ORIGIN AND FORMATION OF ORGANIC MATTER. IN PLANTS. 905 



the electric light for forcing plants. Siemens in England, Bailey in 

 America, and the author in France have made experiments of this 

 character. Since there is no doubt that with the increasing use of 

 water power for the production of electricity, a large supply of electric 

 light can be economically obtained, it is highly interesting to learn 

 what its action is on plants. All observers have found that rays from 

 an arc lamp without a globe exert an injurious influence, blackening the 

 epidermis of the leaves. During the author's experiments in 1881 the 

 epidermis exposed to the direct rays became black, while the parts 

 protected by the upper leaves preserved their beautiful green color. 

 The line of demarcation was as sharp as in a photographic plate. The 

 injurious influence ceased as soon as the lamp was surrounded by a 

 white glass globe through which the ultra-violet rays passed with diffi- 

 culty. To understand the influence which the heat rays situated at 

 the other extremity of the spectrum exert on vegetation, we must 

 recall to mind that in respiration leaves, like all other plant organs, 

 absorb oxygen and exhale carbon dioxid, a process which is precisely 

 the opposite of that which occurs in assimilation. 



It must also be remembered that the activity of respiration increases 

 with elevation of temperature, while rise in temperature has only a 

 very slight effect on assimilation. Maquenne and the author 1 some 

 years ago made a careful study of the action of both light and heat 

 rays on leaves. In this research two sources of light were used, the 

 Drummond light, which is obtained by rendering a piece of quicklime 

 incandescent by means of the oxyhydrogen blowpipe, and the Bour- 

 bouze lamp, which is composed of a cylinder of platinum wire gauze, 

 which becomes incandescent when heated with illuminating gas, the 

 combustion of which is promoted by a strong current of air. The 

 leaves were introduced into tubes containing an atmosphere of known 

 composition and were placed very near the lights, but were protected by 

 screens containing transparent liquids of varying diathermanous prop- 

 erties. In some cases water was used, which allowed the light rays to 

 pass but retained the heat rays. In other cases the screens were filled 

 with benzin or with chloroform, which are also transparent but much 

 more diathermanous than water. Exposing the leaves to the action of 

 the Drummond light, which is poor in heat rays, and surrounding them 

 with a screen filled with water promoted reduction, the proportion of 

 carbon dioxid in the tube diminishing, while the oxygen increased. 

 When the screens were filled with chloroform, however, and the Bour- 

 bouze lamp was used, which is rich in heat rays, the opposite effect was 

 obtained, i. e. 7 the carbon dioxid increased and the oxygen diminished. 

 In this case the phenomena of respiration took the place of those of 

 assimilation. 



Passing from the study of the chemical and heat rays to that of the 

 light rays in the central part of the spectrum, we find that the latter 



1 Aim. Agron., 5 (1879), p. 401. 



