384 



LIFE MOVEMENTS IN PLANTS 



the record of response of leaflet of Averrhoa, given in Fig. 

 143. Light of moderate intensity from an incandescent 

 electric lamp acted from above : the result was a feeble and 

 short-lived positive response, quickly reversed to strong 

 negative by transmission of excitation to the more excit- 

 able lower side. Illumination from below gave rise only to 

 strong positive response. Thus in AverrJioa the effect of 

 continuous light applied above or below is a downward 

 movement ; in Mimosa the movement is upwards. The 

 explanation of this difference lies in the facl, that in 

 Mimosa leaflet it is the upper half of the pulvinule that 

 is more excitable ; while in AverrJioa and in Biophytum 

 the lower is the more excitable half of the organ. 



As a summary of the tropic action of light I shall give 

 diagrammatic representations of various types of phototropic 



ifs^^ 



»-—* 



(a) 



(d) 



% (-¥■') (- 



-) 



c«. © e o 



Fiu. 144. — Diagrammatic representation of different types of phototropic 

 response. (See text.) 



response, including the photonastic (Fig. 144). The direc- 

 tion of the arrow indicates the direction of incident light. 

 Dotted specimens are those which possess transverse conductivi- 

 ty. Thick lines represent the more excitable side of an aniso- 

 tropic or dorsiventral organ. The size of the circles, with 



