308 



PLANT PHYSIOLOGY 



T-tube. The other arm ends in a rubber tube, which is con- 

 trolled by a clamp, while the vertical part of the tube dips into a 

 vessel of water. When air is sucked through the rubber tube 

 and the clamp then closed, the air within the tube and bell jar 

 becomes rarefied, and the water is raised in the vertical arm. 

 If the stomata are open, the air enters through them into the 

 bell jar, and the water column in the tube falls. The rate of 

 falling indicates to some extent the degree of opening of the 



stomata. When they are com- 

 pletely closed, the water column 

 remains stationary for many 

 hours at the height to which it 

 was raised. 



By using any of the described 

 methods, the daily changes of 

 the stomatal movements may 

 be observed. It has been noted 

 that in bright but not too dry 

 or hot weather, there are fairly 

 regular diurnal changes. In 

 the majority of plants, the 

 stomata open at dawn, or even 

 earlier. The maximum open- 

 ing is usually attained in the 

 W morning hours. About noon, 



Fig. 99. — Rate of fall of water T^" in , i , j. i tj. u • ^ 



tube T indicates rates of passage of air ^^6 stomatal sllt begms tO nar- 



into bell glass G through leaf B (after rOW slightly, and it closCS USU- 



"^^ ally a little before sunset. 



When the sky is overcast, the openings are not so wide 

 as in bright light. In very dry weather, the slits open in the 

 morning but soon close again as a result of loss of water by the 

 leaf. Frequently about noon, or between ten and eleven o'clock 

 in the morning, the stomata may have shut completely. Under 

 conditions of maximum heat and dryness, they remain closed 

 the whole day long and open only for a short time during the 

 early morning hours. 



The behavior of stomata varies with different plants. In 

 some, like the potato, cabbage, beet, etc., they are usually open, 

 and under optimum conditions of water supply they tend to 

 remain so, throughout the day and night. In other plants, 



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