487 
Wager . — The Perception of Light in Plants. 
cannot be definitely determined, but it is suggested that the curvature 
of the lens cells of the epidermis may be found to bear some relation 
to the thickness of the cell-wall and cuticle. It is possible that this 
turgidity may be the starting-point for an adaptation to (1) either light 
perception or, as Haberlandt suggests, to (2) the more efficient illumination 
of the chlorophyll grains, or (3) both, but the evidence is not very con- 
clusive. 
12. The papillate epidermal cells of petals exhibit a very pronounced 
convergence of light, with a clear differentiation on the basal wall of 
a central bright area, and a dark peripheral zone. 
13. It is only in a very few leaves, where the cells are highly papillate, 
or where there is a well-marked local thickening of the cuticle, that we 
get the differential illumination of the basal wall required by Haberlandt’s 
hypothesis. In some, it is not visible at all under any conditions, in others 
only when a small stop is used, and in a large number of leaves, probably 
the majority, there is no differential illumination as defined by Haberlandt, 
but only an unequal illumination of the basal wall when the light falls 
obliquely. 
14. The experiments which have been made upon the elimination 
of the lens function by submerging the leaves in water, or by covering them 
with a layer of paraffin oil, have given results which are so contradictory and 
unsatisfactory that a much more complete investigation is necessary before 
any definite conclusion can be based on them. 
15. In a few special cases, the lens cells appear to bring about a con- 
centration of the light on the chlorophyll grains. In some leaves the 
general arrangement of the lens cells with respect to the chlorophyll grains 
seems to indicate that they are effective in promoting a more efficient 
illumination of the chlorophyll grains. 
16. Haberlandt suggests that the stimulus may be brought about 
by the difference in pressure exerted by the light upon the cytoplasm ; but 
this is so very slight that it is hardly probable it can be effective. 
17. There seems to be no good reason why the epidermal cells should 
be the percipient cells more than the chlorophyll-containing cells, except 
that the presence of chlorophyll would interfere with the incidence of 
the light upon the percipient protoplasm. 
18. There is, however, some evidence that the perception of light 
is bound up with its absorption by the chlorophyll grains, in which case the 
palisade cells would be the percipient cells, and the chlorophyll grains with 
the cytoplasm in connexion with them the actual percipient organs. The 
evidence for this is as follows : — The heliotropic response depends mainly 
upon the quality of the light and not upon its intensity ; the rays which are 
active are those which are absorbed by the chlorophyll ; of these the more 
refrangible rays are the most important ; if it were merely the intensity and 
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