430 Vines. — On Epinasty and Hyponasty. 
The explanation which I would offer as a substitute is 
based on the assumption that the leaves are diageotropic ; 
that is to say, that their reaction to the action of gravitation 
is to place themselves in the horizontal plane with their 
morphologically upper surfaces facing the zenith. From this 
point of view, the change in position of the upper youngest 
leaf is ascribed to epinasty acting together with diageotropism ; 
and the change in position of the lower youngest leaf to 
diageotropism acting in opposition to epinasty. It may 
be objected with reference to this last statement, that it 
is inconsistent with the fact which I have brought prominently 
forward, that the young leaves of Helianthus show marked 
epinastic recurvature in darkness, which can only be explained 
on the assumption that here epinasty has overcome diageo- 
tropism. This objection is, however, without weight. The 
two statements are found to be perfectly consistent when it 
is remembered that the effect of gravitation depends upon the 
angle at which it acts. Now, in the case of a plant in the 
normal position, the effect of diageotropism on the horizontal 
leaf is at its minimum, increasing as the leaf-surface forms 
a larger angle with the horizontal, and attaining its maximum 
when the leaf points vertically downwards. Hence a leaf, 
under such conditions, sinks below the horizontal because 
epinasty is more powerful than diageotropism in that position, 
and continues to do so until a position of equilibrium is 
reached. In the special case now under consideration, the 
leaf, pointing vertically downwards, is in that position in which 
diageotropism exerts its greatest influence, an influence, as 
it appears, sufficiently strong to prevent epinastic curvature 
and even to raise the lamina somewhat towards the horizontal 
position. The phenomena presented by the two youngest 
leaves can, therefore, be explained more satisfactorily on the 
theory of diageotropism than on that of negative geotropism. 
The phenomena presented by the leaves of the second whorl, 
cannot be at all explained on the theory of negative geotro- 
pism, but only on that of diageotropism ; this is sufficiently 
obvious to render any detailed discussion unnecessary. 
