Flammarion found in his experiments with sen sitive plants that red light accelerated growth the most, this being followed by green, white and blue light, in the order named. His experiments were made in a small conservatory behind clear and colored glass, which, however, did not in all cases furnish strictly monochromatic light. Other obser vers have shown that plants grow more vigorously in orange rays and that they resemble those which grow in darkness, while those subject to blue light resemble plants grown in daylight. While orange light produces effects similar to those in plants grown in darkness,—that is, they develop small leaves and elongated internodes, resembling etio lated plants,—their leaves are green. On the other hand, blue light prevents the expansion of the cotyledons in some cases, and, since it does not induce photosynthesis, there is little need of their expanding.
The effect of orange light on the growth of fungi is similar to that brought about by darkness. For example, the aerial hyphen of Pilobolus become greatly elongated when grown in darkness or in orange light. Blue light, however, induces irritable movements or heliotropic curvatures. Sachs found that the elimination of the ultra-violet rays has an effect on the production of flowers, causing a less luxuriant development of them. The accurate ex periments of Englemann, Reinke and Timiriazeff have shown that photosynthesis in green plants reaches its maximum in the red and orange rays of the spectrum between the lines B and C. In the case of the red algm, however, the region of maxi mum assimilation is somewhat different, since the greatest photosynthetic activity is shown between the yellow and green bands, while in the blue-green algm this occurs between the orange and yellow. There is some reason to believe that such pigments as phycoerythrin, found in the red algm, may pos sess some ecological significance, since the plant frequently grows at considerable depths in the ocean. The most active assimilation is caused in the purple bacteria in the infra red rays or those rays having wave lengths of 800 to 900 g g.
Some investigators have noted an injurious effect of the green rays on certain plants. This may be accounted for by unlike methods used in experi menting, although it is well known that different plants respond in a different way to the same light stimulus. Plants respond to the ultra-red and ultra violet rays, which are well known to make no impression on the retina ; and the same may be held to be true in regard to other forms of radiant energy. It has been shown that electrical radia tions characterized by wave lengths vastly longer than the last visible red rays are able to produce certain physiological effects on plants, but whether this will apply to the Röntgen and Becquerel rays has not been definitely proved.
There is little likelihood of monochromatic light being employed to advantage in growing crops, since plants are best adapted to mixed rays, such as occur in sunlight.
Literature.
J Reinke, Untersuchungen fiber die Einwirkung des Lichtes auf die Sauerstoffausscheidung der Pflanzen, I. Mitt. Bet. Ztg., XVI, also, II, Mitt. Bet. Ztg. XLII; T. W. Engelmann. Various papers in Arch. f. d. Ges. Physiol., Vols. XXV, XXVI, XXVII, XXIX, XXX ; Cf. Bot. Ztg., Bd. XLI, XLII, XLVI ; Timiriazeff, Ann. de Chim. et de physiq., Vol. XII, Comp. Rend., Vol. CX, 1890 ; J. W. Draper, On the Decomposition of Carbonic-acid Gas by Plants in Prismatic Spectrum, American Journal of Science, XLVI ; J. W. Draper, Scientific Memoirs, 1878 ; C. Flammarion, Etude de ('action des diverses radiations du spectre solaire sur la vegetation, Comp. Rend., CXXI, 1895.
Some of the more important literature on artifi cial light in its relation to the growth of plants is as follows: L. H. Bailey, Some Preliminary Studies of the Influence of the Electric Arc Lamp upon Green house Plants, Bulletin No. 30, Cornell Experiment Station, August, 1891 ; Second Report upon Elee tro-Horticulture, Bulletin No. 42, September, 1892; Third Report upon Electro-Horticulture, Bulletin No. 55, July, 1893. (Subsequent studies have never been published.) G. Bonnier, Influence de la lumiere electrique continue sur la forme et la structure des Plantes, Revue general de botanique, Tome VII, 1895 ; F. IV. Bane, Electro-Horticulture, Bulletin No. 37, West Virginia Experiment Sta tion, July, 1894 ; C. IV. Siemens, On the Influence of Electric light on Vegetation, and on Certain Physical Principles Involved, Proceedings Royal Society, XXX, 210-219 ; and Some Further Obser vations on the Influence of Electric Light on Vegetation, Proceedings Royal Society, XXX, 293 295, by the same author ; C. W. Siemens, On Some Applications of Electric Energy to Horticultural and Agricultural Purposes, Report of British Asso ciation of Advanced Science, LI, 474-480 ; P. P. Deherain, Untersuchungen fiber des Einfluss d. Elektrischen Lichtes auf das Wachsthum d. Han zen, Annales Agronomiques, T. VII, 81, P. 551-575 ; Wollny, Forschungen auf d. Geb. der Agricultur physik, Bd. V, p. 488; M. J. Iorns, Acetylene Light for Forcing Plants, Cornell Countryman, May, 1906 (from this article Figs. 45 and 46 are redrawn).