480 
Wager . — The Perception of Light in Plants. 
The evidence before us, therefore, seems to justify the conclusion that 
in special cases the lens-shaped epidermal cells may be functional in the 
more efficient illumination of the chlorophyll grains, either by concentrating 
the light upon them, or by a more efficient dispersal of it among them. It 
is possible that this may be also bound up with light perception in the 
manner suggested in the following section. 
Chlorophyll Grains as Organs for Light Perception. 
In formulating his hypothesis that it is in the cytoplasm lining the 
walls of the epidermal cells that the light perception takes place, Haber- 
landt based it fundamentally upon the statement of Noll (1878), that the 
outer layer of the cell protoplasm is the seat of sensitiveness for light and 
gravity. In order to explain this sensitiveness, Haberlandt suggests that it 
may be due to the mechanical pressure exerted by light. The stimulus set 
up by the differential illumination of the plasma layer on the basal wall of 
the epidermal cells would thus be brought about by the difference in pres- 
sure between the illuminated and the dark areas. But this must be 
extremely small. Clark Maxwell showed that, on a perfectly reflecting 
surface on which the pressure would be the greatest, sunlight should exert 
a pressure of about 08 milligram per square metre, and on a black surface 
about 0-4 milligram per square metre. As a writer in the Botanical 
Gazette remarks 1 : ‘ The pressure on a cell (from figures calculated by 
Maxwell and determined experimentally by Nicholls and Hull) o-oi mm. 
square in full sunlight would scarcely amount to 7 x to -11 milligram! To 
believe that a plant could discriminate between o and 0-000,000,000,07 mg. 
pressure makes a severe test of one’s credulity.’ It is well known that 
heliotropism depends upon the quality of the light rays, and not merely 
upon variation in light intensity. The less refrangible rays are important 
in photosynthesis, the more refrangible rays are more important in growth 
and formative processes, irritable movements, and curvatures. Haberlandt 
himself has shown, by the use of chlorophyll screens, that the only rays 
which are functional in heliotropism are those which are absorbed by the 
chlorophyll ; and, as we know from the researches of Sachs, it is only the 
more refractive half of the visible spectrum, the blue and violet rays, which 
produce the stimulus. If it were merely a difference in the illumination, we 
ought to obtain a result in red or yellow light. 2 This points to the con- 
clusion that the light perception is bound up with the absorption by the 
chlorophyll. Haberlandt, indeed, considers this possibility, but dismisses 
it for the reason that certain plant organs which do not contain chlorophyll are 
1 C.R.B. Bot. Gaz., xxxviii, 157, 1904. 
2 Cf. E. Pringsheim, junr., Ber. d. d. bot, Gesell., 556, 1908. 
