ANIMAL AND VEGETABLE 135 



In regard to the movement of protoplasm in plants 

 some interesting facts are given by Green. In cells from 

 the leaves of Elodea and the staminal hairs of Tradescantia, 

 to take two examples, the current appears to circulate, 

 as will be seen from the two figures on the preceding page. 



The same author has much to say upon the subject of 

 rhythmic movement in plants. " If we look back," he 

 writes, " to the behaviour of the contractile vacuole of 

 chlamydomonas, we are struck by the fact that its pulsations 

 occur with a certain definite intermittence so long as 

 they are not interfered with by external conditions. 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 con- 

 stitutes 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 proto- 

 plasmic motility which lead to the variations of turgidity 

 in different organs, particularly in those which are growing. 

 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. 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 ceils 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 in turgidity 

 which cause the circumnatation. 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 



