380 VEGETABLE PHYSIOLOGY 
The vacuole dilates slowly, reaches a certain size, and 
suddenly disappears; then is gradually formed again, and 
the series of events is repeated. This regular intermittence 
constitutes what is often spoken of as rhythm. 
The rhythm which is so easily seen in the case of pul- 
sating vacuoles is characteristic also of those less obvious 
changes in protoplasmic motility which lead to the varia- 
tions of turgidity in different organs, particularly in those 
which are growing. We have already seen that during 
the growth in length of a symmetrical organ, such as a 
stem or root, the apex points successively to all points of 
the compass, the successive changes of position being 
spoken of as circumnutation. This is the result of a 
rhythmic variation of the turgidity of the cells of the 
cortex. If we consider a longitudinal band of such cells, we 
find that at a certain moment the cells are at their point of 
maximum turgidity, and the growing apex is made to bend 
over in a direction diametrically opposite to this band. 
The turgidity of this band then gradually declines to a 
minimum, and again increases slowly to a maximum. If 
we conceive of the circumference of the organ as divided 
into a number of such bands, we can gain an idea of the 
changes of turgidity which cause the circumnutation. 
Each band is in a particular phase of its rhythm at any 
given moment, and the successive bands follow one another 
through the phases of their rhythm in orderly sequence, so 
that when one is at its maximum, another diametrically 
opposite to itis at its minimum. The phases of maximum 
and minimum turgidity thus pass rhythmically round the 
organ, and the apex is consequently compelled to describe 
a spiral line as it grows. If the stem or root is not circular 
in section, but is flattened in any direction, the steady 
sequence of the rhythmic changes will cause the projection 
of this spiral to assume the form of an ellipse instead of a 
circle, and if the flattening is extreme the movement will 
be a backward and forward one. 
Modifications of the distribution of maximum and 
