CHAPTER XIV 



TROPISMS 



WE have now to refer to the various tropic movements 

 of plants in response to the multifarious stimuli of their 

 environment ; the stimulus may be (i) of touch, in conse- 

 quence of which tendrils twine round their support ; (2) of 

 the action of light, under which the plant-organs move 

 sometimes towards, and at other times away from, light ; 

 (3) of the action of gravity, which causes opposite move- 

 ments in the shoot and the root, the shoot moving upwards 

 and the root downwards. There are also numerous other 

 complicated movements associated with the recurrence of 

 day and night. The intricacies and apparent contradictions 

 of the responsive movements are so baffling that no con- 

 sistent explanation appeared possible. This led to the 

 supposition that a particular movement was due to some 

 unknown specific sensitiveness ; organs possessed of positive 

 sensitiveness moved towards the stimulus, while others 

 characterised by negative sensitiveness moved away from it. 

 Such use of merely descriptive phrases is, however, no 

 real explanation of the phenomena. The idea of specific 

 sensibility is, moreover, quite untenable when we find cases 

 where, under continued stimulation, an organ moves at first 

 towards the stimulus and afterwards away from it. An 

 identical organ cannot evidently be possessed of both the 

 positive and the negative sensibility. 



Bose pursued for many years the quest of discovering 

 some fundamental reaction which was at the basis of 



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