DIA-lIELIOTKOl'ISM AND DIA-(;EOTROl'ISM 655 



SUMMAR\ 



The position ultimately assumed by a leaf is determined 

 by heliotropic action and other subsidiary factors. 



These subsidiary factors are: (i) the natural movement 

 of the organ, due to epinasty or hyponasty ; (2) the differ- 

 ential excitability to gravitational stimulus of the two halves 

 of the organ ; (3) the effect of suctional activity and of 

 turgescence ; and (4) the modification of effect by the 

 characteristic limits of flexibility of the organ. 



As regards the action of light, the lamina is not the 

 perceptive organ, and there is no specific dia-heliotropic 

 sensitiveness possessed by the leaves. 



The sensibility of the pulvinoid of a leaf is essentially 

 similar to that of a pulvinus. There are two main types of 

 response given by pulvinoids : (i) that exhibited when the 

 conductivity of the organ is feeble, and vertical light induces 

 movement upwards, ox positive response ; and (2) that exhibited 

 when conductivity is great, and the lower half is the more 

 excitable, inducing movement downwards, or negative response. 



The various attitudes assumed by the leaves are the joint 

 effects of these responsive actions, and their modification by 

 epinasty or hyponasty, the differential action of gravity, the 

 turgescent condition of the plant, and the limits of flexibility 

 of the pulvinus or pulvinoid. 



