PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 629 



FlG. 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. 



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 ; 



but as, under natural conditions, 



light always acts from above, the 



leaflets are found, when the acting 



daylight is sufficiently strong, to 



exhibit what is known as diurnal 



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. 



if) 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 Aparajita). 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 Fig. 254. Positive Helio- 

 , , r , . , , ! ,. tropic Response of Leaf- 



the rays of light, and continues so leti ! f of Er j thrina indica 

 from about 1 1 A.M. till about 3 P.M. The leaflets form a holIow 

 The negative heliotropic type cup with direction of 



incident sunlight as axis. 



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 



