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 

 cyhndrical 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 ; D^ 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 



