SUMMARY. 49 



even nullified by increase of evaporation, thus producing day closure 

 and night opening, as in the alfalfa group grown under similar 

 conditions. 



7. The stomatal movement of each plant studied tends to follow 

 a regular course under optimum conditions, opening and closing 

 progressing smoothly and uniformly until complete. As conditions 

 become less favorable, however, the rate of movement becomes more 

 and more irregular, opening, for example, progressing rapidly one 

 hour, slowly or not at all the next, and rapidly again the third. 

 When evaporation becomes extreme, movement consists of alternate 

 opening and closing, the one following the other at hour or even 

 shorter intervals. If the general trend is toward opening, the degree 

 of each opening exceeds that of closing, but if toward closing, the 

 reverse is true. The amplitude of these changes is rarely great and 

 a difference of 40 per cent from the smoothed curve is unusual. 



8. The stomata of the upper and lower surfaces of the leaves in 

 most plants are different in their structure or in their relation to the 

 rest of the leaf. Hence the stomatal movement is dissimilar, even 

 though each surface be exposed to the same environment. However, 

 in some plants, such as the cereals studied and the sugar-beet, the 

 stomata are sufficiently alike in structure and in relation to the other 

 tissues to produce like behavior under identical conditions. In some 

 of the cereals with leaves blown about and alternately illuminated on 

 each surface, the stomatal behavior is similar under most conditions. 

 In other cereals and in sugar-beet, differences in illumination and in 

 exposure to other factors normally cause considerable divergence in 

 the behavior of the upper and lower stomata, although this is not 

 as great as in those plants with dissimilar stomata. 



9. The stomata on the stems, when such are present, usually differ 

 materially from those of the leaves in structure and relation to water- 

 supply, and as a consequence in their behavior as well. 



10. The marsh plants studied have permanently open stomata. 



