Influence of Solar Radiation on Plants. 



63 



The subject may be, however, further elucidated by referring 

 to some of these. Dr. Draper, in his elaborate investigation of 

 the forces which exert a controlling influence on the growth of 

 plants, records a series of experiments on peas. He placed them 

 just after they begun to grow in blue, red, and yellow light, and 

 also in the dark, and in the open air. His observations were 

 confined to the third and fourteenth days. At the former period 

 he found that under the red the plant had attained 4*5 times its 

 original size, and had produced double the number of leaves ; 

 under the blue, three times its original height, with also double 

 the number of leaves. In the dark there was about the same 

 increase of altitude, while in the open air only twice the original 

 height had been attained, and there were no fresh leaves ; and 

 under the yellow light, a still smaller advance had been made. 

 On the fourteenth day he found all his pea-plants green, though 

 varying a little in the character of the color, except those which 

 had been placed in the dark, which were of a pale whitish yellow, 

 the plants vigorous, thirteen times their original height, but with, 

 no fresh leaves. On the whole, then, as far as Dr. Draper's ex- 

 periment goes, it is in accordance with, my results. 



M. Senebier describes an experiment performed by him on 

 lettuce-seeds sown in little cups and placed respectively in the 

 open air in full light of day, in darkness, and under glass vessels 

 filled with colorless, yellow, red, and violet fluids. " Observing 

 then the effects produced by the different portions of light which 

 were thus permitted to act, he found that the plants illuminated 

 by the yellow rays grew most rapidly in height ; next, those in 

 the violet rays ; afterwards those in the red rays. The plants 

 which grew in light transmitted through water were still smaller 

 and approached in size to those which flourished in the open air, 

 while those in perfect darkness attained the greatest height of 

 all. These last plants perished on the eighth day, and those in 

 the yellow light on the ninth day, while all the others continued 

 to vegetate. At the end of about five weeks, the plants growing 

 under the red vessel were 4 inches and 9 lines in height ; under 

 the violet vessel 3 inches and 3 lines ; under the water vessel 2 

 inches and 10 lines, and 1 inch and 3 lines in the open air. With 

 respect to the general appearance of the plants, the leaves of 

 those which grew in red light were smaller and less smooth than 

 those of the plants in violet light, or than the leaves of the 

 plants confined under water, or than the leaves of those which 

 grew in the open air. As to color, the leaves exposed to 

 yellow light were at first green and afterwards became yellow ; 

 those in red light appeared green and preserved a tinge of that 

 color ; those in violet light were quite green and their color aug- 

 mented with their age ; while those raised in obscurity possessed 

 no verdure at all." These experiments were repeated on French 



