Wager . — The Perception of Light in Plants. 
477 
no differential illumination of the basal walls was visible. When the leaf- 
blade was cut off, the leaf-stalks also curved towards the light, but not so 
quickly or so definitely as when the leaf-blade was present, and the move- 
ment was soon inhibited. These experiments show clearly that, although 
the lens function is eliminated, the heliotropic response of the leaves of 
Ranunctilus Ficaria is almost as pronounced in water as in air. 
Adoxa moschatellina. Both in water and in air, a one-sided illumination 
causes the leaf-stalk to curve towards the 
light so as to bring the leaf-blade into 
a position more or less at right angles 
to it. The position taken up by the sub- 
merged leaves was exactly like that given 
by Guttenberg in his figure illustrating the 
heliotropic response of the leaves in air 
(loc. cit„ Fig. 5, PI. II). In one experiment 
which I made, a submerged leaf, which 
had curved sharply to the light, was turned 
completely round so that its upper surface 
was turned away from it. Almost im- 
mediately a second curvature was induced 
a little higher up the leaf-stalk, and the 
leaf again curved towards the light (Text- 
Fig. 1). The upper epidermal cells exhibit 
a beautiful convergence, due to a small 
projecting papilla in the middle of each 
outer wall, as Guttenberg (loc. cit.) has 
shown, which disappears entirely in water. 
Saxifraga Geum. The old leaves turn 
towards the light very slowly in air, but 
in water apparently not at all. The young shoots, in air, turn very 
markedly towards the light, so as to bring the young leaves into a position 
more or less at right angles to it, but are not responsive in water. The 
minute papillate projection in the middle of the outer wall of each of the 
upper epidermal cells brings about a very distinct bright spot of light on 
the basal wall, which disappears entirely in water. 
In both these plants the lens function disappears entirely in water : 
there is no differentiation visible even in oblique light, and yet when sub- 
merged in water there is a definite heliotropic response in the one, but not 
in the other. 
Text-Fig. i. Leaf of Adoxa moscha- 
tellina submerged in water and exposed 
to a one-sided illumination. The double 
curve is due to the leaf having been com- 
pletely turned round from its first position 
and exposed a second time to the light. 
