STOMATAL REGULATION 



171 



between 9 and 10 a.m., a closure round noon, a second open- 

 ing at 2 p.m. followed by a second closure, and a third 

 slight opening in the evening. The stomata finally close at 

 9 p.m. The more considerable opening of the maize 

 stomata may be related to the fact that it is better suited to 

 dry cHmates than the other cereals. 



(2) In the potato growing in conditions of good water 

 supply and low evaporation, the stomata close only for the 

 three hours before midnight, and are open for more than 

 twenty hours. As the water supply fails they close earlier, 

 and may be closed from 4 p.m. till i a.m. If along with 



6 



A.M 



12 



NOON 



6 



Fig. 18. — Daily stomatal movement in potato ; 100 = full open ; 

 A, water supply good, atmosphere humid ; B, water supply good, atmo- 

 sphere dry ; C, water supply low, atmosphere dry. (After Loftfield, 

 modified.) - 



poor water supply there occurs high evaporation, closure 

 takes place as early as 9 a.m. and persists throughout the 

 day, the opening movement starting at 10 p.m. This 

 behaviour, which is illustrated by Fig. 18, is characteristic 

 of plants with rather fleshy leaves like the tulip, onion, and 

 cabbage, and of some with thin leaves. It may be noted 

 that F. Darwin (1898), on the whole confirmed earlier work 

 of Leitgeb's (1886) and Stahl's (1897), which showed that 

 the leaves of marsh plants and of plants showing sleep 

 movements are characterised by possessing stomata which 

 remain open day and night, a condition which is approached 

 by that of the potato in a moist environment. E. B. Shreve 



