THE ANGIOSPKRMAE : LEAVES ,009 



the focused spot of light and the rest of the cell surface. When the leaf 

 surface is in the stable diaphototropic position, i.e., normal to the direction 

 of the sun's rays, this spot is central, but it shifts laterally with any movement 

 of the leaf or of the sun, thus disturbing the equilibrium and 'providing a 

 stimulus which is supposed to result in phototropic adjustment of the leaf. 

 This hypothesis is supported by observations that if phototropic leaves are 

 plunged in water or if their upper surfaces are painted with liquid parafHn, 



Fig. 997. — Diagram to show the refraction of light 

 rays by the curved outer wall of an epidermal 

 cell. A shows position of the light focus with 

 vertical lighting. B shows the lateral shift of 

 the focus with oblique lighting. {After 

 Haberlandt.) 



thus altering the conditions of refraction at the leaf surface, the phototropic 

 reaction is apparently abolished. It is possible, however, that this mav be 

 due to the disturbance of some other condition in the leaf, and the hypothesis, 

 though very interesting, cannot be regarded as proven. 



In a few cases, specially enlarged cells of the epidermis with papillate 

 outer walls, are seemingly the organs of this light perception. These have 

 been called ocelli, a name borrowed from Zoology. Particularly good examples 

 occur in Lithops [Mesemhryanthenmm) pseudotruncateUa, a small South African 

 succulent (Fig. 998), on the leaves of which the ocelli appear as minute dark 

 spots, due to the greater size and depth of the ocellar cells (Fig. 999). 



33 



