172 THE BIOLOGY OF FLOWERING PLANTS 



(191 6) found the stomata of Cactuses more widely open at 

 night, a condition like that of the potato in arid conditions. 



(3) Most thin-leaved plants examined, such as alfalfa, 

 clover, tomato, dandelion, showed, under favourable con- 

 ditions, a gradual opening in the morning, complete from 

 about II a.m. to about 3 p.m., and then a gradual closure 

 complete from 9 p.m. to 5 a.m. This we might regard as 

 the normal behaviour of the mesophyte. With lower water 

 supply, a temporary mid-day closure appears, which, with 

 increasingly arid conditions, extends both ways until the 

 stomata remain closed all day. With this there is associated 

 an increasingly prolonged opening at night. The assimila- 

 tion relations under such conditions would again be an 

 interesting study. In a number of trees examined, cherry, 

 peach, pear and poplar, no mid-day closure was ever found. 

 Loftfield inclines to relate this to the potential water balance 

 in the trunk. 



Loftfield gives many graphs showing the relation of the 

 march of transpiration to these movements. In some cases 

 a close relation is evident, in others discrepancies occur. 

 Loftfield 's conclusion is that stomatal regulation is 

 important ; his own words may be quoted : " Although 

 factors concerned in evaporation have great influence upon 

 transpiration, this influence is definitely controlled by the 

 stomata. When the stomata are wide open, or nearly wide 

 open, transpiration is the result of the action of the factors 

 of evaporation alone, since the stomata in no wise interfere 

 with the action. As the stomata close the influence of the 

 factors is lessened, but until closure has reduced the apertures 

 to 50 per cent, or less, stomatal regulation is still largely 

 overshadowed by the control exerted by these. When 

 closure is almost complete, the regulation of water loss by 

 the stomata is very close, and the eflFect of the factors 

 overshadowed by the eff"ect of even very small changes of 

 the opening." 



This seems a fair statement of the case so far as our 

 present knowledge goes. It means, however, that with the 

 stoma over 50 per cent, open a dangerous depletion of water 



