200 MOVEMENTS AND EXCHANGES OF FLUIDS 



Preparations of this kind may be attached to a plant in several 

 places, and it may be placed under different conditions of mois- 

 ture, temperature, light, or the amount of moisture in the soil 

 may be varied. 



273. Use of a Differential Hygrometer. Construct a differen- 

 tial hygrometer as follows : Secure a piece of copper or iron 

 wire 12 mm. in diameter and 25 cm. long. Thrust directly 

 through the center of a cylindrical cork I cm. in diameter and 

 2 cm. long. Bend a section 4 cm. long at right angles and bring 

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



FIG. 100. Differential hygrometer. A, strip of film with layer of gelatine on up- 

 per side. B, cylindrical cork into which one end of the film is thrust. Z>, scale, 

 over which the indicator has moved two divisions, showing open stomata, and trans- 

 piration, in the leaf on which the instrument rests. 



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

 first. Secure a film plate sold by dealers in photographic sup- 

 plies, which consists of a thin sheet of celluloid coated with 

 gelatine. Cut a strip 8 cm. long and 5 mm. wide, which will be 

 curved owing to the contractility of the gelatine. Attach a stiff 

 bristle or fiber to one end of the strip by means of glue, and 

 thrust the other into the cork as in Fig. I oo. Now bend the 

 free end of the long arm of the wire so that it will support a 

 curved scale made of paper. The gelatine of the strip is very 

 delicately sensitive to all changes in atmospheric moisture. 



Adjust the strip by turning the cork on its axis until the film 



