Dr. Gardner on the Action of Light upon Vegetables. 17 



(42.) The coincidence shown to exist between the illumina- 

 ting power, activity of decomposition, and greening effect of yel- 

 low light, is conclusive of the discussion respecting the rays 

 which are favorable to the growth of vegetables. Blue light 

 cannot be the best, as originally affirmed by Senebier, and sub- 

 sequently 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 attention 

 of physiologists to the remarkable fact, proved in the second part 

 of this paper — that indigo light possesses a soliciting power, ca- 

 pable 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 color of the sky, therefore, regulate the upright 

 growth of stems to a certain extent 1 Is it not in virtue of the 

 soliciting force therein, that plants continue to grow erect, when- 

 ever other disturbing forces are in equilibrio ? These questions 

 might be investigated with profit, were not this communication 

 too extended already. 



(44.) It is proper to state, however, that DeCandolle's theory 

 of the bending of plants towards light, has been fully disproved. 

 in the context,* in as much as it is effected by the indigo rays 

 which have not power to decompose carbonic acid and produce 

 lignin, &c. (See Mem. Soc. d'Arcueil, 1809, p. 104.) 



In conclusion, it appears that the following facts have been es- 

 tablished. 



1st. That chlorophyl is produced, by the more luminous rays, 

 the maximum being in the yellow. 



2d. This formation is due to pure light, an imponderable dis- 

 tinct from all others. 



* De Candolle advanced a theory to account for the bending of plants towards 

 light, on the following grounds. That as the side of any plant nearest the light 

 was acted on thereby, whilst the distant portions were unilluminated, carbonic 

 acid would be decomposed, and lignin, &c. produced on one side, and not the 

 other. The plant becoming firmer on the part thus furnished with woody fibre, 

 bent over towards the luminous source. 



Vol. xlvi, No. 1.— Oct.-Dec. 1843. 3 



