268 
PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 
leaves. All the young leaves turned themselves over by moving on the footstalk, either 
bending it downwards or turning it over at its base. Out of the thirteen leaves 
which had turned themselves over, nine repeated the same process, chiefly by a tor- 
sion on the base of the footstalk : the others dried up. On reversing the plant a 
third time, the leaves did not turn themselves over any more, but became dry. The 
experiments were made during the autumn, the least favourable season for them ; for 
in the spring the same leaf turns itself over as many as fifteen times. When, there- 
fore, the turning over of most leaves can be freely effected, it takes place in most 
cases by the motion of the footstalks ; but even in plants where it is effected by 
this means, it can take place in the flat part of the leaf itself. Tlius, I placed a 
geranium leaf, covered with a black paper screen fastened on its upper surface, 
so as to have it illuminated underneath by means of a mirror. It is evident that if the 
footstalk had been bent or inflected, the position of the leaf with respect to light 
could not have been more favourable than it was. The screen would have been placed 
between the mirror and the upper surface, just as it was between it and the sky ; and 
the light of day would iiave fallen on the under surface then turned upwards, as the 
light of the mirror fell. In this dilemma, the flat part of the leaf bent itself down- 
wards, turning from the screen as far as the pin which held it would allow ; and 
the edges, continuing their motion so as to unite below the screen, gave a globular 
shape to the leaf, and thus exposed a considerable portion of the upper surface to the 
reflected light, and shaded from it the under surface. 
In the same experiment, a young leaf which was accidentally shaded on its upper 
surface by the screen, though at some distance, bent its footstalk to avoid it. Thus 
on the same plant, and at the same moment, each leaf took a different, but the most 
appropriate mode to place itself on the best possible position with regard to the light. 
The same experiments tried on the Polemonium cceruleum gave very similar results. 
Many other experiments exhibited the faculty possessed by the flat part of leaves to 
turn itself over independently of the footstalk. Thus, after having ascertained that, 
as had been already seen by Bonnet, leaves turn themselves over when immersed in 
water, just as they do in the air, I placed in a glass full of water some leaves of gera- 
nium, French beans, &c., of which the footstalks were passed through a hole bored 
in a stick, and in such a way as to leave outside the flat part alone of the leaf. When 
its under surface was presented to the light, the flat part of the leaf turned itself over 
in curling up. The same thing happened with leaves whose footstalks had been 
entirely removed, and replaced by a small wooden pin fastened in the hole of the 
stick to make the flat part of the leaf steady. A leaf deprived of its footstalk and 
freely suspended in water so as to expose to light its under surface only, had curled 
itself up and had taken a form somewhat globular, so as to cause a great portion of 
its upper surface to receive the light. 
These experiments seem to show that the flat part of the leaves is far from being 
passive in their turning over, as Dutrochet had supposed. The turning over takes 
