482 Wager . — The Perception of Light in Plants. 
flagellum itself, and thus brings about those modifications in its vibrations 
by which the direction of movement of the organism is regulated. 
At the red end of the spectrum, Engelmann has shown that the curve 
of assimilative activity corresponds very nearly with the curve of absorption ; 
but at the blue and violet end, a comparison of the two curves shows that 
the amount absorbed is much greater than can be accounted for by the 
assimilative activity, and the suggestion may be made that the excess of 
light absorbed at the blue end of the spectrum is partly functional in 
heliotropism. This would perhaps partly explain the heliotropic activity 
of young seedlings, which are of a yellower colour than fully developed 
leaves, for the yellow colouring matter readily absorbs the blue end of the 
spectrum. 
One important difference between the light-perceiving organs of 
animals and those of plants is the presence of pigment in the former. The 
exact function of pigment in the animal eye is still uncertain. Some 
physiologists think that it serves merely for the absorption of superfluous 
light rays, and, taking this view, Haberlandt suggests that there is no need 
to explain its absence in the plant cell, for light perception could very well 
take place without it. He points out, however, that the chlorophyll grains 
might act as a screen to keep extraneous light away from the sensitive 
percipient layer of cytoplasm, and that, in Selaginella Martensii and 
similar forms, there is a chlorophyll grain at the base of the epidermal cell 
which could be compared to a pigment layer, the cytoplasm between it and 
the cell-sap being the percipient organ. But it is doubtful whether the 
absorption of superfluous light is the sole function of the pigment in the 
animal eye. The alternative view is that the pigment is not merely concerned 
with superfluous rays, but that the rays which are active in vision are ab- 
sorbed by it and produce a chemical change in it, and that it is this change 
which affects the rods and cones and causes the stimulus of light perception. 
There is in the eyes of many vertebrates a substance called visual purple, 
associated with the percipient organs, which is possibly derived from the 
pigment layer and is very sensitive to light. How far the experiments 
made by Kiihne 1 and others upon the changes which take place in this 
pigment may be found to have a bearing upon the problem, we cannot 
say, but the destruction or bleaching of chlorophyll by light is perhaps 
somewhat analogous to the changes in colour and bleaching of the visual 
purple. The most active rays in the bleaching of visual purple are those 
of the yellow-green part of the spectrum, and these are the rays which are 
most readily absorbed by the colour itself. 
Cannot this be taken as the basis of a more satisfactory hypothesis for 
light perception in plants? It is difficult to believe that the sensitive 
1 On the Photochemistry of the Retina and on Visual Purple. Eng. trans., edited by Sir M. 
Foster, 1878. See also Text Book of Physiology by Foster and Rivers, Part IV, p. 1369, 1900. 
