PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 
265 
but towards the end of autumn it ceases to manifest itself. As Bonnet believed 
that the leaves turn themselves over in the dark as well as in the light, he imagined 
that this phenomenon is the consequence of the under surface of leaves containing 
fibres liable to contract themselves by humidity, and their upper surface fibres that 
contract themselves by the influence of heat. He went so far as to construct, in 
support of his hypothesis, artificial leaves, of which the upper surface was of parch- 
ment and the lower of linen, and he states that the heat and the damp produced in 
them the same motions as in natural leaves. According to him, the dampness of the 
earth determines the under surface of the leaves to turn themselves on that side, 
and the heat of the sun acts in the same way on the upper surface. This opinion 
was the more singular in Bonnet, from his having observed that the leaves turn 
themselves over in water as well as in the air, and from his believing that this change 
of position in the two surfaces of leaves is the result of the contortion or the inflec- 
tion of the footstalk. 
Dutrochet indeed attributes to the influence of light the turning over of leaves, 
but he considers the flat part of the leaf as passive, and the footstalk as the only 
agent in its motion. He quotes besides no other experiment in proof of the influence 
of light but those of Bonnet, and especially the one in which some leaves of a cherry- 
tree, placed under the shadow of a table so as to receive light laterally, put them- 
selves in a vertical position with the end downwards. 
He gives for the chief ground of his opinion respecting the action of light in this 
phenomenon, the fact that having placed a Convolvulus in the circumference of a 
wheel rapidly revolving, the leaves after eighteen hours presented their upper surface 
to the centre of rotation and their under one to the circumference. He concludes 
from this experiment that leaves present naturally their upper surface to the light ; 
but, as Decandolle justly observes, it would have been more natural to conclude 
from it that they present it to gravitation. He recapitulates his opinion in these 
words: — “I have attempted without success to discover how light acts in deter- 
mining the torsion of the footstalk when the leaf is turned over. It seems evident 
that in this circumstance the influence of light is exercised on the flat part of the 
reversed leaf, and that this influence is transmitted to the footstalk whose contor- 
tion it produces ; but I do not see here what is the connection between cause and 
effect. The turning over of a leaf depends on two different causes : — first, the dis- 
position of the footstalk to raise itself towards the sky when it has been accidentally 
bent towards the earth ; secondly, the disposition of the footstalk to bend towards 
the light, but only when it presents its upper surface to it ; the flat part of the leaf 
is entirely passive.” 
I know no more recent paper on this subject, which appeared to me to be far from 
being sufliciently elucidated, and I have therefore endeavoured to add something to 
what was already known. 
I thought it was necessary first to ascertain clearly whether light was really the 
