TROPISMS 



185 



The many cases of the lightward movement of plant- 

 growth of which every one must have noticed some, as of 

 plants grown in a window may next be understood in the 

 main ; since the light acts upon the stem and leaf-stalks 



FIG. 22. Effects of Direct and Indirect Stimulus. 



(a) Stimulus applied Directly at the growing region inducing retardation 



of growth or contraction as represented by dotted line. Stimulated 

 area represented in this and in following by shade. 



(b) Stimulus applied Indirectly (at some distance from growing region) 



gives rise to acceleration of growth and expansion. 



(c) Stimulus applied to right side of organ causes contraction of that 



side and expansion of the opposite side, thus giving rise to positive 

 curvature towards stimulus. 



(d) Excitation transmitted to the opposite side causes neutralisation. 



(e) Excitation caused by intense stimulation is transmitted across and 



thus reverses the normal curvature to negative, i.e. away from 

 stimulus. 



just like the touch of the support upon the tendril. For 

 in this case again the directly stimulated side is contracted 

 and the opposite side is expanded, so bending the shoot 

 light wards. 



When the light is very strong and long continued the 

 over-excited plant-organs may begin to turn away : how is 

 this effected ? Bose's experiments show that the strong 

 excitation percolates into and traverses the stems and 



