465 
Wager. The Perception of Light in Pla7its . 
as in velvet leaves, or when a specially differentiated small lens is produced, 
as in Vinca major , &c., then these contrivances fail completely on the 
under side. In such cases he considers it is clear, therefore, that these 
structures are connected with a definite function which is peculiar to the 
upper epidermis, and this can only be the function of light perception. 
Moreover, the local sense organs for light (ocelli) are found exclusively 
on the upper side of the leaf. 
Seefried, however, points out (loc. cit.) that in Aquilegia vulgaris optically 
efficient papillae are found on the outer walls of the epidermal cells of the 
lower epidermis as well as on the upper. The papillae of the lower 
epidermis are, however, not so regularly formed, and are placed more 
excentrically. From this he would seem to imply that they cannot 
be functional in Haberlandt’s sense. So far as my own observations go, 
this is not the case. The papillae are much the same in structure on both 
sides, as indeed Seefried’s own figure shows ; the lower epidermal cells are 
more irregular in outline than the upper, but in both cases the papillae may 
be excentrically placed, although not so much so as in Adoxa moschatellina , 
as figured by Guttenberg (loc. cit.), to which no exception is taken. 
In Viscum album , according to Albrecht (loc. cit.), epidermal swellings 
occur on both sides of the leaf ; and he adduces numerous instances in 
which papillae are found on leaves which do not assume the fixed light 
position. He also shows that special cells or groups of cells (ocelli) may 
occur on the under sides of leaves. 
Haberlandt’s contention, that special lens cells or lens thickenings 
fail completely on the under sides of the leaves, therefore falls to the 
ground, as also does his conclusion that they are specially adapted for 
the perception of light. Two other examples which show this very clearly 
may here be given. In Mesembryan themurn cordifolium we have an excel- 
lent example of local lens cells. The leaves and stem of this plant are 
covered with glistening spots which can be quite easily seen by the naked 
eye. On the leaves these consist of enlarged epidermal cells, more or less 
regular in outline (Fig. 2) and of a biconvex shape in section ; on the stem 
they are in the form of spherical cells, as in the ice-plant, which stand out 
from the surface. The cells appear to contain a mucilaginous substance, 
which may occur in the form of bright refractive granules (Fig. 4), and 
probably serve to store up water. These cells are very efficient lenses 
(Fig. 5) ; they occur equally on both sides of the leaf and in both cases 
equally good images can be photographed through them. In Fig. 3 is 
shown — somewhat indistinctly, owing to the interference of the mucilage 
granules — a photograph of some houses taken through the cells on the 
under side of the leaf. Between these large cells there are the much 
smaller normal epidermal cells. These also bring about a very clear 
convergence of the light rays (Fig. 4). The leaves always place them- 
