30 



THE DAILY MARCH OF STOMATAL MOVEMENT. 



The tendency of potato stomata to remain constantly open may 

 be ascribed to two causes, namely, to a large amount of leaf- water 1 

 in the thick and rather fleshy leaves and to efficient roots and higlu 

 root-pressure. To show the difference in root-pressure, two mercury 

 manometers were attached to potted plants of alfalfa and potato 

 and the plants were heavily watered. The potato reached its max- 

 imum in 5 hours, while alfalfa took 12 hours, and the maximum 

 for potato was three times that of alfalfa. When tried with plants 

 growing in the open, the same experiment failed, as both plants 

 showed negative pressure, probably due to inability to raise the 

 water-content sufficiently during the experiment. While too much 

 reliance can not be placed upon this experiment, it is believed that- 

 it is fairly indicative of the relative efficiency of the roots of the two 

 plants. Although conditions during the two series described were 

 very unlike in most respects, the stomata of potato behaved in 

 essentially the same manner. This was due to the high water-content 

 of the soil during both experiments. When the water-content drops 

 below the amount necessary under given conditions of humidity 

 and temperature, potato stomata, as well as alfalfa stomata, react 

 to conserve water. When the water-content is but little below the 

 amount required, closure of the lower stomata occurs in the late 

 afternoon, followed by partial or total closure of the upper stomata. 

 As soon as the leaves regain turgor, the stomata again open and stay 

 open the remainder of the night. In all cases studied the upper 

 stomata remained open longest and were the first to reopen when the 

 lost leaf-water was replaced. 



NOON I 2 3 4 5 6 7 8 9 10 II MT. 



23*66789 10 II NOON I 



Fio. 13. Series 20, weather data for August 25-26, 1916; sunlight (A), 

 temperature (B), humidity (C). 



This is shown by series 20 (fig. 14), which was begun at noon August 

 25, 1916, and ended at 1 p. m. the following day. On both days the 

 light was strong, although a certain amount of haze was present, the 

 temperatures high, and humidity low. The soil had dried to a 

 water-content of 15 per cent, which was not quite sufficient, in view 

 of the high evaporation. At noon, when the experiment started. 



