94 Plant Physiology 



sions of interest, although these studies also make it evi- 

 dent that there is yet much room for quantitative work 

 in this field. In general, it may be said that, contrary to 

 many early opinions, the stomata do not open and close in 

 direct response to the varying conditions of the atmosphere 

 which may inhibit or promote transpiration. When the 

 plant is provided with sufficient moisture, the stomata are 

 commonly open, but as Brown and Escombe and others 

 have shown, maximum transpiration does not necessarily 

 correspond with maximum opening. Wilting effects a 

 closing of the pores, but according to Lloyd there can be 

 no closing in anticipation of wilting. Again, the stomata 

 may remain open when the humidity is extremely low, 

 provided only sufficient water is available for the plant. 



Many investigators have shown a primary relation be- 

 tween stomatal opening and the time of day; and it is 

 believed that possibly through the possession of chloro- 

 phyll, and the relation to organic food-materials, there 

 may be found in the turgor of these cells an important 

 factor in stomatal regulation. 



56. Modifications tending to check excessive transpira- 

 tion. - - Closure of the stomata is in all cases a means of 

 checking the excessive transpiration, as already discussed. 

 However, this check may be insufficient in extreme cases. 

 It may prove also a menace to other activities of the plant. 

 In any event many plants exhibit a structure peculiarly 

 fitted to limit excessive transpiration. This is important 

 in the occupancy by plants of arid habitats, and it is 

 certain that many delicate species are unable to survive 

 under conditions necessitating the most excessive trans- 

 piration. This may be due in part to the incapacity on 



