Wager . — The Perception of Light in Plants. 471 
so as to expose their upper surfaces to the light, but this appears to be 
due to epinastic growth. 
• The young leaves of Hyacinthus romanus grow towards the light if 
they are subjected to a lateral illumination. They have a dorsi-ventral 
structure and soon begin to bend outwards and downwards so as to 
present their upper surfaces to the light. This appears to be due to 
epinastic growth, and by its means the leaf is brought into such a position 
that its upper surface is more or less at right angles to the rays of incident 
light. The upper epidermal cells are optically active and converge the 
light as in Iris (Fig. 20). The lower surface, which is the only one exposed 
to the light when the leaves are heliotropically responsive in the early 
stages, possesses elongate cells which are flattened and show no trace of 
convergence. 
The hypocotyls of the seedlings of many dicotyledons and the leaf- 
stalks of many leaves, such as Ranunculus Ficaria , Eranthis hyemalis , 
Cyclamen , &c., all of which are heliotropically responsive, possess well- 
developed astigmatic lens cells similar to those already described (Fig. 20). 
Young buds of various bulbous plants, such as Chionodoxa , Tulip, 
Freesia , &c., are heliotropically sensitive, but the adult leaves of these plants, 
although they possess well-marked optically active cylindrical cells, are 
not, or only very slightly, sensitive to light. 
In Freesia refracta, var. alba , the young sheathing leaves bend strongly 
towards the light. The leaf-blade is not heliotropically sensitive and 
grows in the direction in which it issues from the bud. As in Grasses, 
the young sheathing leaves and the basal sheathing position of the 
ordinary leaves are resposible for the heliotropic response. The epidermal 
cells of all parts of the adult leaf and the sheathing leaves show a very 
distinct differential illumination of the basal walls due to thickenings of 
the cuticle. On the basal sheathing part of the leaf and on the outer 
sheath leaves the epidermis, both on the inner and the outer surface, 
consists of elongate astigmatic cells, which have an elongate cuticular 
thickening in the middle of the outer wall of each cell. In the upper 
non-sensitive parts of the leaf, there are, instead of these elongate cuticular 
thickenings, one to three lens-shaped thickenings of the cuticle in each 
cell which produce clear circular spots of light on the basal walls. These 
are quite as well developed and as clearly marked as in many of the 
special cases described by Haberlandt, but are not functional as light- 
perceiving organs. 
Aphotometric Leaves. 
If it could be shown that aphotometric leaves were destitute of light- 
converging epidermal cells, it would be distinctly favourable to Haberlandt’s 
hypothesis. Only one case is, however, adduced by Haberlandt (loc. cit., ’ 05 , 
K k 
