486 
Wager . — The Perception of lAght in Plants. 
Summary. 
1. Haberlandt has shown that the cells of the upper epidermis of 
dia-heliotropic leaves are so constructed that they cause a convergence 
of the rays of light, and has suggested that they are functional as ocelli or 
primitive eyes, capable of setting up a stimulus which results in the 
heliotropic orientation of the leaf. 
2. The central and peripheral areas of the layer of cytoplasm lining 
the basal walls of the epidermal cells are, according to Haberlandt’s 
hypothesis, attuned to light of different intensities. When the leaf is in 
equilibrium, the central area is bright, the peripheral zone dark. When 
the light is oblique, these relations are altered, and it is this variation 
which sets up the stimulus by which the orientation of the leaf is brought 
about. 
3. The efficiency of these lens cells is shown by the fact that in many 
leaves they are able to form clear images of objects focused through them, 
which may be easily seen under the microscope and can be photographed. 
It is not probable that the plant can perceive these images. 
4. Haberlandt’s hypothesis is open to criticism both on morphological 
and on physiological grounds. The phenomenon of the convergence of 
light by the cells of plants is, of very widespread occurrence. Not only 
the epidermal cells of leaves, but all cells which through turgidity assume 
a spherical or cylindrical form are capable of bringing it about. 
5. Lens cells are present in many cases on leaves and other organs 
where there can be no question of their functioning in light perception. 
6. The cells of the lower as well as the upper epidermis of leaves are 
in most cases capable of light convergence. 
7. Special lens cells and lens-shaped thickenings of the cuticle often 
occur on the lower as well as the upper epidermis. The position of the 
lens-shaped thickenings of the cuticle in Garrya elliptica , which occur 
on both sides of the leaf, has no relation to the position of the epidermal cells. 
8. According to the form and outline of the cell, the rays of light may be 
converged to a local point, a focal line, or to an irregular figure intermediate 
between these. Cells with very irregular outlines, as in Eranthis hyemalis , 
commonly have more than one series of converging rays. As many as 
three or more may be seen, each producing a bright spot of light. 
9. Cylindrical cells which bring rays to a focal line are present on some 
orthotropic organs, such as stalks of leaves, pedicels of flowers, and hypo- 
cotyledons of seedlings, also on the long, narrow, non-sensitive leaves 
of such plants as Grasses, Hyacinth, Freesia , &c. 
10. Papillate cells and lens-shaped thickenings of the cuticle are found 
on leaves which are not heliotropically sensitive. 
11. The extent to which the phenomenon of light convergence is 
simply a result of cell-turgor and not an adaptation to light perception 
