METHOD FOE PLANTS HAVING NO DEFINITE WILTING POINT. 49 



from the lower part of the ring. Four thumbscrews working upon 

 rubber packing serve to clamp the glass pot securely into position. 

 A threaded brass rod extends downward from the lowest point of 

 the ring and carries a pointer at its lower end. This rod also carries 

 one or more cylindrical weights, which are screwed up or down on 

 the rod until the desired sensibility is obtained. A second coun- 

 terpoise carried on a threaded rod parallel to the plant serves to 

 counterbalance the excess weight of the soil over the plant and pro- 

 vides a convenient means of balancing the system. The exact 

 position of this counterpoise can be recorded from day to day, if 

 desired, by a simple scale measurement from the ring to the edge 

 of the counterpoise. A sheet-metal cylinder closed at one end 

 surrounds the glass jar and is held in position by friction on the 

 outside ring. This cylinder is either nickel plated or covered with 



Fig. 3.— Simple apparatus for determining the wilting coefficient for plants with water-storage tissues. 

 The system is suspended from a piece of knitting needle which is cemented to the walls of the pot. 



aluminum paint and serves to equalize the temperature of the pot. 

 The surface of the soil is covered with wax composed of a mixture 

 of beeswax and tallow or of paraffin and petrolatum. 



When small plants are to be used the apparatus can be very 

 simply set up with the aid of a short length of rod and some suitable 

 cement. An apparatus of this kind is shown in figure 3. The 

 cactus is potted in an ordinary thin-walled drinking glass, the plant 

 being placed at one side of the center and the surface of the soil 

 covered with a layer of wax. As a substitute for knife-edges a 

 piece of a knitting needle is cemented across the top of the glass with 

 De Khotinsky's hard cement, 1 the latter being reenforced with loops of 

 wire cemented to the walls of the pot. A brass rod or heavy wire 

 about 3 millimeters in diameter and 60 or 70 centimeters long is then 



1 This cement softens somewhat in direct sunlight, and the pot should not be left suspended under heat 

 conditions. 



8477°— Bui. 230—12 4 



