Dr Gardner on the Action of Light on Vegetables. 93 



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 

 continue to grow erect, rvhenever 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 De CandoUe's theory 

 of the bending of plants towards light, has been fully dis- 

 approved in the context,* inasmuch as it is effected by the indigo 

 rays which have not power to decompose carbonic acid and pro- 

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



In conclusion, it appears that the following facts have been 

 established : — 



1^^, 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 

 distinct from all others. 



3fl?, That the ray towards which plants bend occupies the 

 indigo space of Fraunhofer. 



Ath, This movement is due to pure light, as distinguished 

 from heat and tithonicity* 



bth. That pure light is capable of producing changes which 

 result in the development of palpable motion* 



Qth, The bleaching of chlorophyl is most active in those 

 parts of the spectrum which possess no influence in its produc- 

 tion, and are complimentary to the yellow rays. 



1th, This action is also due to pure light. Wehave^ there- 

 fore, an analysis of the action of every ray in the luminous 

 spectrum upon vegetation. The several effects produced are 

 not abruptly terminated within the limits of any of the spaces, 

 but overlap to a certain extent, a fact which coincides with 

 our experience of the properties of the rays. Whilst heat and 



* 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, Ciirbonic acid would be decomposed, and lignin, &c., pro- 

 duced on one side and not the other. The plant becoming firmer on 

 the part thus furnished with woody fibre, bent over towards the lumi- 

 nous Source. 



