DAILY MOVEMENT IN POTATO. 27 



when conditions approach those under which this Opuntia grows. 

 The latter has adapted itself to such conditions, and hence such move- 

 ment is typical of its stomata. Alfalfa, under favorable conditions, 

 has a stomatal curve typical of the ordinary mesophyte, but it can 

 adjust itself to changing conditions until its stomatal curve is like 

 that of an extreme xerophyte. 



This naturally raised a question as to the factors concerned. The 

 accepted view is that light is the all-important factor in stomatal 

 movement. Where the plant is not subjected to extreme conditions 

 this is largely true, as many investigators have shown. However, 

 night opening and day closure of the stomata can not possibly be 

 attributed to light, but must be due to some factor or group of factors 

 which counteract the effect of sunlight. Moreover, night opening 

 and day closure must be related, since in every series an increase in 

 one was accompanied by an increase in the other. The most prom- 

 ising explanation seems to be that both result from the same factor 

 changes, and a study of the data for the group of series pointed to 

 two factors, in which changes were parallel to those in the daily 

 movement of the stomata of alfalfa. These two factors, evapora- 

 tion and water-content, act upon the plant singly or together to 

 produce a water shortage or "incipient wilting," which brings about 

 closure of the stomata during the day. Changes of no other factor 

 could be correlated with day closure of the stomata, but in all series 

 increased evaporation or decreased water-content, or the combina- 

 tion of the two, increased day closure and night opening. 



THE DAILY MOVEMENT IN POTATO. 



A set of upper and lower epiderm from the leaves of potato was 

 included in series 10, June 8 and 9, 1916, in order to compare the 

 stomatal movement of this crop-plant with that of alfalfa. The 

 potato plants were still very young and it was difficult toward the 

 end of the series to find leaves sufficiently mature to strip. The 

 soil conditions were not the same for the two plots, as the potato 

 plot had a water-content of 29 per cent, which was very high for 

 the type of soil, while the alfalfa plot had but 16 per cent at 2 feet. 

 However, the roots of the alfalfa plants undoubtedly reached to the 

 moister soil just above the water-table, which was at that time only 

 6 feet below the soil surface. The weather conditions* were the same 

 for both plots (fig. 7). 



The stomatal behavior in potato was entirely different from that 

 found in alfalfa. At the start of the series the stomata of both 

 surfaces of the leaves were wide open and remained open throughout 

 the day. After sundown the lower stomata were closed for 3 hours, 

 from 9 p. m. until after 11 p. m. At midnight they were half open- 

 and reached the maximum again at 1 a. m The upper closed to 50 



