PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 
257 
^ 2. On the Inclination of Stems towards the Light. 
Every one knows that the branches and green parts of plants have an invariable 
tendency to direct themselves towards the most luminous quarter of the place in 
which they grow. The experiments of M. Teissier, who caused plants to vegetate 
in a cellar where they received air by an opening that did not give light, and light 
by a windovv which did not admit air, have shown that it was really light and not 
air that the plants were in search of. Decandolle imagined that he could ac- 
count for this phenomenon by the inequality of the development of a plant that re- 
ceives light only on one side. The lighted side, he says, decomposes more carbonic 
acid, solidifies a larger portion of carbon in its tissue, and becomes more solid ; at 
the same time it exhales more water, and consequently contains more of the inor- 
ganic matter left behind, which contributes to harden it still further. The other 
side remains softer, the fibres are more elongated, and the consequence is the bend- 
ing of the branch towards the side which is least elongated, that is to say, towards 
the light. This illustrious botanist has extended the same reasoning to the curling 
of tendrils ; but I have already attempted to show that it could not be applied to 
this rapid physiological action in those of the Tamus. 
It was necessary to investigate if there exists a special attraction exercised by 
light on the green parts of a plant, and in particular if a plant so placed as to be 
able to move, could be brought to change its position under the influence of light. 
My experiments on that head have been made under two circumstances of a different 
nature. 
The subjects of the first were some special plants of the family of the Nayadese, the 
Lemna minor and Lemna jjolyrhiza (duck-weed), which naturally float on the water. 
These plants form a kind of anomaly in the vegetable kingdom, in as far as they con- 
sist of a single floating leaf, having submerged roots and producing at their margin 
new leaflets that often detach themselves from the mother plant, like the polypus, to 
form a separate individual ; they are in consequence viviparous. I have nevertheless 
ascertained by experiment that they receive from the action of light the same in- 
fluence as other plants ; thus, if left on the water in complete darkness, the new leaves 
that shoot appear blanched and yellowish. If immersed entirely in spring-water and 
exposed to the rays of the sun, they remain green and vigorous, and emit a great 
quantity of bubbles of oxygen gas that gathers by degrees in the upper part of the 
bell. 
I arranged an oval and elongated vessel so that, being half-full of water, one-half 
of it was kept in darkness by means of a diaphragm which was placed in the middle, 
and kept one line above the surface of the water. The part of the vessel that was 
intended to be kept dark, was carefully covered over with many sheets of black 
paper : the other half of the vessel was freely exposed to the influence of light. 
The apparatus was arranged in a locality perfectly free from disturbance and from 
