86 Dr Gardner on the Action of Light on Vegetables. 



it diminishes almost to zero. Tlie action of light in produc- 

 ing movement seems therefore to be transient ; that is, it is not 

 accompanied with a permanent change of structure in the stem. 



(27.) From all the foregoing experiments, it is demonstra- 

 ble, that the force which constrains the movements of plants to^ 

 wards lights 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 che- 

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

 out the spectrum, and terms the agent which produces it, 

 Tithonicity. Is the bending of plants produced by the tithonic 

 rags ? by heat ? or by light ? 



(29.) The investigation of these important problems has 

 cost me much labour ; but the following results will shew 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 side of a suitable box. 

 The proper place for the hole 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 interval 

 between them, corresponding to the place of the indigo light. 

 The inactive space was marked on the wood, and a perforation 

 made in its centre, without deranging the adjustment, so that 

 the aperture continued to admit detethonised light. Plants 

 placed in this box were bent in two hours, whilst a crop illu- 

 minated by indigo rays, which had not been transmitted 

 through the solution, did not move with much more activity, 

 although one crop was exposed to the maximum of the indigo 

 tithonic rays, and the other placed in detithonised light. 



(30.) Solution of bichromate of potash intercepts nearly all 

 tithonic matter, but permits the free passage of luminous rays. 

 A crop of turnip seedlings was introduced into a box and illu- 

 minated by the yellow rays of the spectrum, analysed by this 

 solution. A daguerre plate was also introduced, to serve as a 

 test of chemical action. In two hours and a half the plants 



