and of Indigo Light on Plants. 9 



green, were used in the preceding observations ; but the sen- 

 sibility 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 phsenomenon probably 

 ceases in parts which are ligneous. In the seedlings submitted 

 to examination, the motion was found to take place in conse- 

 quence of an action impressed upon the stem only, for the re- 

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

 remarkable fact was observed in all the cases examined; 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 to a 

 minimum. The action of light in producing movement seems 

 therefore to be transient, that is, it is not accompanied with a 

 permanent change of structure in the plant. 



27. From all the foregoing experiments it is demonstrable, 

 that the force which constrains the movements of plants toxvards 

 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 effect. Dr. Draper has discovered the existence of 

 chemical action, distinct from the rays of light or heat, through- 

 out this spectrum, and terms the agent which produces it ti- 

 thonicity. Is that bending of plants here considered produced 

 by the tithonic rays? by heat? or by light? 



29. The investigation of these important problems has cost 

 me much labour, but the following results will show that a 

 satisfactory solution has been attained. 



A trough of plate glass containing persulphocyanide of iron, 

 which has the property of absorbing the tithonic rays of the 

 indigo space, and allowing indigo light to pass, was placed 

 before a small aperture made in the end of a suitable box. 

 The proper place for the whole was determined by receiving 

 the analysed spectrum on a Daguerre plate resting against the 

 box. In a few minutes two stains were observed, with an in- 

 terval corresponding to the indigo light between them. The 

 inactive space was marked upon the wood, and a perforation 

 made without changing the adjustments. Plants placed in the 

 box were bent in two hours, whilst a crop illuminated by 

 indigo rays, which had not been transmitted through the so- 

 lution, did not move with much greater activity, although one 

 crop was exposed to the maximum of the indigo tithonic ray 

 and the other placed in detithonized light. 



