G22 



SENSORY SYSTEM 



furnished with a silicified lens situated at the centre of its outer wall ; 

 the point of special interest, in this case, is the fact that the lens 

 consists of an independent cell. While the inner wall of the lens-cell is 

 thin, its outer wall is bi-convex, and so thick that the cell-cavity 

 is almost entirely obliterated, appearing as a mere slit in transverse 

 section. The minute peg-shaped process attached to the middle of 

 the outer wall indicates that the lens represents a modified trichome. 



Fig. 25S. 



Local optical specialisation of outer epidermal walls. A. Adaxial foliar epidermal 

 cell of Campanula persicijblia. B. Adaxial foliar epidermal cell of Petraea volubilis. 

 C. Adaxial foliar epidermal cell of Vinca major. D. Adaxial foliar epidermal cell of 

 Lonicera fragraniissima. See text. 



The efficiency of these minute membrane-lenses is illustrated by the 

 adjoining micro-photograph (Fig. 259), which shows the differential 

 illumination of the inner epidermal walls in the leaf of Aquilcgia, as 

 seen in a lens-experiment. 



Given a particular form of epidermal lens-cell and a suitable degree 

 of refraction, it may happen that the focal length of the lens coincides 

 very nearly, or exactly, with the vertical height of the cell ; in this case, 

 more or less well-defined images of external objects will be projected 

 upon the sensitive ectoplast. The conditions referred to are realised, 

 for example, in the case of Anthurium Warocgueanum. One of the 

 accompanying illustrations (Fig. 2 GO) is a reproduction of a micro- 

 photograph taken in the course of a lens-experiment with this plant ; 



