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



placed so as to receive the compound light in its centre — the 

 plants being illuminated in part by the red, indigo and purple 

 rays. In two hours the movements were considerable, and some- 

 what complex. Every plant lighted by the indigo rays, was in- 

 clined directly to that radiant. Those which received red light, 

 were bent to the central purple, and none to the red radiant. But 

 many seedlings at first in the red, inclined themselves towards 

 the purple, and after being fully illuminated thereby, commenced 

 a lateral movement towards the indigo radiant, so that, at the 

 close of the experiment, their stems exhibited two inclinations, 

 one in a vertical, and the other in a horizontal plane. 



(26.) Plants raised in darkness, as well as those which were 

 green, were used in the preceding observations — but the sensibil- 

 ity of the former greatly exceeds that of the latter. Indeed, 

 plants that have been exposed to light for several days, become 

 sluggish in their movements, and the phenomenon probably ceas- 

 es in parts which are ligneous. In the seedlings submitted to 

 examination, the movements were found to take place, in conse- 

 quence of an action impressed upon the stem only, for the removal 

 of the leaflets did not alter the result. A still more remarkable 

 fact was discovered in all the cases observed — that after complete 

 bending, plants erect themselves again, when placed in darkness, 

 at least in situations so dark, as to appear entirely deprived of 

 light. This effect is best seen in seedlings which have never 

 been exposed to the direct rays of the sun ; for, after full and 

 lengthened exposure, it diminishes almost to zero. The action 

 of light in producing movement, seems therefore to be transient, 

 that is, it is not accompanied with a permanent change of struc- 

 ture in the stem. 



(27.) From all the foregoing experiments, it is demonstrable — 

 that the force, which constrains the movements of plants towards 

 light, has its maximum in the indigo ray. 



(28.) But the solar beam contains a number of agents, one of 

 which more especially developes itself in this part of the flint- 

 glass spectrum, acting upon argentine compounds with great ef- 

 fect. Dr. Draper has discovered the existence of chemical action, 

 distinct from the rays of light, or heat, throughout the spectrum, 

 and terms the agent which produces it Tithonicity. Is the bend- 

 ing of plants produced by the tithonic rays ? by heat ? or by 

 light ? 



