplasm is quite sus- 
ceptible to change, and since its permeability is supposed to 
result from the character of a living membrane surrounding 
the protoplast, one must naturally expect this membrane to 
be easily capable of mechanical injury. The following ob- 
servations bear on this phase of our problem: 
When one places vigorous filaments of Spirogyra setiformis 
(Roth) Kiitz., which have been growing in pond water into a 
strong solution (solutions about two times molar were used) 
of potassium or sodium chloride and observes them with a 
low-power microscope, the cells do not appear to be plas- 
molysed, but when they are observed more closely and 
stronger magnification is used one sees that they are very 
much plasmolysed. The protoplast now occupies only a 
small fraction of its original volume and forms one or more 
spherical masses within the cavity surrounded by the chloro- 
phyll bands. If the cells are carefully observed during the 
course of this experiment it will be seen that when the 
strong chloride solution surrounds them they become plas- 
molysed very quickly. The chlorophyll bands often occupy 
exactly the same position after plasmolysis that they occu- 
pied when the cell was in its normal condition. If a weaker 
solution is used, plasmolysis takes place more slowly and 
the chlorophyll bands are displaced toward the center of 
