170 TEXTBOOK OF PLANT PHYSIOLOGY 



interesting to note that in plants showing stomata on both sides of 

 the leaf, those on the upper surface open later and for a shorter 

 time. Likewise, the stomata near the tip of the leaf open earlier 

 and close later than those at the base. 



54. The Regulation of Transpiration. — The transpiration 

 process consists of two phases: evaporation proper, which takes 

 place in the intercellular spaces; and diffusion of the water vapor 

 through the stomatal openings into the surrounding air. The 

 intensity of transpiration is usually under the controlling influence 

 of two kinds of factors: changes in the diameter of the stomatal 

 pores, and changes in the degree of saturation of the cell walls. 



Since changes in the degree of opening of the stomata may be 

 observed directly, the stomatal movements were, until recently, 

 regarded as the only means of regulating transpiration. This 

 controlling influence of the stomata is based on the fact that when 

 the stomata are closed, outward diffusion of water vapor is entirely 

 stopped. The air in the intercellular spaces then becomes satu- 

 rated and water is no longer evaporated, thus permitting only 

 cuticular transpiration to take place, which is comparatively 

 insignificant. The marked decrease in transpiration at night, 

 mentioned in Art. 51, depends not only on environmental condi- 

 tions but also on the fact that in the majority of plants the stomata 

 are completely closed at night. 



But the fact that the complete shutting of stomata stops 

 transpiration, does not permit the conclusion that simply a narrow- 

 ing of the stomatal pore will lead to a corresponding check in water 

 loss. The epidermis with its stomata may be viewed as a multi- 

 perforated septum. Because of the small dimensions and great 

 number of these openings, diffusion takes place almost at the same 

 rate as if a septum did not exist (Arts. 11 and 52). Within certain 

 limits, changes in the stomatal pores, therefore, induce a slowing 

 down of the diffusion rate. An exact correlation between the 

 degree of opening of the stomata and the intensity of transpiration 

 has not yet been established. Recent investigations, however, 

 make it veiy probable that a decrease of 50 to 75 per cent in the 

 diameter of a stomatal pore does not seem to influence transpira- 

 tion very greatly. A further narrowing of openings, however, 

 will result in a perceptible check in water loss. 



Light, which plays such an important role in transpiration, pro- 

 motes the opening of the stomata. They close in light only with 



