37 
them. Yet the share of the rest of the protoplast in this discrimination 
is not to be overlooked; and since it is impossible to analyze the action 
of each part, we may for convenience consider the protoplast as a mem- 
brane between the vacuole and the outer world. But for substances in 
the protoplast itself the ectoplast may act alone. 
Selective action. The chemical composition of the cytoplasm being 
almost wholly unknown and doubtless variable, no clear statement can 
be made as to the mode of its discriminative action. It is known only 
that it allows many substances to pass through readily and debars others; 
and further, that some substances, which are usually denied passage, 
are permitted to pass under other conditions. These relations are best 
explained by the theory that solubility in the membranes is prerequisite 
to osmosis. If so, a change in composition of the cytoplasm might ac- 
count for the change in permeability that is observed on occasion. 
It is quite possible that local differences in the composition of the cytoplasmic 
membranes (a sort of mosaic composition) may permit the passage of different sub- 
stances at different places. 
Variable selection. The welfare of the organism is largely dependent 
on the discriminative action of the cytoplasmic membranes, for sub- 
stances requisite to food-making are allowed to enter; and foods are not 
permitted to diffuse out and be lost. Chemical transformations of the 
most varied kind occur within the plant, both among the substances that 
enter it and are elaborated into foods, and also among the foods that 
are assimilated . Of course each change in chemical nature changes the 
relations of the substance to the protoplast and may modify thereby its 
diffusibility through it. Moreover, without known chemical change, 
the mere presence of one solute may greatly modify the behavior of 
another, either by changing the membranes, or by its direct influence 
upon the other solute. With membranes capable of change, and solutes 
capable of change, and the almost unknown extent of the influence of 
one solute on another, the complexity of the phenomena of osmosis 
has almost baffled investigation hitherto, but some hopeful progress has 
been made recently in the discovery of factors determining the permea- 
bility of protoplasm. 
It cannot be too strongly emphasized that the " selection " above described has 
in it no element of choice, nor does it depend upon the " needs " of the plant. On 
the contrary, it is purely physical, and depends solely upon the mutual relations of 
the substances (membranes and solutes) which the conditions bring into contact. 
