474 Wager . — The Perception of Light in Plants. 
got a distinct turning towards the fixed light position. This, he explains, 
is due to the fact that the inner walls of the cells are curved, and that there 
was still, therefore, a sufficient light differentiation to produce the given 
result. 
Kniep 1 eliminated the lens function by covering the leaf with a thin 
layer of paraffin oil under a thin sheet of mica or a thin piece of paper. The 
refractive index of the oil (1-476) is greater than that of water, and tends 
to produce a divergence of the light rays instead of a convergence, and in 
some cases he obtained, instead of a bright central spot of light, a dark 
central area and a lighter peripheral zone. Notwithstanding this, the leaves 
responded to the light stimulus just as ordinary leaves. 
I find that this reversion of the differential illumination only takes place 
when the lens function is very pronounced. It is very easily seen in Saxi- 
fraga Geum, which produces under normal conditions a very marked central 
spot of light on the basal wall. But it is not visible in Tradescantia flumi- 
nensis , where the epidermal cells, although very efficient as lenses, show no 
differential illumination on the basal wall. 
In discussing the results obtained by Kneip, Haberlandt 2 points our 
that, theoretically, the divergence produced by the concave oil layer 
over the curved wall of the epidermal cell ought still to result in 
a slightly brighter central region on the basal wall, but that actually what 
is seen under the microscope is the reversion described by Kniep. The 
brighter peripheral zone appears to be due to reflection of the outer rays of 
light by the lateral walls and to the passage of rays through the walls from 
neighbouring cells. By experiments on leaves of Begonia semper jlorens he 
confirms Kniep’s observations. When the leaves are covered with a layer 
of paraffin oil, they respond to the light just as ordinary leaves ; when they 
are covered with water, however, they cease to respond. It is evident, 
therefore, according to Haberlandt, that the power to respond, which is 
maintained in the oil-covered leaf, must still be connected with the differen- 
tial illumination of the basal wall, although under these conditions it is 
completely reversed. Instead of, as he formerly suggested } 3 the central 
region of the basal protoplasmic layer being attuned or sensitive to light of 
higher intensity, he now concludes that this sensitiveness cannot be con- 
sidered as an unchangeable inherent property of the different parts of the 
plasma layer, but only as an acquired adaptation phenomenon (analogous to 
the local adaptation of the human retina) capable of modification, when 
placed under conditions which bring about an inverse illumination. 
It is, however, not clear how the plasma layer is able to adapt itself 
to such extremes, which are unlikely to occur in nature. Moreover, the 
difference in illumination is nothing like so pronounced with the paraffin oil, 
1 Ueber die Lichtperzeption der Laubblatter. Biol. Centralbl., xxvii, 97, 1907. 
2 Bedeutung, &c., p. 289. 3 Liehtsinnesorgane, & c. 
