648 PLANT RESPONSE 



the position of the leaflet in space — that is to say, its relation 

 to the vertical line — is largely modified not only by the limits 

 which the anatomical structure of the laminal pulvinus or 

 pulvinoid imposes upon its flexibility, but also by the angle 

 which the petiole makes with the stem ; for there is, 

 generally speaking, a second pulvinus or pulvinoid at the 

 junction of the petiole and the stem, and this petiolar 

 pulvinus is, in its turn, more or less sensitive to stimulation 

 (p. 59). We have thus obtained some idea of the anatomical 

 elements, regarding the petiole and its joints, which enter 

 into the complex question of the final position assumed by 

 the leaf in space. 



Discrimination of the part played by heliotropism in 

 the movement of the leaf. — In studying the heliotropic 

 effect we are concerned only with the action of light itself, 

 and not with the resultant effect, due to various co-operating 

 factors. When the action of each of these is definitely 

 understood, it becomes a simple problem to understand the 

 effects due to their combination. The difficulty of this 

 investigation has hitherto lain, as already said, in the fact 

 that, owing to the generally sluggish nature of respon- 

 sive movements in ordinary leaves, a long time must be 

 allowed to elapse before they become measurable, and during 

 this long period other factors may become operative in 

 unknown ways. The effect of light, however, can easily be 

 discriminated by the use of the Optic Lever for record, for this 

 allows us any degree of magnification which may be desired. 

 Thus the natural curve gives us the resultant effect of all the 

 pre-existing factors under normal tonic conditions, and its 

 subsequent variations under the incidence of light at once 

 exhibits the distinctive action of that stimulus. On the 

 withdrawal of light, again, the recovery from the induced 

 variation affords an additional corroboration of the inference 

 that the variation itself had been due to the action of light. 

 In consequence, moreover, of the delicacy of this means of 

 continuous record, the characteristic effect can be detected 

 in the course of a few minutes, thus eliminating the unknown 



