Vines . — On Epinasty and Hyponasty. 433 
raise the lamina slightly above the horizontal. Coming next 
to the leaves inserted above the axis, leaves 2, 5, and 7, the 
explanation becomes more difficult. In the case of leaf 2, 
hyponasty must be strong, and yet the only effect is the 
raising of the leaf slightly from the surface of the soil, when it 
might have been expected to cause the leaf to curve over 
downwards in the same way as it caused leaf 1 to curve over 
upwards. I believe the cause of the different hyponastic effect 
in these two leaves, of nearly the same age, is their relative 
position. The tendency of the hyponastic growth of leaf 2 
was to bring the morphologically superior surface of the leaf 
into such a position that it would face downwards. Now this 
tendency is in direct opposition to the most striking effect of 
diageotropism, which is to bring the morphologically upper 
surface of the leaf in such a position that it faces the zenith. 
Hence the hyponasty of leaf 2, strong as it undoubtedly must 
have been, was insufficient to counteract diageotropism to any 
considerable extent in this position ; and, as torsion seems to 
have been physically impossible, no change of position took 
place. If this explanation is adequate in the case of leaf 2, it 
must apply also to leaves 5 and 7, in which, as they were older, 
hyponasty was less powerful. In the two lateral leaves, 
Nos. 3 and 4, both hyponasty and diageotropism produced 
their full effects, the one in curvature, the other in torsion. 
Similar results were obtained with Taraxacum Dens-Leonis. 
It may be urged, in criticism of these views, that I have 
assumed diaheliotropism without adducing any evidence in 
support of the assumption. In reply I would submit that my 
observations do afford definite evidence on this important 
point. It appears to me to be quite impossible to explain 
the return to the horizontal, when exposed to sufficiently 
intense light, of members which have become curved either 
epinastically or hyponastically in consequence of having been 
kept in darkness, in any other way than by attributing it to the 
influence of the light on their diaheliotropic irritability. It is 
true that gravitation co-operates in inducing this return to the 
horizontal, but the fact that hyponastic or epinastic curvature 
