310 PLANT PHYSIOLOGY 



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 saturated, and w^ater is no longer evaporated; thus 

 there can take place only cuticular transpiration, which is com- 

 paratively insignificant. There results a considerable decrease 

 in the total water loss. The marked reduction in transpiration 

 at night, mentioned in Art. 69, depends not only on environ- 

 mental conditions, 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 a mere 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 multiperf orated septum. On account of the small dimen- 

 sions and great number of these openings, diffusion takes place 

 almost at the same rate as if a septum did not exist (Arts. 44 

 and 70). Within certain limits, changes in the stomatal pores 

 therefore do not always 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 

 estabUshed. Recent investigations, however, make it very 

 probable that a decrease of 50 to 75 per cent in the diameter of a 

 stomatal pore does not seem to influence transpiration very 

 greatly. A further narrowing of openings, however, will result 

 in a perceptible check in water loss. 



In estimating the regulatory role of the stomata, it must be 

 remembered that light, which so greatly influences transpiration, 

 promotes the opening of the stomata. They close in light only 

 with great difficulty. This influence of light on the movements 

 of stomata may seem incomprehensible as regards water econ- 

 omy. It is understandable, however, when it is remembered 

 that the stomata are the channels through which carbon dioxide 

 must enter the leaf; and that 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 ceU walls within the 



