74 
Francis Darwin 
he makes a highly ingenious suggestion as to how plants may be 
conceived to perceive the direction of incident light. His evidence 
is taken from the light-reactions of leaves, and to this part of the 
subject I shall confine myself. 
Leaves (as I shall show in greater detail in my next lecture) 
have the power of placing themselves at right angles to incident 
light. This is effected by appropriate torsions and curvatures which 
are grouped together as dia-heliotropic or transverse lieliotropic 
movements. In many cases,—possibly in the majority,—the act of 
perception is performed by the leaf-blade while the movement is 
confined to the stalk. In such plants it is clear that if sense-organs 
for light exist they must be sought on the leaf-blade. We should 
further expect that such organs would be found on the surface, more 
especially since the superficial layer of cells has as a rule no chloro¬ 
phyll bodies which would obviously interfere with the optical 
behaviour of the tissue. It is, as a matter of fact, in the epidermis 
that Haberlandt finds the organs which, as having some resemblance 
to a primitive eye, he names ocelli. 
The simplest kind of ocellus consists of an epidermic cell 
having a dome-like outer wall. Imagine one of these cells forming 
part of the epidermis of a horizontal leaf illuminated from above. 
As the light passes from the air to the fluid of the cell, it is refracted 
or bent; the cell in fact acts like a plano-convex lens and throws a 
spot of light on the basal cell wall. We have therefore the two 
elements of which the simplest eye can be imagined to consist, a 
lens and a sensitive protoplasmic membrane on which the light may 
be focussed. 
We have imagined the leaf to be horizontal and under a top 
light, that is to say in a position of equilibrium. Now imagine 
the direction of illumination to be changed, by subjecting the 
leaf to oblique light. The consequence of this will be that in 
each of the epidermic cells the bright spot on the basal wall will 
be no longer central. We can imagine this acting as a stimulus and 
causing the leaf to move in such a way that it tends to become at 
right angles to the light: and when that position is actually reached 
the fact that the spots of light are once more in the centre of each 
of the basal walls will act as an inhibitory stimulus and the leaf will 
remain in a condition of equilibrium. It is certain therefore that 
domed epidermic cells may conceivably act as sense organs 
1 Annals of Botany, Vol. XIX., p. 75. 
* Lichtsinnesorgane der Laubbliitter, 1905. 
