483 
Wager . — The Perception of Light in Plants. 
chlorophyll grains, which in some plants show definite movements of orien- 
tation in response to light, have no other function in light perception than that 
of a screen. If the light had a merely mechanical action such as is suggested 
by Haberlandt, it is difficult to see why the most energetic rays, which are 
at their minimum in the blue, but rise rapidly to a maximum in the yellow- 
green, and extend through the whole of the red and far beyond into the 
infra red, should not be functional. The rays which are active are, in fact, 
the chemical rays, and it appears to me to be much easier to explain the 
heliotropic reaction upon the assumption that it is due to chemical activity 
than that it should be merely due to mechanical action. 
Perhaps the most important and attractive feature of Haberlandt’s 
hypothesis is that it affords, for the first time, a definite explanation of how 
a plant is able to perceive the right direction in which its leaf must be 
turned to bring it into the most advantageous position for the illumination 
of the chlorophyll grains. It is true that he has not brought forward any 
evidence to show whether the movement takes place at once in the right 
direction, or whether there is anything in the nature of a trial and error 
movement on the part of the leaf before the right direction is perceived. 
From the general trend of his observations we are, I think, led to conclude 
that the leaf at once perceives and turns in the right direction. 
It is an important question, therefore, whether the leaf would be able 
to orient itself as readily and as surely if the chlorophyll grains were the 
percipient organs. There seems to be no reason from the evidence available 
why it should not do so. The chlorophyll grains are probably much more 
sensitive to light than the protoplasm, and any differential illumination 
of them would be likely to set up a much more definite stimulus than 
a differential illumination of the cytoplasm lining the basal walls. 
To bring about such a stimulus, an unequal illumination of the chloro- 
phyll grains would be necessary in oblique light. There is no difficulty in 
seeing that this would be the case (Text-Fig. 3). The passage of rays 
of light obliquely through the epidermal cells into the palisade tissue would 
bring about a partial illumination of the chlorophyll grains. The rays 
of light would be bent on passing into the epidermal cells, and would then 
pass on in straight lines to and through the palisade cells. The chlorophyll 
grains on one side of each palisade cell would be more strongly illuminated 
to an extent depending upon the angle of the incident rays, than those on 
the other side, which would remain in obscurity, or at least would only be 
exposed to rays which had already passed through one, and possibly more 
than one, layer of chlorophyll grains, being thereby much weakened in the 
active heliotropic rays. This would set up the stimulus required to bring 
about the orientation of the leaf in a direction at right angles to the incident 
light, in which position the chlorophyll grains in each palisade cell would be 
equally illuminated. That this unequal illumination of the chlorophyll 
