296 PLANT RESPONSE 



only one way of evading the difficulty of finding out their 

 actual cause. When we see these responsive indications 

 given by a moving leaf or leaflet, we can but feel it 

 necessary to trace them to the impulses, internal or externa], 

 by which they must have been occasioned. How are such 

 periodic impulses caused, and where is their seat ? The 

 investigations which follow are intended to throw light on 

 this, one of the most obscure problems in Physiology. In 

 previous chapters I have demonstrated the fact that the 

 antecedent cause of a periodic effect need not itself be 

 periodic. A stimulus may remain long latent in a tissue, 

 and this latent stimulus may subsequently give rise to 

 periodic excitations. 



Such periodic expression of the absorbed energy is not 

 without analogy in the world of physics. For example, we 

 take a glass tube of moderate diameter and push into it, to a 

 certain distance from the free end, a piece of wire gauze, 

 which is then heated over a Bunsen burner. On removal of 

 the heating flame, we observe the after-effect of this absorbed 

 thermal energy, in pulsating movements of the air-column, 

 which give rise to a musical note, whose pitch is determined 

 by their periodicity. We have here a physical analogue to 

 a case previously described, in which the thermal energy 

 absorbed by a tissue of Biophytum was afterwards manifested 

 in long-continued periodic movements of the leaflets. In the 

 experiment with the glass tube, the periodicity of aerial 

 pulsation is determined by the size, shape, and temperature of 

 the pulsating column. Similarly, the periodicity of multiple 

 response in the Biophytum leaflet is determined by the various 

 constants of the cell-complex which is the seat of the movement. 



We saw in Biophytum that the region to which strong 

 external stimulus was applied, became the place in which it 

 was accumulated, and the source of subsequent excitation. 

 We further saw that there is no rigid line of demarcation 

 between plants which exhibit multiple responses and those 

 which show autonomous movements, the same plant passing 

 from one category to the other, according to circumstances. 



