616 SENSORY SYSTEM 



when the blade is darkened, provided that the pulvinus is exposed to 

 light. In any case, it may safely be asserted that the lamina is the 

 principal, or even the only heliotropically sensitive part of the leaf in 

 many, and perhaps in the majority of plants with diaheliotropic leaves, 

 while the petiole chiefly represents a motor-organ. Here again, there- 

 fore, the perceptive and the reactive regions are separated in space. 



The question next arises, as to whether the power of perceiving 

 photic stimuli is common to all the tissues of leaf-blades, or whether it is 

 confined to a particular tissue, or to special cells or groups of cells ; a 

 priori the latter alternative seems the more probable. The blades of 

 euphotometric leaves respond to differences in the intensity of illumina- 

 tion produced by comparatively small deviations from the stable position. 

 But in all the tissues internal to the epidermis, including even the palisade- 

 cells, the inevitable reflexion, refraction and absorption collectively cause 

 a great amount of dispersion and a serious diminution in the intensity of 

 the incident illumination ; these internal tissues thus seem very badly 

 fitted for the task of perceiving the original direction of light which is 

 generally not very intense in the first instance. It seems, therefore, 

 highly probable that the power of perceiving photic stimuli is vested in 

 the upper epidermis. As a matter of fact, the structure of the upper 

 epidermis of euphotometric leaves is characterised by features which are 

 correlated in some cases without any doubt, in others with a high 

 degree of probability with the light-perceiving function of that layer. 

 It is convenient in this connection to recognise several distinct types 

 of light-perceiving epidermis (sensory epithelium), and to discuss each 

 of them separately. 



1. Papillose epidermal cells. 



The most widely distributed, and at the same time the most perfect 

 type of light-perceiving sensory epithelium, is represented by those forms 

 of adaxial foliar epidermis, in which the outer walls are more or less 

 papillose, while the inner walls are approximately flat and placed 

 parallel to the leaf-surface. When a pencil of rays falls upon such a 

 papillose epidermal cell, perpendicularly to the leaf-surface, the outer 

 and inner walls, and the ectoplast attached to them, are illuminated in a 

 particular manner, in accordance with well-known optical laws. The 

 rays meet the middle of the protuberant portion of the outer wall at 

 right angles, or nearly so, whereas, towards the margin of this wall, the 

 angle of incidence becomes more and more acute ; the underlying ecto- 

 plast is therefore most brilliantly illuminated over a limited central 

 area, while its marginal region remains dark in comparison. This 

 differential illumination becomes even more pronounced on the inner 

 walls. For every papillose epidermal cell acts as a plano-convex or 



