28 MISSOURI BOTANICAL GARDEN. 
but can take up only sodium chloride and ammonium sul- 
phate from dilute solutions. Fluri (5) claims that aluminum 
salts render the protoplasm of Spirogyra cells permeable to 
ordinary plasmolytic agents. Wachter (18) found that 
sugar diffuses out of sections of onion in distilled water or 
in hypotonic sugar solutions, but that the addition of a trace 
of a potassium salt to the solution prohibits the diffusion of 
the sugar from the onion tissue. Osterhout (12) has con- 
cluded that the antagonistic action of various salts as regards 
their toxic effects is largely or entirely due to the fact that 
they hinder the entrance of each other into the protoplasm, 
7. é., that each salt affects the permeability of the protoplasm 
for other salts. True and Bartlett (17) have shown that the 
rate of absorption and secretion by the roots of peas varies 
with the concentration of the solution in which the roots are 
immersed and with the nature of the salt or salts dissolved in 
the solution. McClendon (10) thinks that those substances 
that stimulate growth do so by affecting the permeability of 
the protoplasm of the cells stimulated. Czapek (2) found 
that tannin diffuses from cells of Echeveria whenever the sur- 
face tension of the medium in which the cells are immersed 
is lowered to approximately two-thirds that of water. It is 
claimed by Ramann and Bauer (16) that the time of 
maximum absorption of an element by the roots of trees 
varies with the season. The pine, for example, absorbs ni- 
trogen most rapidly in June, while it takes up calcium most 
rapidly in August. 
These observations, made by a number of different workers, 
show that the membrane, which has been assumed to sur- 
round living cells, must, if it exists, alter its permeability 
under many different conditions. The permeability of non- 
living membranes is not capable of such alterations. The im- 
portance of the supposed membrane surrounding living cells 
depends entirely, so far as absorption and secretion are con- 
cerned, on its degree of permeability. Since the living 
membrane differs from the non-living membranes in this im- 
portant characteristic, it naturally follows that we should be 
very careful when we compare the one with the other. The 
. ee we ee | 
