618 SENSORY SYSTEM 



being taken to keep the bulging outer walls dry. The cover-slip is 

 then inverted over a moist chamber placed on the stage of a microscope. 

 The preparation is illuminated by diffuse light, reflected from the plane 

 mirror of the microscope, so as to meet the epidermis approximately 

 at right angles. On focussing upon the inner epidermal walls, one at 

 once sees, in each cell, a bright central area surrounded by a dark 

 marginal zone (Fig. 254). 



This well-marked differential illumination may also be demonstrated 

 in a somewhat different way, by placing the severed piece of epidermis, 

 with its outer surface uppermost, on a piece of sensitised paper moistened 

 with water, and by then exposing the preparation to vertical illumina- 

 tion. If the resulting photograph is examined, after fixation, with the 

 aid of a powerful lens, or as an opaque object under the microscope, 

 the contrast between the completely blackened central area and the 

 lighter marginal zone, in the image corresponding to each cell, may 

 often be seen with astonishing distinctness. 32 ' 2 



When the incident light falls obliquely, instead of vertically, upon 

 the leaf-surface, the distribution of light intensities is no longer sym- 

 metrical with reference to the median radial plane of each cell. The 

 protruding outer walls are now more brightly illuminated on the side 

 facing the source of light, than they are at their apices or on the 

 opposite side. Consequently, the bright area on the inner wall shifts 

 from the centre towards the side furthest removed from the source 

 of illumination (Figs. 253 c f f, and 255). This change can be readily 

 demonstrated in connection with the above-described lens-experiment, 

 by tilting the mirror to one side (after removal of the condenser, if 

 one is being used) so as to produce oblique illumination. 



After what has been stated above, there can be no doubt that 

 a leaf-blade with a papillose epidermis is furnished with an excellent 

 optical apparatus, which enables it to orientate itself with reference 

 to the incident illumination. In the author's opinion, the action of 

 this apparatus is as follows : To begin with, it is necessary to assume 

 that the ectoplast lining the inner epidermal wall is sensitive to 

 photic stimuli. This sensitiveness involves a twofold power of dis- 

 crimination ; the ectoplast distinguishes not only between brightness 

 and darkness, but also between symmetrical and asymmetrical illumina- 

 tion of the cell (or, in other words, between concentric and excentric 

 distribution of light intensities on the inner wall). Heliotropic 

 equilibrium prevails, when the cells are vertically and hence sym- 

 metrically illuminated ; oblique and, therefore, asymmetrical 

 illumination, on the other hand, induces a heliotropic response of 

 the petiole, which continues until illumination is once more symmetrical, 

 or, in other words, until the lamina has regained its heliotropically fixed 



