14 Action of Yellow and of Indigo Light on Plants, 



are active in forming chlorophyl. Upon longer exposure the 

 subordinate action along the yellow, &c. occurs, but not until 

 the other portions are perfectly bleached. 



In Sir John Herschel's experiments there remained a sal- 

 mon colour after the discharge of the green. This is not 

 seen when chlorophyl is used, and is due to a colouring matter 

 in the leaf soluble in water, but insoluble in aether. 



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

 autumnal tint of leaves is due to the residual after the de- 

 struction of the green colour. The xanthophyl, which imparts 

 the yellow, depends on an organic change of chlorophyl, 

 which Berzelius could not imitate (Journ. de Pharm., Juillet 

 1837). 



Some observations made with a view of determining the 

 action of indigo light on the green of living plants, brought 

 me to the conclusion that it faded into a yellowish green 

 colour under its influence ; 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 of the colour of seed- 

 lings raised without light. In this result my experience is at 

 variance with the statement of Macaire Princep : " les feuilles 

 d'une plante conservees a l'abri de la lumiere s'en detachent 

 colorees vert" (in Berzelius, Chimie, t. 6. p. 42). 



41 . In the bleaching of chlorophyl, as well as in its produc- 

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

 medium excluding tithonicity, and the action has no connec- 

 tion with the maxima of the calorific spectrum. 



V2. The coincidence shown between the illuminating power, 

 activity of decomposition of carbonic acid, and greening effect 

 of yellow light, is conclusive of the discussion respecting the 

 rays which are favourable to the growth of vegetables. The 

 blue rays cannot be the best, as originally affirmed by Sene- 

 bier, and subsequently maintained by Mr. Hunt, nor would a 

 conservatory glazed with cobalt glass answer the expectations 

 of Professor Johnston. 



4-S. It is impossible to conclude without calling the atten- 

 tion of physiologists to the remarkable fact proved in the 

 second 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, 

 incomparably feeble in comparison with the yellow rays. The 

 blue of the atmosphere is scarcely less intense when compared 

 with the sun's beams. Does not the colour of the sky, there- 

 fore, regulate the upright growth of stems to a certain extent ? 

 Is it not in virtue of the soliciting force therein that plants con- 

 tinue to grow erect whenever other disturbing forces are in equi- 



