FORMATION OF IMAGES 



G23 



inset in the bright circle (the image of the diaphragm-opening) 

 on the inner wall of each epidermal cell, is a tiny image of a 

 microscope-stand, which was interposed at a suitable height between 

 the window that served as the source of illumination, and the plane 

 mirror from which the light was reflected on to the epidermis. In such 

 cases the possibility of a perception of external objects on the part of 

 the plant cannot be denied. But there is no reason to suppose that 

 any perception of this kind actually takes place. For leaves are concerned 



Fig. 259. 



Lens-experiment with the adaxial foliar epidermis of Aquilegia vulgaris. (Micro- 

 photograph, after Seefried.) 



not with the perception of external objects, but with exact self-adjust- 

 ment to their heliotropically fixed position ; and this result can be 

 attained, certainly with equal and perhaps with greater ease, if the 

 focus of the epidermal lenses falls behind the sensitive ectoplast. 



It is more particularly those leaves which are constantly exposed 

 to feeble illumination, that stand in need of an upper epidermis with a 

 well-developed power of light-perception ; for it is leaves of this type 

 that require to orientate themselves as accurately as possible in relation 

 to the brightest available light. Hence, it might have been expected 

 that, in one and the same species, shade-leaves would be more perfectly 

 adapted in this respect than sun-leaves. As a matter of fact, Gaulhofer 

 finds that this expectation is realised in a great many cases. 323 Thus, if 



