356 VEGETABLE PHYSIOLOGY 
a more or legs 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 Dionwa, 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 
