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any one of these forces acts with increased or decreased intensity 

 upon the plant. The behavior of stomata to these factors is ex- 

 ceedingly various however. Thus some stomata open when the 

 leaf is placed in water, while others close ; some stomata open in 

 light, while others close under the effect of the sun's rays. Again, 

 weak electric shock gives rise to one result, while a strong shock 

 exercises the reverse action. 



Any study of stomata by which their action is observed by 

 means of a microscope will be vitiated with many errors, because 

 in taking the epidermis from a leaf and mounting it for examina- 

 tion, stimuli are set up, which may cause the stoma to open or 

 close before its original condition can be observed. 



Practically all of the water given off by a leaf in transpiration 

 passes through the stomata in the form of vapor, and the best 

 method of ascertaining whether the stomata are opened or closed 

 is to use some means of detection of watery vapor. This may be 

 done in two ways, viz., by the cobalt method, in which paper 

 saturated with cobalt nitrate placed on the leaf changes from a 

 bluish to a reddish color in the presence of watery vapor ; the 

 second method consists in the use of a hygrometer. Several 

 types of these instruments are in use in physiological laboratories. 

 In one the variations in length of a strand of human hair with the 

 changing humidity moves a lever carrying a pen which gives a 

 constant record of the proportion of watery vapor in the air. 

 This form has not been made suitable for testing the action of 

 leaves. Another hygrometer consists essentially of an awn of 

 some grass, like Stipa, which twists or untwists with the varia- 

 tions in humidity of the atmosphere. This type has been found 

 very useful in some forms of investigation. A third form con- 

 tains a thin strip of some material which curves and straightens 

 with the varying humidity, and the best example of this type is 

 the horn hygrometer of F. Darwin, in which the sensitive mate- 

 rial is made of a thin strip of pressed horn. The simpler forms of 

 hygrometer sold in the market for general use have a sensitive 

 strip composed of two layers of material of different hygroscopic- 

 ity, and the writer has devised one for testing the action of stomata 

 which is based upon this principle. It may be made as follows : 



