460 
Wager . — The Perception of Light in Plants. 
Various observations which have been made by recent observers to 
test the truth of this explanation would seem to show that, in many respects, 
both the morphological and the physiological evidence upon which it is 
based is unsound ; and in any case it appears to me Haberlandt has not 
sufficiently considered (1) that the orientation of the leaf may be bound up 
with the absorption of light by the chlorophyll grains, and not with the action 
of light upon the protoplasmic layer of the epidermal cells, and (2) that the 
light rays which are brought to a focus in the tissues of the leaf may 
be functional, in part at any rate, in the more efficient illumination of the 
chlorophyll grains. 
The Optical Behaviour of the Epidermal Cells. 
The various modifications by which, according to Haberlandt, a differ- 
ential illumination of the basal wall is brought about in the epidermal cells 
of diaheliotropic leaves may be grouped as follows : — 
Type I. Outer wall flat ; inner or basal wall curved or angular. 
Type II. Outer wall curved ; inner wall flat. 
Type III. Combination of I and II. Both outer and inner walls 
curved. As Seefried 1 points out, Haberlandt does not 
give this as a separate type, but it is convenient so 
to regard it. 
Type IV. Papillate projections of a portion of the upper cell-wall, 
which act as lenses. These are included by Haberlandt 
under papillate epidermal cells. 
Type V. Lens-shaped thickenings of portions of the upper cell-wall, 
which act as lenses. 
Type VI. Local light-perceiving organs of various kinds, reduced 
or rudimentary hairs, special lens cells, oil drops, and 
tannin-containing receptacles. 
To these may perhaps be added the special cases described by 
Gaulhofer 2 as : — 
Type VII. Flat epidermal cells with very thick walls which bring 
about a differential illumination of the basal proto- 
plasmic lining layer through the passage of the rays 
of light from the highly refractive cell-wall into the less 
refractive cell-sap. 
1 Ueber die Lichtsinnesorgane der Laubblatter einheimischer Schattenpflanzen. Sitzungsber. 
d. k. Akad. d. Wiss. in Wien, cxvi, 1907. 
Die Perzeption der Lichtrichtung in den Laubblattern mit Hilfe der Randtupfel, Randspalten 
und der windschiefen Radialwande, Sitzungsber. d. k. Akad. d. Wiss. Wien, cxvii, pp. 153-90, 1908. 
