356 VEGETABLE PHYSIOLOGY 



a more or less regular pulsation, assuming slowly the 

 appearance of a nearly spherical cavity and then 

 suddenly disappearing, recalling the active contraction 

 of animal protoplasm. These pulsating or contractile 

 vacuoles can be seen very well in Chlamydomonas. 



The power of movement which is thus exhibited by 

 many of the lowlier plants may be distinguished, however, 

 from certain of the movements of portions of higher plants 

 which have already been alluded to, and which will be 

 discussed more fully subsequently. These movements 

 include the circumnutation of growing organs, the closing 

 of the leaves of Dioncea, the bending of the tentacles of 

 Drosera, and many others. These are brought about in 

 multicellular organs, and by a mechanism different from the 

 one now under discussion, the movement being secondary 

 and following indirectly on a change in the behaviour of 

 the protoplasm of certain of the cells, which, instead of con- 

 tracting, modifies its resistance to the escape of the water 

 which they contain. In one or two cases, as in the curving 

 of certain tendrils and in the drooping of certain leaves in 

 response to stimulation, the hydrostatic disturbance seems 

 to be attended by, and perhaps partly dependent upon, a 

 contraction of the protoplasm of certain cells. These 

 phenomena will be discussed in a subsequent chapter, 

 and need only be alluded to here as possibly showing the 

 inherent power of contractility residing in the protoplasm. 



Though the power of locomotion, which we have seen 

 in many cases to exist, is an evidence of certain powers of 

 movement or contractility possessed by living substance, 

 it must not be inferred that only organisms which are free 

 to move are possessed of these or similar properties. Loco- 

 motion is impossible to the great majority of plants on 

 account of their relationship to their environment. There 

 is, however, a certain amount of evidence to show that the 

 instability which, in the cases discussed, finds its expression 

 in movement, is a property of living substance in general. 

 We find many cases in which movement of the living 



