860 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 it is 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 



