468 Wager . — The Perception of Light in Plants . 
plants, but there is no evidence that it plays any part in orienting the flower 
with respect to the light. 
Dia-heliotropic Leaves which do not possess Contrivances for 
Centric Differential Illumination. 
Albrecht (loc. cit.), on the ground of numerous observations on the micro- 
scopic structure of the epidermis, comes to the conclusion that in many 
cases, in the absence of contrivances for light perception, the leaves are still 
found to assume the fixed light position. Haberlandt, 1 however, considers 
that all the cases cited by Albrecht which he examined have contrivances for 
differential illumination of the basal wall, and are thus capable of light 
perception. 
The problem to be determined, however, is not merely the presence of 
curved outer or inner walls to the epidermal cells, but to what extent these 
are capable under normal conditions of producing a definite and visible 
differential central illumination. In the case of markedly papillate cells 
where the rays of light are actually brought to a focus on or very near the 
basal walls, the central brighter area of illumination is quite clear and 
distinct, but in a very large number of plants the focus of the rays is so 
far below the basal wall that although theoretically a differentiation is 
possible, in practice, even with the most careful illumination, no such 
differentiation is observed. Thus in Saxifraga hirsuta or 5 . Geum , the 
papilla on the upper surface of the cell produces a very distinct spot of 
light, whether the diaphragm is open or no, and a corresponding irre- 
gular illumination of the field when the light is oblique. Figs. 7, 8, 9, 
show this. In Tradescantia fluminensis , on the other hand, no differentia- 
tion of the lower wall can be observed when the light is perpendicular 
(Figs. 13, 14). When oblique, one side of the wall is illuminated, the other 
dark (Fig. 16). This would of course be sufficient to explain the orientation, 
but in a slightly different manner from that given by Haberlandt. When 
the leaf is at right angles to the incident light, the whole of the basal 
protoplasm is equally bright ; there is no differentiation into a light central 
spot surrounded by a darker zone, and, consequently, there is no necessity 
to regard different portions of the cytoplasmic layer as sensitive to light 
of varying intensities. The whole of the basal plasma layer may be 
regarded as attuned to light of higher intensity and no part of it to dark- 
ness. When one portion of it is illuminated whilst the other remains 
dark, as in the case cited (see Fig. 16), this variation may act as the 
stimulus by which the orientation is brought about. It occurs much more 
commonly than the centric illumination required in Haberlandt’s hypo- 
thesis, and will probably be found very widely distributed among plants 
1 Ueber die Verbreitung der Lichtsinnesorgane der Laubblatter. Sitzungsber. d. k. Akad. d. 
Wiss. Wien, mathem.-naturw. Klasse, cxvii, 1908. 
