1 88 



THE STOMATAL MECHANISM 



of their opening than in the time of their closing. The aggregate diffusive 

 capacity of all of the stomates on a plant must therefore be thought of in 

 terms of the two factors of the degree of opening of the individual stomates 

 and the number of stomates that are open. 



It is probable that the stomatal mechanism of every species of plant ex- 

 hibits certain distinctive features in the way in which it reacts to various 

 combinations of environmental factors; hence only broad generalizations can 

 be formulated regarding the daily periodicity of stomatal behavior. The 

 results of a number of investigators indicate that as a rule, under the condi- 



FiG. 48. Daily variation in certain environmental factors on a "standard day." 



tions prevailing on a "standard day," the stomates of most mesic species 

 of plants are open all or most of the daylight period and closed at night, 

 their maximum diffusive capacity being attained about mid-day, or little be- 

 fore. The stomates open in the morning under the influence of the light 

 factor and soon attain nearly their maximum diffusive capacity. Under 

 "standard day" conditions, however, the water content and turgor of the leaf 

 cells usually decrease progressively during most of the daylight period. Be- 

 cause of this internal water deficit which develops in the leaf, stomatal closure 

 usually begins about noon or a little before. Virtually complete closure of the 

 stomates often takes place considerably before the advent of darkness because 

 of the predominant effect of the water factor over the light factor during 

 the afternoon hours. 



Innumerable other types of daily cycles of stomatal behavior are possible, 

 a few of which will be described briefly. The diffusive capacity of the stomates 

 sometimes rises to a mid- or late morning maximum, decreases markedly dur- 

 ing the mid-day hours, rises to a secondary maximum during the afternoon. 



