210 
Walter Stiles 
while in the dark sugar was absorbed. Since the behaviour is observed 
in plants that have been maintained for days in absence of carbon 
dioxide it is concluded that the behaviour is not related to carbon 
assimilation, but is due to changes in the permeability of the cells 
to sugar. But this explanation appears to me hardly to be in agree¬ 
ment with the facts, for a mere change in permeability of a mem¬ 
brane will not account for the reversal in direction of the movement 
of the solute. A change in the physical or chemical constitution of 
the cell contents, brought about by a photochemical reaction, appears 
a much more credible explanation of the observed facts. 
Influence of Dissolved Substances on Permeability 
In addition to the antagonistic effects described in the last chapter 
there is a certain amount of evidence that substances in solution 
external to plant cells influence the rate at which substances pass into 
or out from plant cells. The action of toxic substances in this respect 
has already been discussed. Apart from these undoubted cases there 
are a number of observations on record which are alleged to indicate 
the influence of dissolved substances in altering permeability. Thus, 
Wachter (1905) found that the passage of sugars out from the bulb 
scales of the onion was prevented by the presence of potassium nitrate 
in the liquid external to the plant. A rather similar observation is 
that recorded by Bayliss (1915) to the effect that while a 0*31 M 
solution of sodium chloride brings about exosmosis of the pigment 
from the cells of the root of the red beet, no such exosmosis takes 
place into a solution containing the same concentration of sodium 
chloride and also 0*17 per cent, of calcium chloride. Both these 
observations can be explained on the basis of a reduction of permea¬ 
bility of the cell to a cell constituent, the reduction being brought 
about by the presence of a solute in the external solution. In the 
second case the observation can be brought into line with the 
antagonistic effects noted in the last chapter. 
Fluri (1909) thought he had obtained evidence that aluminium 
salts brought about an increase of permeability of plant cells to 
glucose and a number of salts. When filaments of Spirogyra were 
immersed for two or three days in a o-oi per cent, solution of alumi¬ 
nium sulphate or nitrate, the cells were no longer plasmolysable in 
hypertonic solutions of glucose or a number of salts. Fluri explained 
this as due to the increased permeability of the cell to these sub¬ 
stances. On transferring the filaments back to water the cells recover 
their original condition, so that the effect is reversible. 
