76 Sense Organs for Gravity and Light. 
to the type known as “velvet leaves” in which the epidermic cells 
project as papillae above the general surface. It is now suggested 
that this form of epidermic cell serves to prevent the leaf being 
“blinded” by the continuous rain of the regions where the plants in 
question chiefly occur. 
Other, and more specialised, types of what are believed to 
function as ocelli are described by Haberlandt. Some years ago 
Heinricher described in Campanula persicifolia some minute 
papillae (for which he could suggest no function, apparently 
dwarfed or reduced hairs). It is certain that they might function as 
ocelli, for they produce a bright spot of light when the leaves are 
examined under the microscope as above described. This type is 
interesting in as much as the refractive quality of the ocelli depends 
on the rounded mass of silica deposited in the external cell walls. 
In another well-marked type, that of Fittonia Verschaffelti 
(Acanthaceae), the refraction, as in the first case described, depends 
on the form of the cell and the fact that it contains cell-sap. In 
Fittonia the epidermic cells are flat-topped and cannot function as 
ocelli, but among them are a number of two-celled trichomes of 
which the upper one has the form of a bi-convex lens, the broad 
lower stalk-cell serves as a support for the lens, and also doubtless 
supplies the protoplasmic membrane on which the spot of light is 
thrown. It is a remarkable fact that in Impatiens mariannce, a plant 
belonging to a different Natural Order, closely similar ocelli occur. 
Although Haberlandt’s theory cannot be considered as 
absolutely established, it is deserving of every consideration. One 
feature of it is especially interesting, namely its broad general 
resemblance to the statolith theory. In both cases the directive 
element in the mechanism depends on the stimulus being applied to 
certain regions of the protoplasm lining the cell-walls. The organ 
recognises, as it were, that it is not vertical, or that it is not at 
right angles to the light, by a definite change in the position of 
the stimulated region. 
