PROPERTIES OF VEGETABLE PROTOPLASM 381 
cell, as we have seen, by the formation of various osmoti- 
cally active substances in its interior, which have an attrac- 
tion for water, the quantity which enters depending upon 
the nature and amount of such substances present. We 
have seen already that the regulation of osmotic material is 
controlled by the protoplasm. But besides this another 
important factor exists in the greater or less difficulty with 
which water is enabled to pass through the protoplasmic 
membranes. The power of altering’ its permeability by 
water is a property of protoplasm which is of the highest 
importance in the mechanics of the cell. It takes the place 
practically which is held by the power of contractility in 
the living substance of animals. No doubt it can be called 
into play during life under constant conditions, but it 
becomes much more marked when the plant is subjected to 
particular kinds of stimulation. A ready instance of its 
employment under the former conditions is afforded by 
the variations of turgidity and subsequent growth which 
we have already spoken of as inducing circumnutation 
(p. 815). Instances of its following upon stimulation will 
be discussed more appropriately in a later chapter. 
The facts thus briefly narrated impress upon us the 
belief that all protoplasm is the seat of active molecular 
movement, the intensity or vigour of which, as well as the 
forms of its manifestations, varies very greatly in different 
cases. Indeed, the life of the protoplasm is intimately 
bound up with such a motile condition. The manifesta- 
tions are in all cases appropriate to the manner of life and 
the surroundings of the organism under observation; they 
may take the form of locomotion, of contractility, or of 
variation of permeability, leading to the regulation of 
turgescence. 
If we look back to the behaviour of the contractile 
vacuole of Chlamydomonas, we are struck by the fact that 
its pulsations occur with a certain definite intermittence 
so long as they are not interfered with by external condi- 
tions. The vacuole dilates slowly, reaches a certain size, 
