342 LIFE MOVEMENTS IN PLANTS 



intensity was raised to four units. Thus increasing 

 intensity of light induces an increasing retardation of 

 growth at the proximal side of the organ. This aided by 

 the effect of indirect stimulus at the distal side brings 

 about an increasing positive curvature. 



Kxperiment 1-W. — The flower bud of Crinwii. was used 

 for the experiment, the source of light being a small arc 

 lamp. The duration of exposure was one minute. In- 

 creasing intensity of light gave rise to increasing positive 

 curvatures (Fig. 125) in the ratio of 1 : 2*5 : 5 under 

 increasing intensities which varied as 1 : 2 : 3. 



THE EFFECT OF INCREASING ANGLE. 



The quantity of light which falls on an unit area of 

 the responding organ varies as sin 6 where is the direc- 

 tive angle i. e. the angle made by the rays with the 

 surface. Some allowance has to be made for the amount 

 of light reflected from the surface, this being greater at 45° 

 than at 90°. 



Tropic response of pulvhius of Deamodium gyrans : 

 Experiment ISl. — For application of light at various angles 

 an incandescent electric lamp was mounted at one end 

 of a brass tube, a collimating lens being placed at the 



^ c i^ 



Fig. 126.— The Collimator. 



other (Fig. 126). The parallel beam of light from the 

 collimator could be sent at various angles by rotating the 

 collimator tube round an axis at right angles to the tube. 



