4.66 Wager . — The Perception of Light in Plants. 
selves with the upper surface exposed to and more or less at right angles 
to the rays of light. In the leaves of Garrya elliptica both the upper and 
the under epidermal cells are covered by a thick cuticle, in which lens- 
shaped thickenings occur more or less regularly spaced (Fig. 23). These 
function as very efficient lenses and produce clear spots of light about the 
level of the basal walls of the epidermal cells. Fig. 24 shows the image 
of a cross photographed through them. The images formed are equally 
clear on both sides of the leaf. So far, however, from producing a centric 
differential illumination of the basal walls of the epidermal cells, they 
appear to have no definite relation to them. They are sometimes placed 
in the middle of the cell- wall, but are just as often near one side, and 
frequently occur just over the partition wall between two cells (Fig. 23). 
In these circumstances it is difficult to conceive that their function is that 
of light perception. 
If we consider both upper and lower epidermal cells and also ortho- 
heliotropic as well as dia-heliotropic organs, we find that we can distin- 
guish three different forms of cells according to their optical behaviour. 
Form I. Cells with more or less regular outline, which bring the 
incident rays of light to a focal point — spherical or stigmatic lenses. 
Form II. Cylindrical cells which bring the rays to a focal line instead 
of a focal point, and may therefore be designated astigmatic lenses. These 
occur mainly on orthotropic organs. 
Form III. Cells with very wavy or irregular outlines, intermediate 
between I and II, each one capable of bringing rays of light to a focal 
point in one or more regions of the cell, and to a focal line in other parts 
of the same cell. These occur commonly on the under sides of leaves, 
but are also found occasionally on the upper surface (e. g. Eranthis hye- 
malis). 
In cells with an irregular or wavy outline, such as occur on the leaves 
of Eranthis hy emails. Ranunculus Ficaria , Veronica Beccabunga , various 
species of Anemone, many Ferns, and numerous other plants, a distinct 
differential illumination can be seen when a small diaphragm opening, 
2 to 3 mm., is used. The light area is, however, very irregular (Fig. 17), 
and, although not incapable of explanation by Haberlandt’s hypothesis, 
introduces complications in the light perception which the plant must 
find some difficulty in disentangling. In some cases the variation in 
curvature of the outer cell-wall results in the formation of two or more 
bright spots of light in each cell, each of which is capable of forming an 
image of an object focused through it. 
Anything which brings about a state of turgor in transparent cells 
tends to the development of a more or less spherical or cylindrical form, 
and the cells may thus become optically active. This occurs in many 
cases where it can be no question of the lens function being of special 
