Stimulation and Correlation. 



125 



vaseline. The pollen tubes are generally directed away from 

 contact with air, being negatively chemotropic to oxygen. 



137. Movements in response to shock.— Secure 

 seeds of Mimosa from a florist, and germinate in a pot of 

 rich, loose soil in a warm room. As soon as the seedlings 

 have begun to develop foliage leaves, cover the pots with 

 a bell jar, and ventilate in such manner that the air is kept 

 in a humid condition. It will be still more convenient if 

 the seeds are planted in several small pots. Keep the plants 

 under observation, and note positions of the pinnules at vari- 

 ous times in the day. 

 All experiments 

 should be made with 

 the temperature at 

 20° to 25° C. Select a 

 plant with expanded 

 leaves, and jar it by 

 striking a quick, 

 sharp blow on the 

 pot in which it is 

 growing. Note re- 

 sult. How long is 

 the reaction position 

 maintained ? Touch the tip of an expanded leaflet lightly 

 with a pencil, and follow the resulting movements. Re- 

 peat this test in another form by snipping the terminal 

 pinnules with a pair of scissors. Allow an ample length of 

 time after each stimulation, and observe the extent of the 

 reactions. Does a reaction take place in any part of the 

 plant which has not been directly stimulated ? If so, make 

 careful note of the time necessary for the impulse or effect of 

 the stimulus to be transmitted from the point at which the 

 stimulus was given to the distant part of the plant in which 

 the reaction occurs. Test various forces as stimuli, such 



Fig. 98. — c, flower of Ofiuntta with stamens in 

 normal position ; d, with stamens after stimulation. 

 After Tourney. 



