472 Wager . — The Perception of Light in Plants. 
p. 126), and this does not appear to hold good for all specimens. He 
states that the leaves of Convallaria majalis are aphotometric, and that 
both the inner and outer walls of the epidermal cells nearly perfectly 
even. This apparently implies that they have little power of light con- 
vergence. I find, however, that this is not so ; the cells of the upper 
epidermis, in all the specimens which I have examined, act as cylindrical 
lenses, and show an unmistakable convergence (Fig. 21) which, in an 
oblique light, is capable of producing a very definite variation in the 
illumination of the basal walls. 
The leaves of Iris , Gladiolus , Montbretia , Tradescantia virginica , and 
numerous other plants possess elongate cylindrical lens cells capable of 
bringing the light rays to a focal line, but appear not to be heliotropically 
sensitive. 
In support of Haberlandt, Seefried (loc. cit. , p. 17) has some inter- 
esting observations on Pirola secunda and P. chlorantha. Both species 
often occur near one another in the shade of woods. The leaf-blade of 
P . secunda is pan-photometric and turns irregularly towards the light 
on both sides. P. chlorantha , on the other hand, is euphotometric, and 
takes up the fixed light position. The latter is clearly dorsi-ventral 
with well-developed palisade tissue ; the upper epidermis has biconvex 
lens-shaped cells ; the cells of the lower are flattened. In P. secunda the 
leaf is iso-lateral with no, or only slightly developed, palisade tissue and 
flattened epidermal cells, a few only of which show any light convergence. 
Many Fern leaves, which appear to possess no power of response to 
light, show in their epidermal cells a distinct and often very beautiful 
convergence, capable of producing in oblique light a distinct difference 
in the illumination of the basal walls of the cells. 
The only leaf known to me which is aphotometric, and does not 
possess epidermal lens cells, is Aspidistra lurida. The cell-walls, both inner 
and outer, are quite flat, and the outer wall is very thick. They appear not 
to have any power of causing a differential illumination. In many of the 
cells, however, there is another means by which this can be brought about. 
Such cells occur generally in groups, and are not visible over all parts of 
the leaf. Each cell contains a well-marked finely punctate nucleus more or 
less spherical in shape, generally lying on the lower wall. Its diameter 
is about J to § that of the cell itself. Sometimes it is at one end of the cell 
sometimes near the middle. It is more highly refractive than the sub- 
stances around it, and is capable of causing the convergence of light rays 
and of producing bright spots of light. Images can be focused through 
the nuclei, and with care photographs can be obtained (Fig. 11). We 
have here an efficient apparatus for convergence of light, but it is obvious 
that it is not functional in any way in causing the orientation of the leaf. 
In the leaves of the Carnation ( Dianthus Caryophyllus ) both the upper 
