18 



Secure a straight piece of iron or copper wire 2 mm. in diameter 

 and 25 cm. long, and bend a section 8 cm. long at right angles. 

 Thrust this short arm through the axis of a cylindrical cork 1 5 

 mm. long and 8 mm. in diameter and bend the terminal 5 cm. at 

 right angles and parallel with the long arm. Cut a strip from a 

 developed film plate, such as are supplied by photographers, 8 cm. 

 long and 5 mm. wide. Cut a slit in the cork parallel to the axis and 

 thrust one end of the film in the slit. Now fasten a bristle 5 or 6 

 cm. long to the free end of the strip of film, which should have its 

 convex surface uppermost. Bend the free end of the long arm 

 of the wire upwards and at right angles, affixing a cork to the 

 tip to which a suitable scale may be attached with glue (Fig. 1, D). 

 Turn the cork on its axis until the strip would lie within 2 mm. 

 of any surface on which the apparatus might be placed ; note the 

 position of the pointer, and place on the under surface of 

 leaf which has been laid on a table upside down. If the stomata 



Fig. I. Differential hygrometer. A, strip of film. B, cork. C, portion of 

 wire bent upward to hold scale. D, scale over which the indicator has moved two 

 divisions showing open stomata in the leaf on which the instrument rests. (Illus- 

 tration from "A Practical Text-Book of Plant Physiology," by MacDougal, now in 

 press. By permission of Longmans, Green & Co. ) 



are open the gelatine of the film will absorb watery vapor in- 

 stantly and the strip will begin to straighten so that a movement 

 of the indicator may be noticed inside of ten seconds. Set the 

 hygrometer aside for a few minutes and place on the opposite 

 side of the leaf. If it is free from open stomata no movement will 

 be seen. 



