Vines. — On Epinasty and Hyponasty. 417 
may be gathered from what he says with reference to the 
thallus of Marchantia. The plagiotropism of the thallus of 
Marchantia is, he says, to be ascribed to (negative) geotropism, 
to positive heliotropism (of the under side), and to the epinasty 
of the upper side. In the earlier part of his paper he speaks 
of the negative heliotropism of the thallus, but he explains 
that this apparent negative heliotropism is in fact the hyponasty 
of De Vries. Sachs’ conclusions, therefore, go to confirm De 
Vries’ dissent from Frank’s theory of diaheliotropism. 
Wiesner’s 1 conclusions may next be briefly stated. He is 
of opinion that the ultimate position of dorsiventral leaves is 
the resultant of the opposing forces, negative heliotropism and 
negative geotropism : the leaf, which at first tends to rise by 
negative geotropism, is brought into the most favourable 
light-position by negative heliotropism, and is retained in 
that position because these conditions of illumination are 
those which most strongly resist negatively geotropic curva- 
ture. 
The next paper on the subject, that by F. Darwin 2 * * , is one 
which marks a distinct advance. By experiments with the 
clinostat he ascertained that dorsiventral leaves tend to place 
their normally upper surfaces at right angles to the incident 
rays of light. The importance of these observations is, that 
they go far to invalidate De Vries’ argument against Frank, 
inasmuch as they show that the position in question is assumed 
by dorsiventral leaves under conditions in which gravity, 
whether as regards geotropic stimulation or the balancing of 
the parts, is inoperative. The conclusion 6 that the power 
which leaves have of placing themselves at right angles to the 
incident light is due to a specialised sensitiveness to light — 
diaheliotropism — which is able to regulate or govern the ac- 
tion of other external forces, such as gravitation, or of internal 
1 Wiesner : Die heliotropischen Erscheinungen im Pflanzenreiche, Denkschr. d. 
Math.-Naturwiss. Klasse d. K. Akad. d. Wiss. Wien XXXIX, Part 2, p. 58, 
1880. 
2 F. Darwin : On the power possessed by leaves of placing themselves at right 
angles to the direction of incident light. Journal Linnean Society, XVIII, 
London, 1881. See also ‘ Movements of Plants,’ 1880. 
