GROWTH 



175 



glass plate ( c ) by means of a small amount of plaster of 

 Paris. The upper needle is fastened within the spring by 

 means of shellac, while the lower one is adjustable by means 

 of the screw (i). The flower-pot (n) containing the root 

 set as in the figure in plaster of Paris is firmly pressed into 

 the iron ring (777) fastened by two screws to the upright 

 ( s ) . The small plaster block ( b ) is now fitted to the root- 



no. 11. 



Figure 11. Pfeffer's apparatus for measuring the force exerted by grow- 

 ing roots or stems. (Reduced from the original figure.) 



tip and fastened by plaster of Paris to the glass plate (c) 

 so that the root-tip is over the middle of the spring. The 

 screws ( h ) are now turned up so that the two plaster blocks 

 (a and b) are pressed lightly together. The saw-dust in 

 the pot should be kept moist. The plaster blocks may be 

 constantly moistened by wrapping them with filter-paper 

 wet through a strip of paper connecting with a reservoir. 

 The horizontal rod (o) pressed down on the pot (n) and 



