DIA-HKLIOTROPISM AND DIA-GKOTROPISM 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, /;;. Or we may 



=:i-:::?^r ^ -' ~:: ^ 



a 



Flc. 259. Difl'erent Limits of Flexibility 



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

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

 causes movement downwards below this neutral line through iSo°. 

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

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

 Erythrina iitdica 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 



1 80° 

 thus . Now, when this leaflet is acted upon bv light 



90° 



from above, it may become almost doubled upon the petiole 



upwards, just as we found the terminal leaflet of Desmodiuiii 



to be almost doubled downwards. Hence we see that 



though the heliotropic eflect of light is always the same, \et 



