Lecture 4 



— 85 — Movement and Distribution 



the complexity of causes and effects involved in these 

 processes. In the studies referred to observations 

 were made of root pressures in decapitated sunflower 

 or other plants kept under a nearly constant environ- 

 ment: that is, in dark chambers with controlled tem- 

 perature and humidity. Despite this constancy of 

 external environment, the plants passed through reg- 

 ular 24 hour cycles of variation in root pressure [com- 

 pare also White's (reviewed 1942) studies on single 

 roots grown from root tips and earlier work by 

 Barenetsky (1877)]. The peaks of pressures — or 



NOON NOON NOON NOON 



Textfigure 23. — Periodicity in exudation and 

 effects of auxin application to Helianthus on the 

 rate of exudation from decapitated plants at dif- 

 ferent periods in 24 hour cycles. (From Skoog, 

 Broyer and Grossenbacher, 1938, redrawn). 



rates of exudation — were attained at periods usually 

 not far removed from noon and the low points in 

 periods around midnight. A protoplasmic rhythm is 

 suggested, which governs the cell activities affecting 

 salt and water movement into or out of the conduct- 

 ing system. By growing plants in continuous arti- 

 ficial light and cutting the stems at appropriate times, 

 maxima and minima of pressure could be inverted, 

 but the 24 hour cycle persisted. 



