THE PAPILLOSE EPIDERMIS 



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condensing lens. The rays falling upon the convex outer wall in the 

 direction parallel to the optical axis of the lens, are refracted so as to 

 converge upon the middle of the inner wall ; hence, this central region 

 is brightly lighted, whereas a marginal zone of varying width is not 

 directly illuminated at all, but merely receives a small amount of 

 reflected light from the mesophyll. The accompanying diagram 



Diagram illustrating the lens-action of a papillose epidermal cell, abdc. In 

 vertical illumination there will be a bright central area, ef, on the inner tangential 

 wall, cd, surrounded by a relatively dark marginal zone, ce, and fd. If the light 

 falls obliquely from the left as indicated by the broken lines the bright area will 

 shift from the centre towards the right, to e'f. 



(Fig. 253) represents the paths of the rays in an epidermal cell of this 

 type ; for the sake of simplicity the outer wall is represented as part of 

 a spherical surface, and the mathematical construction is based upon 

 the refractive index of water (T33). 



The differential illumination of the inner walls of papillose 

 epidermal cells may also be directly observed by means of the following 

 very simple physical experiment, which has been termed the " lens- 

 experiment" by the author. A piece of the epidermis is carefully 

 removed with the aid of a razor, and at once mounted (with the 

 cut surface downwards) upon a slightly moistened cover-slip, care 



