and Laboratory Methods. 



2269 



plant over so that the stem and wires shall be in the same horizontal plane. 

 Make observations at intervals of 15 minutes to determine the time when the 

 stem begins to move out of its place. Record the temperature. 



6. After Effect. When the plant used in the preceding experiment has 

 acquired a curve of 10°-20° revolve it 180° from its former position and imme- 

 diately bend the wires to conform to the curve of the stem. Observe at intervals 

 of 10-15 minutes to observe how long the plant continues to bend in the original 

 direction. The temperature should be the same as in the foregoing experiment. 

 A more exact method of determining the after effect is by using the roots of 

 seedlings. When they have been exposed to the stimulus of gravitation, as in 

 Experiment 1, long enough to cause pronounced curves, they should be trans- 

 ferred to a klinostat and revolved in such a way that gravitation will be neutral- 



FiG. 13. — A damp chamber in use on a klinostat. The power is fur- 

 nished by a small electric motor. The speed is reduced by the 

 intervention of a worm-gear. 



ized. Observations may be made every 10 minutes to determine the length of 

 time the root continues to curve. 



7. Force Exerted by Growing Roots. A satisfactory means of demonstrating 

 that the downward growth of roots is an active, not merely a passive, response, 

 due to the gravity of the root, is shown in Fig. 12. A lupine or Vicia Faba 

 seedling is placed in a horizontal position in a large damp chamber ; immedi- 

 ately beneath the tip of the root is a small dish containing washed mercury to a 

 depth of 1 to 2 c. m. When the root curves downward it penetrates the mercury 

 but is not forced into it by the mere elongation of the root-tissue. 



8. Simulation of Gravitation. Kni'^hfs Experiment. Fig. 18 represents a 

 convenient means of performing this classic experiment by the use of an ordinary 

 damp chamber which can be tightly closed and attached to the klinostat. 



