668 



PLANT MOVEMENTS 



move toward the side having the reduced pressure. As the flaccid cells of 

 the pulvinus regain their turgor the petiole is pushed slowly back into the 

 position occupied before the movement occurred. Frequently the loss of 



Fig. 150. Dia- 

 grams showing dis- 

 tribution of the 

 vascular tissue 

 (shaded) in {A) 

 cross section of a 

 pulvinus, (B) the 

 transition zone be- 

 tween a pulvinus 

 and a petiole (lon- 

 gitudinal section) 

 and (C) cross sec- 

 tion of a petiole. 



Fig. 151. A sensitive plant (Mimosa pu- 

 dica). Upper: leaves in expanded condition; 

 lower: leaves in a collapsed condition as a 

 result of turgor movements. (From Stiles 

 Introduction to the Principles of Plant Physiol- 

 ogy, Methuen & Company, Ltd.) 



turgor occurs very rapidly. In the sensitive plant detectable turgor move- 

 ments have been reported to occur within O.075 second after "stimulation," 

 and the response may be complete in little more than a second. Recovery 

 of turgor commonly occurs in from 8 to 20 minutes. The speed of the reac- 

 tion and the time of recovery vary greatly with the intensity of the causal 



