232 



THE TEACHING BOTANIST 



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course, with its smaller surface, it works far more slowly than the 

 diffusion shell. To the other burette a plant is to be attached 

 as shown in Fig. 23. Select an actively growing plant with 

 a stem about the size of the inner diameter of the burette. 

 Cut it off an inch from the earth, and attach it to the burette 

 by a rubber tube fitting over both it and the plant, and tie it 

 tightly to the plant with a rubber band so there can be no 

 leak. When plant and osmometer are placed side by side, 



as in Fig. 23, the demonstration is very in- 

 structive. A film of oil may be placed on 

 the liquid in both tubes to prevent loss of 

 any of it by evaporation. Of course, very 

 careful records of the rise should be made 

 by the students. Exact measurement has 

 great pedagogic value in itself, and a habit 

 of preferring precise quantitative results to 

 loose generalizations should always be culti- 

 vated. 



Experiment No. 10. - - A simple and effec- 

 tive pressure gauge may be made as follows 

 (Fig. 24) : Take a glass tube with a glass 

 stop-cock at 'one end, and graduate it in 

 millimetres and centimetres with India ink 

 applied with a stretched thread. Select a 

 vigorous plant with a stem about the diam- 

 eter of the inside of the tube, and cut it 

 off an inch from the ground. On the top 

 FIG. 24. Gauge for of the stump fit a short piece of rubber 

 measuring root pres- tubing thick enough to make stump and 



outside of tube the same diameter, and 

 slip another piece of tubing over this and the tube so as 

 to make a water-tight joint. This joint must next be made 



