436 Vines. — On Epinasty and Hyponasty. 
ments which are affected by light, those, namely, of highly 
sensitive members such as the leaves of the Sensitive Plant 
(Mimosa pudica ) and the perianth-leaves of many flowers. It 
is well known that the leaflets of Mimosa close on being placed 
in darkness, and open on being again exposed to light, and 
that many flowers close in darkness, or on diminution of the 
intensity of the light, re-opening when again exposed to light 
or when the intensity of the light is increased, whereas others 
close in light and open in darkness. In the attempt to analyse 
these phenomena, it must be first pointed out that the effect 
of light is quite different in this case from that in the cases 
previously discussed. In those cases it was directive or helio- 
tropic ; in these it is dependent, not upon the direction, but 
upon the intensity of the incident light. This is made clear 
by the fact that exactly the same movements can be induced 
by other means ; in the leaflets of Mimosa , by a touch, in the 
perianth-leaves of flowers by variations of temperature. In 
the next place, it may be inferred from the observations de- 
scribed above, that the position assumed in darkness by a 
dorsiventral member is that which results from the conditions 
of tension inherent in the member. Hence the leaflets of 
Mimosa , and the perianth-leaves of flowers which close in 
darkness, are hyponastic ; whereas the perianth -leaves of 
flowers which open in darkness are epinastic. The opening 
of the former, when exposed to sufficiently intense light, is 
due to the induction of epinastic tension ; the closing of the 
latter, under the same circumstances, is due to the induction 
of hyponastic tension. 
The movements of these leaves under the influence of light 
afford instances of true photo-epinasty and photo-hyponasty, 
as distinguished from those cases ( Cucurbita , Phaseolus ) to 
which Detmer applied this explanation ; the essential dif- 
ference being that, in these cases, light induces a tension 
which is precisely the opposite of that inherent in the leaf, 
whereas in Detmer’s cases light merely rendered possible, by 
inducing phototonus, the external manifestation by growth of 
the conditions of tension inherent in the leaf. 
