DIA-HELIOTROPISM AND DIA-GEOTROPISM 6$$ 



application of light induced an energetic movement upwards— 



that is to say, a positive heliotropic response, the tip of the 



leaf moving through a distance of 35 mm in the course 



of sixty-five minutes — 



that is to say, at an 



average rate of '53 mm. 



per minute. On the 



cessation of light there 



was a slow recovery, 



which was found to be 



only partial. 



Negative type of 

 response : Artocar- 

 pus. — As an example 

 of negative response by 

 the internal diffusion 

 of stimulus, due to the 

 high transverse con- 

 ductivity of the tissue, 

 and the greater excita- 

 bility of the lower half 

 of the pulvinoid, I give 

 here two records, from 

 leaves of different ages, borne on the same plant, under the 

 action of strong sunlight from above (fig. 264). The upper of 

 these two records was taken from a young leaf, which was 

 second in order from the top of the shoot, and the lower from 

 the fourth. It is generally found that motile sensitiveness is at 

 its greatest in leaves which are neither too young nor too old. 

 In the present experiment the upper of the two leaves, 

 which was very young, gave a negative heliotropic response, 

 the tip moving through 15 mm. in the course of eighty 

 minutes, or at an average rate of almost '2 mm. per minute. 

 In the case of the lower leaf the rate of the responsive move- 

 ment was more rapid, being on an average *5 mm. per minute. 



As regards leaves, then, their responsive movements under 

 light have been shown to be very definite, and simply 



Fig. 263. Positive Heliotropic Movement of 

 Leaf of Mangifera indica under Sunlight 

 acting from Above 



Dotted line shows recovery on cessation of 

 light. 



