108 NATURE AND LIFE. 
these little vessels, he proved that the largest production 
of gas occurred in the tubes exposed to the yellow and 
green light; the next,in the orange andred rays. In 1848, 
Cloéz and Gratiolet discovered the singular fact that the 
action of light on vegetation is more powerful when it 
passes through roughened glass than when transmitted 
through transparent glass. Julius Sachs, more lately, con- 
ceived the idea of measuring the degree of intensity of 
light-action upon aquatic plants, by counting the number 
of gas-bubbles released by a cutting of a branch exposed 
to the sun in water charged with carbonic acid. He thus 
observed that the bubbles thrown off under the influence 
of orange light are very little less numerous than under 
white light, while the branch put under blue light throws 
out about twenty times less. These experiments are deci- 
sive. Neither the chemical nor the calorific rays of the so- 
lar beam act on plants. The luminous rays only, and 
chiefly the yellow and the orange, have that property. To 
these clearly-settled results, Cailletet added a new fact, 
that green light acts on vegetation in the same way as 
darkness. He assigns this reason for the feebleness of 
vegetation bathed in green light under the shade of large 
trees. It is true, this discovery of Cailletet has been 
warmly questioned recently, but it has found defenders too, 
Bert among others; and we shall find soon that it harmo- 
nizes with the whole system of the actions of light in the 
two kingdoms of life." 
A year ago, science had gone thus far, when a very 
distinguished botanist, Prillieux, published the result of a 
course of experiments made with an entirely different pur- 
pose, and taking up the study of the action of light from 
a new pointof view. Resting on the twofold consideration 
that the distinctly-colored rays are not equally luminous, 
1 Bert ascertained that green light stops the motions of sensitive 
plants. 

