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TEXTBOOK OF PLANT PHYSIOLOGY 



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benzol and alcohol other liquids, such as benzine, kerosene, etc., may 

 be used. The infiltration method is not exact, but it is simple and 

 well adapted for use under natural conditions. 



For determining the degree of opening of the stomata, Darwin 

 devised a special apparatus called by him the "porometer." It 

 consists of two parts: a bell jar about 1 cm. in diameter, and a 

 T-shaped tube (Fig. 73). The tiny bell jar is glued with its flange 

 to the leaf surface. This leaf chamber is connected by a rubber 



tube with one of the hori- 

 zontal arms of the T-tube. 

 The other arm ends in a rubber 

 tube which is controlled 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 

 W rate of falling indicates to some 



Fig. 73. — Darwin's porometer, showing extent the degree of Opening 

 arrangement oUhe parte ol the porometer Qf ^ gtomata When they 



are completely 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 march 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 opening is usually attained 

 in the morning hours. About noon the stomatal slit begins to 

 narrow slightly, and it closes usually a little before sunset. When 

 the sky is overcast, the openings are not so wide as in bright 

 weather. In very dry weather the slits open in the morning but 

 soon close again, as a result of loss of water by the leaf. Fre- 



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