467 
Wager. — The Perception of Light in Plants . 
importance to the plant for light perception. Such are, for example, the 
cells of the pith of the Rush, cortical cells in the stems of certain plants, 
cells in the pileus of species of Russula, and various hair cells and gland cells. 
Turgidity plays a very important part in the growth of plants and especially 
in the unfolding of leaves. The cells of the epidermis on the concave side 
of many expanding leaves are more turgid than those on the outer convex 
side. This is well shown in Hyacinthus romanus. The young leaves just 
as they appear above ground are very much curved ; the outer epidermis 
exhibits no turgidity whatever ; the inner epidermis, on the contrary, is 
composed of elongated cells which are very strongly turgid and are capable 
of bringing the rays of light to a focus in the shape of a clearly defined 
bright line. The same phenomena can be seen in the strongly incurved 
young leaves of Cyclamen. Both exhibit a pronounced heliotropism in 
the young state. 
In the case of all leaves which remain flat and rigid, the maintenance 
of a certain amount of turgidity in the epidermal cells is doubtless neces- 
sary unless they possess a thick cuticle. It may be suggested, therefore, 
that possibly the amount of turgidity and the arching of the cell-walls 
bear some relation to the thickness of the cell-wall and cuticle. In some 
cases, for example, the leaves show little or no turgidity in their epidermal 
cells, but their flatness and rigidity are maintained apparently (in part, 
at any rate) by much-thickened cell-walls. Such leaves may exhibit 
a very pronounced dia-heliotropism, and yet show no lens action at all 
( Prunus lauro-cerasus). Gaulhofer (loc. cit.) has apparently appreciated 
the importance of this in respect to Haberlandt’s theory, and has brought 
forward a number of observations to show that in such cases a differential 
illumination can be brought about, which gives similar results to those 
described by Haberlandt, by refraction of light through the more highly 
refractive thick cell-walls into the less highly refractive cell-sap. 
The greater turgidity of the epidermal cells of many shade plants may 
be due largely to their thinner cell-walls and to the fact that they have 
probably more water to store up owing to their diminished transpiration. 
Haberlandt refers to turgidity (loc. cit., p. 1 36) in speaking of aphotometric 
leaves, but considers apparently that it is of little importance. If, he says, 
such leaves possess either an arched outer or inner wall in their epidermal 
cells, we should only perceive in them those physiologically unimportant 
early stages due to turgor which formed the starting-point in euphotometric 
leaves of their adaptation to light perception. The problem, however, 
is one which deserves a much fuller investigation from this point of view 
than has yet been given to it. 
The papillate epidermal cells of many petals show a very distinct light 
convergence, and are capable of bringing about a very clear centric illumi- 
nation of the basal walls (Fig. 18). This is well seen in a large number of 
