TURGOR MOVEMENTS 669 



factor, the resulting movement being more rapid and recovery slower when 

 the initiating factor is intense than when it is weak. In many species, how- 

 ever, pulvinal movements are too slow to be noticed without measurement. 



The mechanism causing the sudden changes in turgor of the cells in one 

 part of the pulvinus is not clearly understood. Water moves out of the cells 

 into the adjacent intercellular spaces and some probably enters other nearby 

 cells of the petiole or stem. This outward movement of water from the cells 

 into the intercellular spaces appears to be accompanied by an increase in the 

 permeability of their cytoplasmic membranes and also by a decrease in the 

 osmotically active contents of these cells (Blackman and Paine, 191 8). All 

 of these changes are reversible since turgor may be regained by the flaccid 

 cells of the pulvinus within a short period of time. 



Turgor movements may be initiated in many different ways. In the 

 sensitive plant (Fig. 151) movements result from physical contact, injury, 

 exposure to various gases, electrical shock, jarring, insufficient water supply, 

 the change from light to darkness and vice versa and from other factors as 

 well. 



The sensitive plant also exhibits a difference in reactivity to the various 

 wave lengths of light. Turgor movements result when darkened plants are 

 illuminated by wave lengths of light of suitable intensity in the blue, the 

 long ultraviolet and the long red. No movements occur from exposure to 

 wave lengths in the orange, yellow green, or infrared (Burkholder and Pratt, 



1936). 



The environmental factors initiating the movements may be received by 

 organs at considerable distance from the pulvinus in which the turgor changes 

 causing the movement actually take place. If a terminal leaflet of one leaf 

 of a large sensitive plant is burned by a flame all of the leaves on the entire 

 plant may react with vigorous turgor movements. The conspicuous turgor 

 movements that may easily be evoked in this plant have interested many 

 students and the phenomenon has been exhaustively studied, especially with 

 reference to the mechanism of "stimulus" transmission (Houwink, 1935). 

 When the "stimulus" is mild (cool drops of water applied to the leaflets) it 

 appears to be transmitted only through living cells and the rate of transmis- 

 sion is controlled by the temperature. When a leaflet is injured by burning 

 or cutting, substances appear to be produced at the point of injury, and the 

 transmission of these compounds through the non-living vessels of the vascular 

 system apparently causes the reaction of pulvini located at some distance from 

 the point at which the injury occurred. Whatever may be the mechanism 

 by which reactions are induced at some distance from the point at which the 

 initiating factor acted, the effect is the same: a rapid loss of turgor in the cells 



