22 RELATIONS OF PLANTS TO MECHANICAL FORCES 



If no machine is at hand, imitate its action by a succession of 

 blows with a small wooden rod or pencil. Determine the length 

 of time before the effect of the shock on transpiration can be seen, 

 and the actual amount of such acceleration. 



This experiment may also be carried out by weighing the potted 

 plant during an hour of quiet and also during a second hour after 

 the shock has been given, although the actual amount of differ- 

 ence in the water thrown off may not be easily appreciable by this 

 method. 



30. Contact as Stimulus. Another form of mechanical stim- 

 ulus which differs from shock in degree rather than kind is that 

 of contact. Two distinct phases of the influence of contact upon 

 the plant are observable. One form which may be designated as 

 thigmotropism is exhibited by some roots and the special organs 

 of climbing plants. The sensory elements in such plants may 

 perceive a stimulus made by the weight of a body of a weight of 

 not more than a fraction of a milligram lying in contact with 

 them. It is possible to regard contact stimulation as an instance 

 of summation of an immense number of shock effects, from projec- 

 tions so minute that their separate impact would not be perceptible. 

 Again it is to be said that the perception of contact as a stimulus 

 is developed only in certain specialized forms, in which this form 

 of irritability is of special use in relation to environmental factors. 



31. Reactions to Contact. Secure specimens of Sicyos, Mi- 

 crampelis, Cucurbita, or Passiflora growing at a temperature of 

 25 to 30 C, and select nearly mature tendrils. Make a careful 

 drawing of the profile of the organ and then touch the surface 

 which is slightly concave with a rod of wood or iron. With 

 watch in hand note the number of seconds or minutes elapsing 

 before a curvature ensues. This would range from a second or 

 two in Passiflora to two hours in Vitis. Follow the course of the 

 reaction, making drawings of the profile of the tendril every fifteen 

 minutes. Note the length of time during which the contraction 

 endures, the resting period, and the length of time necessary for 

 the resumption of the original unstimulated position. 



