DIA-HELIOTROPISM AND DIA-GEOTROPISM 647 



above, the leaflet will move continuously upwards till the 

 lamina has reached the neutral position— that is to say, till the 

 midrib constitutes a straight prolongation of the petiole ; but 

 if light act on the leaflet from below, it will bend downwards 

 (fig. 259, a). If the petiole, however, at the beginning of the 

 observation be horizontal instead of vertical, and the lamina 

 be in the neutral position, then vertical light cannot, owing 

 to the anatomical peculiarities of the pulvinar joint, carry the 

 leaflet further above the dorsal line (fig. 259, b). Or we may 



Fig. 259. Different Limits of Flexibility 



Vertical light on terminal leaflet of Desmodium causes (a) up movement 

 till the dorsal line is a continuous straight line ; light applied below 

 causes movement downwards below this neutral line through 180 . 

 In b is shown neutral position, after reaching which there is no further 

 movement upwards. In c is shown movement of terminal leaflet of 

 Erythrina indica upwards, under vertical illumination, through 180 

 above the neutral line. 



again take as an example the terminal leaflet of Erythrina 

 indica (fig. 259, c). The limit of flexibility is in this case 

 represented by an angle of almost 180 above the neutral, 

 whereas downwards its limit is about 90 . The formula is 

 i8o c 



thus 



90 



Now, when this leaflet is acted upon by light 



from above, it may become almost doubled upon the petiole 

 upwards, just as we found the terminal leaflet of Desmodium 

 to be almost doubled downwards. Hence we see that 

 though the heliotropic effect of light is always the same, yet 



