428 Vines.- — On Epinasty and Hyponasty. 
conditions, and to mention one or two experiments on geo- 
tropism which I have made by the way. 
Taking first the case of the epinastic members, it may be 
assumed that there are three forces at work ; their own 
epinasty, the action of light, and the action of gravitation. 
The tendency of epinasty, in the case of leaves, is to bring 
the lamina into the vertical plane, the apex being directed 
downwards. 
When an epinastically recurved leaf is exposed to vertical 
light of sufficient intensity, the effect is to raise the lamina 
into the horizontal plane so that its upper surface is at right 
angles to the direction of the incident rays. From this I 
conclude that the lamina is diaheliotropic. Doubtless the 
influence of gravitation promotes the assumption of the hori- 
zontal position ; but it is clear that this is mainly due to the 
action of light, since gravitation is incapable of preventing 
epinastic curvature in darkness when the plant is in the 
normal position. 
With regard to the influence of gravitation in determining 
the position of epinastic dorsiventral members, I inferred from 
my observations which prove that hyponastic members are 
not negatively heliotropic, that this is true also of epinastic 
members ; in fact, that no dorsiventral member is negatively 
geotropic. Though I do not claim to have fully investigated 
the matter, I have made some observations on epinastic mem- 
bers which tend to prove, not only that they are not negatively 
geotropic, but that they are diageotropic. The following case 
will explain the mode of experimentation. A young plant of 
Helianthus annuus> growing in a pot, its stem being firmly 
secured to a stick to prevent geotropic curvature., was placed 
in darkness with its long axis horizontal. In this position of 
the stem, the two youngest leaves were situated, the one on the 
upper surface, the other on the lower surface of the horizontal 
stem, the apex of the former pointing vertically upwards, that 
of the latter vertically downwards. The phyllotaxis of Heli- 
anthus being opposite and decussate, the two leaves of the 
next whorl were situated one on each side of the horizontal 
