DISCUSSION OF RESULTS 47 



water-content and low evaporation, the stomata are continuously 

 and often widely open all day and night. As the evaporating power 

 of the air increases beyond a certain point, the stomata tend to close. 

 Based upon the time of such closure, three subgroups may be dis- 

 tinguished. Potato stomata at first close just after sunset for a 

 time, but as the water-content decreases this time extends backward 

 into the afternoon. Then, as conditions become extreme, the stomata 

 close a short time after sunrise, accompanied by visible wilting of the 

 entire plant, which persists throughout the day After sunset the 

 plant recovers, the stomata open slowly about midnight, and at 

 sunrise are fully open. A second variation is found in cow-beet, 

 where, under favorable conditions, the stomata are wide open during 

 the day and close very little during the night. At sunrise this 

 closure is arrested and shortly afterward the stomata again become 

 wide open. As conditions grow less favorable, night closure becomes 

 more rapid and is completed before sunrise. Progressively as 

 evaporation increases and water-content decreases, such closure is 

 begun at an earlier time, becoming complete at midnight and then at 

 sunset. Finally, the stomata open widely only an hour after sunrise 

 and close gradually and completely during the forenoon or even before 

 morning is over. The third variation of this form of movement 

 occurs in onion, where the stomata are wide open at night under 

 conditions of high water-content and low evaporation. If water- 

 content alone becomes low, the stomata close at night, but if the 

 evaporation increases instead, the stomata tend to close during the 

 day. Hence, with medium or low water-content, onion stomata react 

 to increased evaporation like the plants in the alfalfa group, showing 

 increasing mid-day closure correlated with increasing night opening. 



The plants having stomatal movement like that of the cow-beet 

 and potato are cabbage (Brassica oleracea), tulip (Tulipa gesneriana, 

 Red Prince), Portulaca oleracea, and probably Encelia farinosa. This 

 latter plant has not been found with open stomata at night in any 

 series, since the latter were not made under the necessary conditions. 

 The general behavior is essentially like that found in cow-beet, and it is 

 probable that under favorable conditions this plant would show open 

 stomata at night. Verbena ciliata is also included here, as plants 

 which were heavily watered showed the stomata 20 per cent open all 

 night. Fouquiera splendens exhibits a rather bewildering behavior,, 

 the stomata of the primary leaves of heavily watered plants showing 

 movement like that of cow-beet, while the secondary leaves showed 

 mid-day closure and correlated night opening, such as found in alfalfa. 



The plants with behavior similar to that of onion are salsify 

 (Tragopogon porrifolius) , Hubbard squash (Cucurbita maxima), 

 crook-neck squash (C. moschata), pumpkin (C. pepo), plantain 

 (Plantago major), lily (Lilium speciosum), and leek (Allium porrum). 



