SENSITIVITY AND MOVEMENT 245 



during which the stimulus must be appHed before any 

 visible movement ensues. This so-called " statolith 

 theory" is a very alluring one, and has attracted the 

 attention of many investigators since it was first put 

 forward, some supporting, some attacking the theory 

 itself, as well as the facts on which it is supposed to rest. 



Knight, you will remember, was also greatly exercised 

 over the perverse behaviour of vine leaves, that would 

 insist on attempting to defy his efforts to make them 

 grow at other angles with incident light than those to 

 which they were accustomed in nature. Here again the 

 discovery of protoplasm presented the " vehicle of 

 irritation," but later on the further question arose of 

 the existence or non-existence of specific sense organs 

 on such leaves as responded to the phototropic stimulus. 

 Once more Haberlandt came forward, in 1905, with a 

 theory to explain the mode of stimulation of the proto- 

 plasm. According to him the epidermal cells, of many 

 leaves at least, possess papillate elevations which he 

 thought acted as lenses, concentrating the incident rays 

 on the sensitive ectoplast. In the phototropic rest 

 position the hght is focussed on the centre of the ectoplast, 

 and in that position produces no response ; but when 

 the angle of the sensitive organ to the light ray is changed 

 the beam of light strikes the ectoplast at some other 

 spot, and induces an excitation, and finally an endeavour 

 on the part of the organ to regain the original orientation. 

 This also is a most interesting line of investigation and 

 worthy of your study, but it would be quite out of place 

 for me to discuss it here. 



When the terminal leaflet of a " Sensitive Plant " is 

 gently touched with a slightly heated wire the successive 

 pairs of leaflets fold together, and when all have so 

 folded the entire leaf droops. The movement may, 

 if the stimulus be sufiiciently intense, extend to other 

 leaves also. It follows, first, that the excitement set 

 up by the stimulus — the " excitation " — must have 



