PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 
269 
place either by the action of the footstalk, or by the motion of the flat part, accord- 
ing as each of these modes will most conduce to the final result. Thus, when I 
immersed in water an entire branch of geranium so as to expose the under surface 
of its leaves to the light, all the young leaves turned themselves over in three days by 
moving on the point of insertion of the flat part of the leaf into the footstalk, and 
without the curling-up of the leaf. It has been the same when the footstalk has been 
kept on a leaf fixed in the bored stick, so as to allow the footstalk perfect freedom of 
motion in placing it in the hole. 
I have endeavoured to ascertain whether this action of light on the surface of 
leaves, which so evidently exists, would be so powerful as to excite in them a real 
physical attraction or repulsion. With this view I placed on a moveable cork-float 
some leaves of the raspberry-shrub {Ruhus idoeus), which are among those whose 
turning over is most energetic. I placed the float on a glass full of water, and 
arranged the leaves so that their under surface alone was exposed to light, all the 
other sides of the apparatus being carefully darkened by means of black paper 
screens. The whole was kept free from all vibration, and an index showed the posi- 
tion of the float by means of a graduated circle. The leaves turned themselves over 
by a torsion of the footstalk at the point where the flat part is inserted into it, but 
there was not the least motion in the float. 
The same experiment was repeated many times with leaves of French beans, maple 
{Acer pseudoplatanus), clover {Trifoliuni pratense) , &c., and gave the same results. 
In a leaf of geranium placed under the same circumstances, the turning over took 
place by a bending of the footstalk, which thus brought down the flat part of the leaf 
so as to present the upper surface to the light. In reversing the apparatus, and 
replacing the leaf in an inverted position, the footstalk raised itself up and regained 
its primitive situation. There was no motion whatever in the float. The same result 
was obtained with a leaf of Camellia japonica. 
When the flat part of the leaf was alone the subject of experiment, it curled up 
without the float changing its position. 
When the plant itself was left floating freely in the water, properly counterpoised, 
the leaves placed in an inverted position with respect to the light turned themselves 
over by a motion of the footstalks ; but the body of the plant itself remained stationary. 
It would appear, therefore, that there is neither attraction nor repulsion, in the ma- 
terial and physical meaning of the word, between light and the surface of leaves. 
In compound leaves, the turning over sometimes takes place on the common foot- 
stalk, as in the raspberry-shrub, and sometimes on the particular footstalk of each 
leaflet, which turns itself either on the point situated near the insertion of the foot- 
stalk to the flat part of the leaf, as in clover, or else on the point of insertion to the 
common footstalk, as in the horse-chestnut. The removal of one or more leaflets 
does not prevent the remaining portion turning itself over when placed towards the 
light in an inverted position. 
2 N 2 
