Wager . — The Perception of Light in Plants. 469 
which possess only a slight or moderate curvature of the outer walls. It 
is, however, only a variation on Haberlandt’s hypothesis. 
But there are numerous other leaves mentioned by Haberlandt, and 
of which some instances are given by Albrecht, where the outer wall is flat 
or only very slightly curved, but the inner wall distinctly angular or arched, 
which assume a distinctly dia-heliotropic position under the influence of 
light, and are more difficult to explain. In these cases, Haberlandt assumes 
that a sufficient amount of light differentiation would be brought about by 
the difference in the area covered by the rays of light passing through the 
cell, and impinging on the lower wall. If the lateral walls were very steep, 
there might be a perceptible difference ; but in all the cases which I have 
examined they show absolutely no light differentiation at all under the 
microscope. Even when the light falls obliquely there is no differential 
illumination. The illumination becomes less bright equally all over the 
basal wall so far as can be observed under the microscope, and as the light 
becomes more and more oblique, and more of it is reflected from the 
surface, the basal wall becomes less and less bright, until finally the light 
is so oblique that none of its rays is able to penetrate the cell. 
The number of species in which epidermal cells of the kind just 
described are found is probably small. The following are a few examples 
of such leaves, which possess flat-walled or only very slightly curved epidermal 
cells : Hedera helix , Prunus lanro-cerasus , Bnxus sempervirens , Rhododen- 
dron, Berber is vulgaris, and Ilex aquifolium. In the majority of other plants 
which I have examined, the outer walls of the epidermal cells are curved 
to some extent and therefore capable of causing, in an oblique light, an 
unequal illumination of the basal wall. 
In Hedera helix , according to Haberlandt, 1 the upper epidermal cells 
of the leaf are even, but in consequence of the presence of a somewhat 
uneven cuticle are not quite flat. He states that the inner walls of the 
cells are more or less angular or arched, and that about 24 per cent, 
possess the structure suitable to a differential illumination, and only these 
cells are able to perceive precisely changes in the direction of light. The 
leaf is clearly dia-heliotropic, but although I have examined a large 
number of specimens I have not been able to observe any differential 
illumination capable of demonstration under a microscope. If it exists, 
therefore, it must be extremely small, and can only be inferred from the 
cell structure as Haberlandt has done. 
In Prunus lauro-cerasus , the most careful microscopic examination, 
both with and without a diaphragm, fails to reveal anything in the nature 
of a differential illumination. Fig. 10 shows this clearly. The basal walls 
were clearly focused ; the lateral walls are thick and the illumination is 
seen to be equal all over the cell. The same results were obtained in 
the other species mentioned. 
1 Lichtsinnesorgane, &c. } 1905. 
