624 



SENSORY SYSTEM 



epidermal papillae are present in both types of leaf {Cercis siliquastrum, 

 Primus Pachis, Fagus sylvatica, etc.), their light-condensing action is 

 more powerful in the case of the shade-leaves. In Cydonia japonica, 

 the outer walls of the adaxial epidermal cells are parallel-sided in the 

 sun-leaves, but distinctly piano- or bi-convex in the shade-leaves. 



Fie 260. 



Lens-experiment with the adaxial foliar epidermis of Anthurium Warocqueanum. 

 Note the image of a microscope in each bright area. See text. 



2. Light -perception hy epidermal cells with fiat outer walls. 



The papillose epidermis is not the only type of sensory epithelium 

 that occurs in euphotometric leaves. There is another type, which is 

 neither so common nor so effective characterised by the fact that the 

 outer walls are flat or nearly so, or at any rate display no tendency to 



develop in a papillose manner. In this 

 case the ectoplast adhering to the outer 

 wall will always be uniformly illumi- 

 nated, whether the light falls perpen- 

 dicularly or obliquely upon the leaf; 

 the intensity of this uniform illumina- 

 tion will, of course, be greatest in 

 vertically incident light. As a rule, 

 the inner walls of such epidermal cells 

 are not flat, but bulge more or less into the underlying mesophyll. Each 

 inner wall may appear as a continuously curved outline (Fig. 261) in 



Flo. 261. 



Portion of the adaxial foliar epidermis of 

 Franciscea macrantha, in vertical section. 



