488 Wager . — The Perception of Light in Plants. 
not the quality of the light, there seems to be no reason why the red and 
yellow rays should not be just as active as the blue and violet rays ; in the 
more refractive half of the spectrum the amount of light absorption is 
greatly in excess of that required for assimilation ; when a chlorophyll 
screen is interposed between the leaves and the light, the heliotropic response 
either ceases altogether or is much reduced ; in motile organisms, such 
as Euglena , the heliotactic response is bound up with the absorption 
of light by the pigment-spot ; in the large majority of animal eyes the 
presence of a layer of pigment in connexion with the actual percipient 
organs seems to be necessary ; light exerts a very definite stimulus upon the 
chlorophyll bodies of some Algae and foliage leaves, resulting in their 
movement into positions in which they can be more effectively illuminated ; 
why should not a similar stimulus bring about the orientation of the leaf 
itself, if the chlorophyll grains are incapable of movement ? the rays absorbed 
by the chlorophyll, which are functional in heliotropism, are the chemically 
active rays ; chemical changes taking place in the chlorophyll would afford 
a more satisfactory explanation of the origin of the stimulus than the 
pressure of light upon the cytoplasm. 
19. With the exception possibly of the few special cases in which 
the light is concentrated upon the chlorophyll grains, there is no satis- 
factory evidence to show that the lens-shaped cells or local cuticular 
thickenings can be regarded as special adaptations, either for light perception 
or for the more efficient illumination of the chlorophyll grains, although 
it is possible that they may be of use for both purposes. 
EXPLANATION OF PLATES XXXI AND XXXII 
Illustrating Mr. Wager’s paper on the Perception of Light in Plants. 
PLATE XXXI. 
The figures are reproduced from photographs taken by means of the Gordon photo-micrographic 
apparatus with the objectives of Leitz Nos, 3 and 4, and oculars 4 and 8. 
Fig. 1. Reproduction of a photograph taken through the upper epidermal cells of Tradescaniia 
Jluminensis. Some of the cells are in much better focus than others. 
Fig. 2. Special cells from the upper epidermis of Mcsernbryanthemum cordifolium. They 
contain a mucilaginous substance, and are equally well developed on the under side of the leaf. 
Fig. 3. The special cells on the under side of the leaf of M. cordifolium , showing distant houses 
photographed through them. These special cells vary in size, and, being irregular in outline, do 
not usually give a circular disk of light. 
Fig. 4. Lower epidermis of M. cordifolium , showing the special cells with highly refractive 
granular mucilaginous substance through which the light is shining. 
