620 



SENSORY SYSTEM 



state the eye is comparable to a symmetrically illuminated epidermal 

 cell. When the image shifts to the right- or left-hand side of the 

 retina, the eye revolves, until the image once more coincides with the 

 yellow spot. 



The structure of the condensing portion of light-perceiving epi- 

 dermal cells varies considerably in different cases. The simplest arrange- 

 ment is that in which the whole outer wall protrudes, forming part of a 

 more or less spherical surface. The degree of condensation naturally 

 depends upon the radius of curvature. As a rule the curvature is not 

 very great (Fig. 256 a); in this case, since the vertical height of 



Fig. 2515. 



Foliar (adaxial) epidermal cells with bulging or papillose outer walls. A. Oxalis 

 acetosella. B. Trifolium incarnatum (cotyledon). C. Riullia Daveauana. D. An- 

 thurium Leuconeurum. 



the epidermal cells is small, the focus of the lens lies far below 

 the inner walls, somewhere within the mesophyll. In the so-called 

 " velvety " leaves, which are on the whole characteristic of plants 

 inhabiting tropical rain -forests (Melastomaceae, Ficus barhata, Cissus 

 discolor, Begonia Rex and other species of Begonia, Philodendron 

 Lindeni, spp. of Anthurium, etc.), the outer wall of each epidermal 

 cell assumes the shape of a more or less steeply inclined conical 

 papilla, with a rounded apex representing the actual condensing 

 lens (Fig. 256 c, d). Here the focus generally falls within the 

 epidermal cell itself, at a varying distance above its inner wall ; in 

 this instance, therefore, the bright central area seen in the lens- 

 experiment represents the cross-section of a divergent pencil of rays. 

 If the surface of* such an epidermis is covered with a thin film of 



