352 VEGETABLE PHYSIOLOGY 



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 amount of such substance present. We have seen 

 already that this 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 plasmatic 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. 316). 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 mtermittence 

 so long as they are not interfered with by external condi- 

 tions. The vacuole dilates slowly, reaches a certain size, 



