1144 Summers.—On the Occurre 7 ice of Lens-cells in the 
days old, which already showed traces of an encrustation, responded to 
heliotropic stimuli but slowly, while the power of response was evidently 
lost altogether by seedlings sixty-five days old. 
In M. pseudotruncatellum we have to do with a plant the leaves of 
which possess lens-cells which are quite efficient in structure, but whose 
function as such is almost wholly shut out by the encrustation of calcium 
oxalate found in the epidermis. As the plant in its native habitat is subject 
to strong solar illumination, the encrustation is a protection when this 
becomes too strong for the plant. The image formed on the basal wall of 
each epidermal cell is a duller, shaded and toned-down copy of the one 
which would be formed there were the encrustation absent. This is borne 
out by the ‘ Linsenversuch * with treated and natural pieces of epidermis. 
Marloth considers the chief function of this encrustation to be the pro¬ 
tection of the plant against excessive insolation, and when insolation and 
Fig. 9. Seedling 20 days old. 
Fig. 10. Seedling 20 days old exhibiting heliotropic curvature. 
illumination are considered together the combination of encrustation and 
lens-cell will be seen to be peculiarly effective and advantageous. 
An encrustation of uniform thickness and colour would be a better 
protection against insolation, but in that case the interior of the plant 
would not be illuminated at all. 
With the development of the papilla Lo~wo °f the upper surface of 
each epidermal cell is available for the collection of light rays, while the 
remainder is under a protective cover. The protection is extended to the 
papillae, which are covered with a thin encrustation, but as a compensation 
are more highly developed as light-ray collectors. 
The best developed lens-cells are in the side epidermis over the assimi¬ 
lating tissue, and these are the most weakly illuminated cells. There 
appears here, therefore, a correlation between the development of lens-cells 
and assimilating tissue on the one hand and the decrease of illumination 
on the other. The direct insolation and illumination of the upper surface 
evidently exercise a repellent influence on the development of chloroplasts 
in the cells immediately underneath, even when the lens function is almost 
excluded from the epidermal cells. Lens-cells of slightly better construe- 
