PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 629 



transverse conductivity is feeble. I give here (fig. 253) a 

 response-record obtained with a leaflet of Robinia under the 

 action of sunlight from above. A similar response, but in a 

 downward direction, is obtained when 

 light is made to act from below ; 



conditions, 

 above, the 

 the acting 

 strong, to 

 as diurnal 



Fig. 253. Positive Helio- 

 tropic Response of Leaf- 

 let of Robinia to Sunlight 

 Acting from Above 



Dotted line represents re- 

 covery on cessation of 

 stimulus. Time - marks 

 represent intervals of five 

 minutes. 



but as, under natural 



light always acts from 



leaflets are found, when 



daylight is sufficiently 



exhibit what is known 



sleep, or the para-heliotropic effect, 



in an upward direction. I have 



obtained numerous similar records 



with other leaflets, which fold upwards 



under the action of sunlight. 



(c) Responsive movements of leaflets 

 of Erythrina indica and of Clitoria 

 ternatea. — For the sake of simplicity 

 I described the movement of Robinia 

 leaflet as upward ; but the actual direction is one which 

 more or less accurately coincides with that of incident 

 sunlight. As further examples of this particular type of 

 diurnal sleep movement, I may mention the leaflets of 

 Erythrina indica and Clitoria ternatea 

 (Indian name Aparajitd). Both of 

 these are so remarkably sensitive that 

 they follow the course of the sun, in 

 such a way that the axis of the cup 

 formed by the folding leaflets at the 

 end of the petiole is coincident with 

 the rays of light, and continues so 

 from about 1 1 A.M. till about 3 P.M. 



The negative heliotropic type 

 of response. — We shall now pass 

 on to the second, or negative, heliotropic type of response 

 due to the internal diffusion of stimulus; and in order 

 to show that there is a continuity between these and the 



Fig. 254. Positive Helio- 

 tropic Response of Leaf- 

 lets of Erythrina indica 



The leaflets form a hollow 

 cup with direction of 

 incident sunlight as axis. 



