92 Dr Gardner on the Action of Light on Vegetables. 



chlorophyl is used, and is due to a colouring matter, insoluble 

 in ether. 



(40.) No ground exists, therefore, for the theory that the 

 autumnal tint of leaves is due to the residual, after the destruc- 

 tion of the green colour. Xanthophyl, which imparts the yel- 

 low, depends on an organic change of chlorophyl, which Berze- 

 lius could not imitate (Journ. de Pharm., JuUiet, 1837). 



Some observations made with a view of determining the ac- 

 tion of indigo light on the green of living plants, brought me 

 to the conclusion, that it faded into a yellowish-green colour ; 

 but I will not speak positively. Plants do, however, lose all 

 their greenness in a dark place, after a greater or less time, 

 and become the colour of seedlings raised without light. In 

 this result my experience is at variance with the statement of 

 Macaire Princep, '' Les feullles d'une plante conservees a I'abri 

 de la lumiere s'en detachent colorees vert." (In Berzelius, 

 Chimie, t. 6, p. 42 ; from Mem. de la Soc. Hist. Nat. de Ge- 

 neve, t. 4.) 



(41.) In the Bleaching of Chlorophyl, as well as in its pro- 

 duction, the active agent is light, for it will take place behind 

 a medium excluding the tithonic rays ; and the points of acti- 

 vity have no relation to the maxima of the calorific spectrum. 

 See Sir John Herschel's paper (Phil. Trans. 1840, part i., 

 p. 51, " On the distribution of the calorific rays of the solar 

 spectrum".) 



(42.) The coincidence shewn to exist between the illumi- 

 nating power, activity of decomposition, and greening effect of 

 yellow light, is conclusive of the discussion respecting the rays 

 which are favourable to the growth of vegetables. 



Blue light cannot be the best, as originally afi[irmed by Se- 

 nebier, and subsequently maintained by Mr Hunt ; nor would 

 a conservatory glazed with cobalt-glass answer the expectations 

 of Professor Johnston. 



(43.) It is impossible to conclude without calling the atten^ 

 tion of physiologists to the remarkable fact, proved in the se- 

 cond part of this paper, — ^that indigo light possesses a solicit- 

 ing power, capable of governing the direction of the stems, 

 peduncles, &c. of plants ; an action accomplished by light in- 

 comparably feeble in comparison with the yellow rays. The 



