464 Wager . — The Perception of Light in Plants. 
curved outer cell-walls. In this case two images are formed at different 
levels, one by the cell as a whole (Fig. 6), the other by the more or less 
central papilla in the upper wall of the cell (Fig. 12). If the epidermal cells are 
focused so that the image formed by this papilla is first of all brought into 
view, then, as the tube is moved upwards, this disappears, and the image 
formed by the whole cell appears. 
The production of such clear images shows that the epidermal cells are 
capable of functioning as very efficient lenses. But it is not probable that 
the plant is capable of perceiving images, nor is such a supposition necessary 
to Haberlandt’s hypothesis. The epidermal cells have very little resem- 
blance to the highly organized eyes of animals, although, as in the case 
of Fittonia Verschaffeltii , instanced by Haberlandt, in which there is a lens- 
shaped cell at the apex of a larger cell, the two have some analogy to 
an animal eye in their general arrangement. Haberlandt points out, if we 
reserve the term ‘ eye ’ only for those organs which bring about an image 
perception, then the foliage leaves and many animals are eyeless. But 
if the term is extended to include organs capable of perceiving a difference 
in the intensity of light, then plants must be said to possess eyes. 
Cases in which the Lens Cells do not function in Light Perception. 
That the lens function is of very common occurrence is shown not 
only by the number of instances adduced by Haberlandt, but by many 
more recent observations, notably those of Seefried (loc. cit.) and Sperlich, 1 
all of which lend support to Haberlandt’s hypothesis. On the other hand, 
there are numerous instances where lens cells and even special cells and 
local thickenings of the cuticle occur, which can have no special function in 
the perception of light for the orientation of the leaf. 
In the majority of leaves which I have examined, the cells of the under 
epidermis show a distinct lens action ; they produce a less regular con- 
vergence than those of the upper epidermis as a rule, but in many cases 
equally good and clear. Albrecht 2 has also shown that papillate cells are 
of common occurrence on the under sides of leaves. This had, however, 
not escaped Haberlandt’s observation. On p. 125 of his chief paper 3 
he refers to it as of frequent occurrence. He states, however, that it 
cannot be considered as an objection to his hypothesis regarding the 
function of the upper epidermal cells. All papillate epidermal cells do not 
necessarily function in this way ; they may serve for very different pur- 
poses such as the prevention of wetting of the leaf. But whenever the 
curved outer wall of the upper epidermal cells is specially clearly marked, 
1 Die optischen Verhaitnisse in der oberseitigen Blattepidermis tropischer Gelenkpflanzen. 
Sitzungsber. d. k. Akad. d. Wiss. Wien, cxvi, 1907. 
2 Ueber die Perzeption der Lichtrichtung in den Laubblattern. Per. d. d. bot. GeselL, xxvi, 
p. 182, 1908. 3 Lichtsinnesorgane der Laubblatter, 1905. 
