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PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 
I wished to ascertain which of the rays of light had the most influential action in 
producing the turning over of leaves. In order to make absolutely conclusive expe- 
riments on this subject, it would have been necessary to operate by means of the rays 
of light analysed by the prism ; but I had not at my disposal the apparatus of a he- 
liostat, &c., which would have been necessary, and had I possessed it, it would pro- 
bably have been difficult to arrange the experiment properly ; I was consequently 
obliged to confine myself to the use of coloured glasses. But with the view of 
ascertaining what degree of confidertce they deserved, I analysed with a flint-glass 
prism a ray of light transmitted thjrough each of the coloured glasses I made use of. 
I could thus easily find out their degree of purity. The following are the results of 
this trial. 
Glass coloured red by protoxide of copper — spectrum perfectly pure. 
Glass coloured blue by cobalt — spectrum contains a little red. 
Glass coloured green by chrome — spectrum pure contains very little yellow. 
Glass coloured yellow by silver — spectrum contains many orange rays. 
Glass coloured violet by manganese — a very fine spectrum, but it contains red rays 
and a few others, chiefly blue. 
For the purpose of observing the effect of coloured glasses on the turning over of 
leaves, I made use of only three colours, — blue, red and violet. 
Three leaves of the same size and age, taken from the same geranium, were placed 
in an inverted position towards the light ; the first behind a blue glass, the second 
behind a red, and the third behind a violet one. In all these experiments, and they 
were repeated a great many times, the leaves turned themselves over in the violet 
and blue rays, and remained motionless in the red. The most remarkable effect was 
in the blue. 
It now remained to investigate the influence of light acting on the two surfaces of 
leaves with relation to the physiological functions of these organs, so important in 
vegetative life. These functions are chiefly the exhalation and the decomposition of 
carbonic acid. I propose now to examine the influence of light under these two heads. 
§ 4. Action of Light on Exhalation. 
Every one knows that plants exhale in the atmosphere a large quantity of water, 
and that this function is chiefly ascribed to the leaves. To these organs, indeed, the 
exhalation of water, properly so called, seems to belong ; the other parts of the plant 
losing only an infinitely smaller portion of water, apparently the result of the com- 
mon evaporation which every humid body experiences in the air. Decandolle calls 
this last-mentioned loss of water deperdition, to distinguish it from exhalation properly 
so called. 
Senebier has alleged that the vegetative exhalation is nothing, or nearly so, in the 
dark. This may be true when the temperature is very low and the atmosphere very 
damp; but I have ascertained, in accordance with Dutrochet’s results, that in summer 
