74 Elementary Plant Physiology. 



Select some species, and make tests of the leaves in shade, in 

 strong sunlight, and at high and low temperatures. 



68. Hygrometer test for transpiration. Construct 

 a differential hygrometer as follows : secure a section of copper 

 or iron wire one to two millimeters in diameter and twenty-five 

 centimeters long. Thrust directly through the center of a 

 cylindrical cork i cm. in diameter and 2 cm. long. Bend a 

 section of the wire four centimeters long at right angles, and 

 bring the cork near the bend on the long arm. Bend the wire 



Fig. 52. Differential hygrometer for testing transpiration. A, strip of film 

 with gelatine on upper side ; B, cork for holding end of film ; Z>, scale. 



again at right angles, beyond the cork, and in the same plane as 

 the first bend. Secure a film plate, sold by dealers in photo- 

 graphic supplies, which consists of a thin sheet of celluloid, 

 which does not absorb water, coated with gelatine, which does 

 so very readily, and swells during the process. Cut a strip 8 

 cm. long and 5 mm. wide, and thrust one end into the cork. 

 Attach a bristle to the other end by means of glue. The strip 

 will be curved, and the long arm of the wire should now be bent 

 so that it will support a small paper scale near the tip of the 

 bristle. This should be arranged so that the convex surface of 

 the hygroscopic strip will lie within 2 mm. of the surface of a 



