266 
PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 
agent that caused the turning over of leaves, and afterwards to study the mode of its 
action on the two surfaces of those organs. 
After I had verified most of the experiments of Bonnet on the turning over of the 
leaves of a great number of plants, as far as relates to the rapidity of the phenome- 
non, the frequency of its reproduction on the same leaves, &c., in leaving the plants 
exposed to the light, I placed many plants in such a manner that their inverted 
leaves remained in complete darkness. Contrary to Bonnet’s observations, I found 
that the leaves in that case did not regularly turn over. It often happened, for in- 
stance in a branch of lilac, in a plant of Polemonium cceruleum, that the footstalks 
and the flat part of the leaves so kept in darkness, turn about first in one direction, 
then in another ; but this happened equally to the leaves left in their natural position. 
This appears to me to be the consequence of the state of uneasiness in which the pri- 
vation of light puts the leaves ; they seem to move about as if in search of this agent, 
and this cause, as well as the elastic reaction of the footstalk, when drawn from its 
natural position by torsion, which process I have always avoided, may have led the 
observer into error. 
With the conviction that light is the only agent whose influence causes the turning 
over of leaves, I made the following experiment. In cold weather (45° Fahr.) I 
placed a vase of geranium in such a manner that one of its leaves, young, though 
fully developed, without altering in the least its position or that of the vase, should 
be entirely covered from the light on its upper surface, by a double screen of black 
paper that did not touch it, while, on the contrary, the under surface was strongly 
lighted by a mirror placed below at the proper inclination. Three days after, the 
flat part of the leaf had begun to turn over ; and after six days, the leaf was entirely 
inverted, so as to present its upper or varnished surface towards the mirror. The 
apparatus was kept in the same state during a fortnight, and the leaf continued in 
the same position, the upper surface downwards towards the mirror, and the under 
upwards towards the screen. The mirror and the screen were then simultaneously 
removed, and the leaf was left presenting to the diffused light its under surface that 
was uppermost. After a few days the leaf turned itself over again, and the torsion 
took place by a motion of the flat part on the point where it is inserted in the foot- 
stalk. This experiment has been repeated many times on various plants with the 
same results, and seems to me conclusive. 
When the two surfaces of leaves are exposed to light at the same time, they do not 
change their position, but seem to suffer; if the under surface be turned towards the 
mirror, while at the same time the upper surface being shadowed by other leaves, 
does not receive as large an amount of direct light as the other surface receives of 
reflected light, the flat part of the leaf contracts itself so as to rendei’ it almost glo- 
bular. The edges of the upper surface are thus presented to the mirror and cover 
over the under surface. 
Some geranium leaves were covered with a screen of black paper attached to their 
