2 Dr. Gardner on the Action of Light upon Vegetables. 



pidium sativum, raised in darkness, by the light of two lamps, 

 and De Candolle obtained the same result with six Argand lamps. 



(3.) The investigation has been subsequently confined to the 

 name of the ray which produces chlorophyl.* Formerly it was 

 tacitly admitted on the authority of Senebier that the chemical 

 or blue ray was most active. Prof. Morren (Annal. des Scien. 

 Nat., Oct. 1832) ascribed it to the luminous colors, more espe- 

 cially the rays which had passed through bright yellow and or- 

 ange glasses. Dr. Daubeny (Phil. Trans. 1836) in his valuable 

 researches arrived at the same conclusion. The next investiga- 

 tor, Dr. Draper, (Jour. Franklin Inst. 1837,) obtained better re- 

 sults in yellow than blue light. Mr. Hunt in 1840 (Lond. and 

 Edinb. Phil. Mag. April) resumed the question, and published the 

 most decided results (p. 272) to the effect, that blue light alone 

 causes the green color of plants, and that the yellow and red 

 rays, " destroy the vital principle in the seedy In 1841, he was 

 one of a committee appointed by the British Association to report 

 on this subject, and in a subsequent conversation at the late 

 meeting of that body, has repeated his statements. Being the 

 last writer, his results have given a prominence to the doctrine 

 that chlorophyl is produced by the blue rays, so as to mislead 

 Prof. Johnston in his agricultural lectures, and Prof. Graham. 

 (Chemistry, p. 1013.) 



(4.) In September, 1840, I repeated Mr. Hunt's experiments 

 in Virginia, and obtained dissimilar results. A known number 

 of turnip seeds were sown, and every grain germinated in the 

 yellow and red rays. The greenest plants were found in yellow 

 light. Every condition was favorable, and the results well char- 

 acterized, but my reason for deferring the publication arose from 

 a conviction that the use of solutions and colored glasses was ob- 

 jectionable — and that no perfect results could be obtained except 

 with the spectrum. Plants exposed to light which has permeated 

 cobalt glass, are not placed in blue rays, but in red, yellow, 

 green, blue, indigo, and violet, in proportions differing with the 

 tone of color, and thickness of the material. The effect may 



* Chlorophyl — the green matter of leaves. It is insoluble in water, but soluble 

 in ether and alcohol. The ultimate analysis has not been made ; but chemists 

 agree that it is of the nature of wax. The yellow color of autumnal foliage is 

 due to a similar yellow wax, called Xanthophyl, supposed to be produced by the 

 action of frost on the former substance. 



