48 THE DAILY MARCH OF STOMATAL MOVEMENT. 



Such plants as Scirpus validus, Equisetum hiemale, and E. palustre 

 showed the stomata continuously wide open, and this seems to be 

 their normal state. Equisetum was found with wide-open stomata 

 during wilting and even after the death of the stems from water-loss. 



SUMMARY. 



1. The daily march of stomatal movement varies more or less 

 from day to day. It is as unusual to find movement identical on two 

 successive days as it is for the weather to be the same. This varia- 

 tion is correlated with changes in weather and water-content, and 

 does not occur when these are sufficiently alike. 



2. In nearly all plants, stomatal opening is correlated with the 

 presence of light when conditions are favorable. When they become 

 unfavorable, the influence of light is modified by the action of other 

 factors, and finally nullified. In a few plants, light seems to have 

 little or no part in producing stomatal opening. 



3. Plants fall into three groups, according to their stomatal be- 

 havior, each of which has several subdivisions. 



4. The first group includes the cereals in which night opening 

 does not occur under ordinary conditions, favorable or unfavorable. 

 Day opening is dependent in duration and degree upon favorable 

 conditions of evaporation, temperature, and water-content. 



5. The second group includes most thin-leaved mesophytes, such 

 as alfalfa. Under favorable conditions their stomata are open all 

 day and closed all night. Alfalfa stomata open in 2 to 6 hours after 

 daylight, remain open from 3 to 6 hours, and then gradually close 

 during a period about twice as long as required for opening. When 

 conditions become less favorable, the stomata close partially or 

 completely for a time during the middle of the day, the period in- 

 creasing to include the whole day under very unfavorable conditions. 

 Night opening appears when mid-day closure occurs and increases in 

 degree and extent with it. Finally, when conditions become extreme, 

 the stomata are closed all day and open all night, the degree of 

 opening being dependent upon the water-content. 



6. The third group of plants, which includes the potato, tends to 

 have stomata open to a greater or less degree throughout the day 

 and night under optimum conditions, especially of water-content. 

 If evaporation increases to a critical degree, the stomata close for a 

 time during the day, when this is greatest. If water-content de- 

 creases moderately, the stomata not only become more responsive 

 to evaporation, but in most plants to light changes as well. Hence 

 the stomata open with the appearance of daylight and close at night, 

 unless evaporation has become serious. If the water-content de- 

 creases to a critical degree, the effect of sunlight can be modified or 



