THE LOSS OF WATER BY THE PLANT 171 



great difficulty. This influence of light on the movements of sto- 

 mata may seem incomprehensible as regards water economy. It is 

 understandable, however, when we remember that the stomata 

 are the channels through which carbon dioxide must enter the leaf, 

 and for successful photosynthesis it is requisite that the stomata 

 be open during the hours when the plant is illuminated by the sun. 

 The closing of the stomata during the daytime, though lessening 

 transpiration, would have an unfavorable effect on the nutrition of 

 the plant. 



Another way of regulating transpiration would be by a reduc- 

 tion of evaporation from the surface of the cell walls within the 

 intercellular spaces. Such decrease may take place only when the 

 cell walls lining the intercellular spaces become drier than usual. 

 Such a dehydration of the walls in turn is subject to a lower degree 

 of saturation of the cells themselves, and, consequently, an increase 

 in their suction tension. As a result of this mechanism, based on 

 the equilibrium between the suction tension of the cell contents and 

 the colloidal cell walls, a water shortage in the leaf, due to excessive 

 transpiration or an insufficient water supply from the soil, is in 

 itself a cause of decreased transpiration. Whether this is accom- 

 panied by a narrowing of the stomatal pores or not is of small 

 moment. This phenomenon has been called "incipient drying" 

 (Livingston). It must be regarded as a most important non- 

 stomatal regulator of transpiration. 



The degree of moisture in the cell walls depends not only on 

 the condition of saturation of the cell, but likewise on the permea- 

 bility of its protoplasm. If the protoplasm is quite impermeable to 

 the substances in the cell sap, then the osmotic pressure of the latter 

 will attain its highest value, and water will be drawn with a maxi- 

 mum force from the cell walls. If the permeability of the proto- 

 plasm increases, the pressure of the sap on the protoplasm, and 

 through it on the cell walls, diminishes. Part of the water is 

 pressed out of the cell by the contracting cell wall, the surface of 

 the wall is covered with water, and transpiration increases. This 

 mechanism is especially conspicuous when cells die, and as a conse- 

 quence the protoplasm loses its semipermeability. Hence, leaves 

 killed by frost or heat, or merely injured in this way, lose water 

 very rapidly, and become dry. Minor fluctuations in permeability, 

 such as are effected by the influence of light or narcotics on leaves, 

 likewise accelerate water loss from the cells. 



