G28 SENSORY SYSTEM 



modified hairs. In certain species of Peperomiei, which have strikingly 

 euphotometric leaves, the principal light-condensing organs are the 

 large vesicular basal cells of special short trichomes. 



4. Experimental observations on the light-perceiving function of the 



upper epidermis of leccves. 325 



The author himself has subjected his theory of the light-perceiving 

 function of papillose (upper) epidermal cells to the test of experiment 

 in various ways. All the methods employed by him are the same in 

 principle. First, the sensory cells are artificially put out of action, so 

 far as their light-perceiving function is concerned ; then, the petiole 

 having been darkened, steps are taken to determine whether the leaf 

 retains its power of perceiving the direction of the incident light and 

 of taking up its heliotropically fixed position. 



Assuming that the refractive index of the cell-sap is roughly equal 

 to that of water, it should be possible to inhibit the light-perceiving 

 activity by wetting the upper surface of the leaf; for this purpose the 

 leaf may be completely immersed in water, or its upper side may be 

 moistened and covered with a sheet of mica, so as to produce a flat 

 liquid surface. It is clear that a total inhibition of the lens-action can 

 only be effected by such means, where the refractive index of the cell-sap 

 is not higher than that of water, where there are no highly refractive 

 cutinised, silicified or wax-impregnated regions of the outer epidermal 

 walls to act as condensing lenses, and where the inner walls do not 

 bulge towards the mesophyll. Even if all these conditions are 

 fulfilled, reflection of the incident light often produces differential 

 illumination of the inner walls, when the epidermis is wetted, although 

 the contrast between bright and dark regions is not so striking as it is 

 under normal conditions. In these circumstances, a sufficiently sensitive 

 leaf may adjust itself to changes of illumination, even when the normal 

 lens-action of its sensory cells is inhibited. Less sensitive leaves, 

 however, lose their power of perception, and consequently become 

 incapable of executing heliotropic movements, when the upper 

 epidermis is wetted. In order, therefore, that the results of experiments 

 with wetted leaves may be correctly interpreted, it is, above all, 

 necessary that the degree of sensitiveness of the leaves employed, 

 towards differences of light-intensity, should be accurately known. 

 The author has accordingly carried out experiments with the aid 

 of incandescent gas-light, reflected from two mirrors of equal 

 size placed on opposite sides of the plant or organ which it is 

 desired to test ; by this method the following values were obtained 

 for the minimum difference of light-intensity perceived by certain 

 plants : 



