434 Vines. — On Epinasty and Hyponasty. 
takes place in darkness when the plant is in the normal 
position proves that gravitation alone cannot determine the 
assumption and maintenance of the horizontal position. 
My observations lead me to the conclusion that the forces 
which are active during growth under normal conditions in 
determining the position of dorsiventral members, are their 
own inherent hyponasty or epinasty; the action of light pro- 
ducing a diaheliotropic effect ; the action of gravitation pro- 
ducing a diageotropic effect. The ultimate position assumed 
is, I believe, a fixed light-position, the influence of light being 
the most powerful factor. Light can, as F. Darwin’s experi- 
ments prove, overcome epinasty or hyponasty; and these can 
overcome the influence of gravitation, as I have shown, except 
when in so doing they would bring the morphologically 
upper surface of the member to face downwards, a possibility 
which is not often realised in plants growing in the normal 
position. 
The conclusions at which I have arrived with regard to 
growing members appear to me to be equally applicable to 
motile foliage-leaves. So far as I am aware no explanation 
has as yet been offered of the differences in the nyctitropic 
movements of leaves ; of why it is that some motile leaves 
rise, and others fall at night. These differences may be ex- 
plained on the assumption that motile leaves, like growing 
leaves, may be either epinastic or hyponastic ; that is to say, 
they may be in a state of epinastic or hyponastic tension, 
though the tension is not associated with actual growth. 
Motile leaves which fall at night do so, apparently, because 
their epinastic tension is no longer counteracted by diahelio- 
tropism ; similarly, leaves which rise at night do so because 
their hyponastic tension is no longer counteracted by dia- 
heliotropism. The former behave like, for instance, the 
growing leaves of Helianthus when placed in darkness ; the 
latter, like the younger leaves of Plant ago. This view is simply 
a natural consequence of the remarkable similarity between 
the nyctitropic movements of growing leaves, as described by 
Batalin, and those of motile leaves ; it can hardly be doubted 
