Wager — The Perception of Light in Plants . 461 
The curved upper surface of the epidermal cells can be as a rule very 
easily seen with a pocket lens, as Haberlandt has pointed out is also the 
case with well-marked papillate cells ; in bright sunlight or near a window 
a bright spot can usually be seen reflected in the curved surface of each 
cell. If examined under a low power by reflected light, images of various 
objects, reflected in the convex surface as in a convex mirror, are visible. 
Tradescantia fluminensis , Orchis bifolia , Adoxa moschatellina , and many 
other plants show this clearly. 
With the exception of Type I, the differential illumination of the basal 
wall is always due to convergence. In Type I, where the outer cell-wall is 
flat, and there are no special thickenings or other contrivances for the con- 
vergence of light, the curvature of the inner wall is sufficient, according 
to Haberlandt, to bring it about. The lateral portions of such a wall 
are placed obliquely to the incident rays, and are therefore less illuminated 
than those portions near the middle which are more or less horizontal. 
How far this arrangement is capable of bringing about a sufficient amount 
of differentiation to set up an orientation stimulus will be discussed later. 
An excellent example of the way in which these convergent rays 
act in the illumination of the basal wall is seen in Saxifraga Geum. The 
curved outer walls of the upper epidermal cells have a well-marked papillate 
projection about the middle of the cell-wall. This, even without a dia- 
phragm, produces a clear spot of light on the basal wall (PI. XXXI, Fig. 7). 
If the mirror is moved a little to one side, this spot of light moves, and is 
now not exactly in the centre (Fig. 8). If the mirror is moved still further 
to one side, the spot of light moves correspondingly further to one side of 
the cell (Fig. 9). In this plant the light is brought to a focus actually on 
or very near the basal wall, and, under all circumstances, the differential 
illumination of the basal wall is very marked ; in the majority of plants it 
is usually not so clear. 
As Haberlandt points out (loc. cit., p. 54), the usual case is for the rays of 
light to be brought to a focus behind (below) the basal wall. The basal 
wall cuts the converging cone of light so as to give a bright middle region 
surrounded by a darker field. The further this is from the apex of the cone 
of light, the larger the middle field, and vice versa. If the diaphragm is taken 
away, the dark zone becomes narrower, but is always visible. If the focus 
is in front of (above) the inner wall, then the clear middle field is formed by 
cutting through the divergent cone of light. In either case, if the mirror is 
moved laterally, so that the light falls at an angle on the epidermis, the field 
of light makes a corresponding movement to one side (Figs. 8, 9). 
Haberlandt points out (loc. cit, p. 56) that if the diaphragm opening is 
1 mm. and this is at a distance of 5 mm. from the epidermis, a cone of light 
is admitted which is equal to a circular opening in the woods of a diameter 
of 1 m. at a distance of 5 m. high from the surface of the leaf illuminated. 
