SCIENTIFIC SUMMARY. 
73 
tlie plants are thus subjected to a constant lateral illumination, while they 
are freed from the disturbing influence of gravitation ; for, owing to their 
being kept in constant slow rotation, there is no reason why they should bend 
geotropically in one direction more than another.* On the same principle 
the behaviour of leaves which place themselves at right angles to the incident 
light has been studied. If a plant with horizontally extended leaves which 
has been illuminated from above, is fixed on a slowly revolving horizontal 
spindle, so that the axis of the plant is parallel both to the axis of rotation 
and to the direction of incident light, we shall have a means of testing the 
opposing theories above mentioned. The plant’s leaves will still be illumi- 
nated by light striking them at right angles ; therefore if Frank’s theory is 
the right one, they ought to remain in this position. But if De Vries and 
Sachs are correct in their views, the leaves ought not to be able to remain at 
right angles to the incident light since geotropism has disappeared, which was 
one of the tendencies necessary to keep the leaves in a position of equilibrium. 
A considerable number of experiments were made with the celandine {Ranun- 
culus ficaria ) , the results of which are decidedly in favour of Frank’s views. 
The leaves of the celandine are sometimes extremely epinastic, so that they 
press against the ground ; and when a plant is dug up it often happens that 
the leaves, being released from the resistance of the soil, curve nearly verti- 
cally downwards. If such a plant is fixed on the clinostat in the position 
above described, the leaves will be pointing away from the light, so that if 
the leaves were apheliotropic, as might be expected according to De Vries’ 
theory, the leaves would remain pointing away from the window. But this 
is not the case ; they move forward until they are approximately at right 
angles to the fight, and then come to rest. Again, if a celandine is placed in 
the dark, its leaves rise up so as to be highly inclined above the horizon ; if 
the plant is then placed on the clinostat, the leaves (which now of course 
point towards the light) again accommodate themselves by curving back- 
wards until they are at right angles to the light. Thus the leaves cannot be 
called heliotropic, or apheliotropic ; we are forced to believe that under the 
influence of light they are able to move in either direction which may be 
necessary to bring them at right angles to the light. The other experiments 
with R. Jicaria, the details of which we omit, lead to the same general result. 
Besides a few observations on Vida, Cucurbit a, and Plantago, a series of 
experiments were made on seedling cherries, and these lead to a somewhat 
different result. A cherry-plant, growing in the open air, has its leaves 
approximately horizontal, and when placed on the clinostat, as above described, 
the leaves are unable to remain at right angles to the light, but curve back- 
wards, so as to become parallel to the stem of the plant. The petioles are 
shown not to be apheliotropic but to be powerfully epinastic, so that they 
move in the manner described when unopposed by apogeotropism. 
It is clear, therefore, that the horizontal position of the leaves of seedling 
cherries growing normally, must largely depend on the balance struck be- 
tween epinasty and apogeotropism, in accordance with the views of De Vries 
and Sachs. But since these forces obviously cannot produce the power of 
accommodating itself to the direction of incident fight which the cherry 
See Sachs in his Arbeiten, Bd. ii., 1879. 
