PIIOTONASTIC PHENOMENA AND DIURNAL SEEK 



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 ; 



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. 



(c) Responsive movements of leaflets 

 of Erythrina indico. 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 

 the rays of light, and continues so 

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



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 



. 253. Positive lldio- 

 Iropic 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. 



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. 



