352 VEGETABLE PHYSIOLOGY 



and suddenly disappears ; then is gradually formed again, 

 and the series of events is repeated. This regular inter- 

 mittence constitutes what is often spoken of as rhythm. 



The rhythm which is so easily seen in the case of 

 pulsating 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 

 minimum turgescence in a radially symmetrical organ may 



