II. RELATIONS OF PLANTS TO MECHANICAL FORCES 



15. Mechanical Shock. Mechanical shock in its various forms 

 is a kind of stimulation to which protoplasm has been subject con- 

 tinuously since its existence began, and it has developed the power 

 of a number of adaptive morphological responses of which the 

 alterations in stems and other structures as a reaction to strains 

 and stresses may be taken as an example. Of the directive and 

 metabolic responses to this class of stimulation but few have a 

 definite economic purpose. The contractile movements of plas- 

 modial forms, the movements of certain organs in carnivorous 

 species, of pollinating mechanisms, and of tendrils are of this 

 number. On the other hand, a large number of plants exhibit 

 marked reactions to shock in the form of movements, metabolic 

 variations and exchanges with the surrounding medium which the 

 most thorough investigation has failed to invest with a purpose. 

 New relations of the plant may be discovered however, which will 

 interpret these reactions. Among the responses of the plant to 

 shock of unknown purpose are the movements of " sensitive " 

 plants, the increase in transpiration and the behavior of stomata. 



Reactions to mechanical stimuli offer well-marked demonstra- 

 tions of the relations between the amount of the stimulus and 

 response, since the energy of the stimulus may be easily measured 

 and the amplitude of the response estimated. The sensory and 

 motor mechanisms involved are also usually highly differentiated, 

 making them most profitable objects for the introduction to the 

 study of irrito-motility. 



16. Contractile Reactions to Shock. Collect fruiting forms of 

 some myxomycete such as Trickia, Arcyria, Stemonitis or Didy- 

 mium and sow the spores on a piece of the decaying wood or other 



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