The Perception of Light in Plants. 
BY 
HAROLD WAGER, F.R.S. 
With Plates XXXI and XXXII, and three Figures in the Text. 
T HE extremely interesting observations by Haberlandt upon the sense 
organs for light in foliage leaves 1 have stimulated anew the interest 
in this question which has been taken ever since the discovery of Charles and 
Francis Darwin, that in grass seedlings ‘ sensitiveness to light is sometimes 
confined to a small part of the plant ; and that this part, when stimulated 
by light, transmits an influence to distant parts, exciting them to bend.’ 
(The Power of Movement in Plants, 420, 1880.) 
It is now well known that the stimulus which causes the foliage leaves 
of many plants to place themselves in such a position as to receive the 
fullest advantage from the incident rays of light, is perceived mainly by 
the le^f-blade, and that this stimulus is transmitted to the petiole, by 
which the turning of the leaf is brought about. 
The question therefore arises : by what means does the leaf-blade, 
or the plant for that matter, perceive that it is or is not in the most 
advantageous position for the incident rays? Haberlandt has attempted 
to explain how this is brought about. He has shown that in many leaves 
the upper epidermal cells are shaped like convex or planoconvex lenses, 
and being filled with a clear sap are able to bring about a convergence 
of the light rays ; in others, special cells or local thickenings of the cuticle 
act in the same way. The epidermal cells have a thin layer of protoplasm 
on their basal walls. When the leaf is at right angles to the light, the central 
portion of this layer in each cell is illuminated, the peripheral zone re- 
maining dark. Under the microscope this appears as a bright central 
disk of light on a dark ground (Fig. 7). In oblique illumination, the 
bright spot of light moves to one side ; and this alteration in the position 
of the light spot, according to Haberlandt, sets up the stimulus which 
results in the orientation of the leaf into a more favourable position. The 
evidence for this conclusion is based upon the optical behaviour of the 
epidermal cells and upon experiments by which the lens function is elimi- 
nated. 
1 Lichtsinnesorgane der Laubblatter, 1905. 
[Annals of Botany, Vol. XXIII. No. XCI. July, 1909.] 
