Transpiration and Water Movement 93 



The preceding table is sufficient to indicate that the 

 number of stomata in a given area is variable. Again, 

 there is no constant relation between the number in a 

 given area and the size of the leaf ; for, in the highest and 

 lowest moisture-content with wheat, both length of 

 leaves and entire weight of tops are approximately 

 equal, yet in the low moisture-content there are only 57 

 per cent of the stomata found in the high moisture. 



FIG. 27. Stoma of Helleborus : position of guard cells open (darker 

 lines) and closed (lighter lines) ; cell contents shaded. [After Schwen- 

 dener and Strasburger.] 



55. The control of water-loss by stomatal movement. 



The mechanism of stomatal movement has been abun- 

 dantly studied. Commonly, wilting releases the tension 

 which forces the guard cells apart, so that closure must be 

 effected. Turgor of the guard and other cells of the 

 stomatal region are therefore primarily important in 

 determining the extent of the opening. The relative 

 positions of the guard cells open and closed are shown in 

 Figure 27, after Schwendener. 



Recent studies upon the relation of stomatal movement 

 to transpiration point to several suggestions and conclu- 



