82 Dr Gardner on the Action of Light on Vegetables. 



duction of chlorophyl ; and that the action diminishes on either 

 side, to the termination of the mean red and blue, 



(16.) In this stage of the subject an interesting question 

 suggests itself — Is the active agent light ? some form of chemi- 

 cal ray ? or heat 1 



To discover whether it was due to Tithonicity,* I placed a 

 crop of turnip seedlings in a box, illuminated exclusively with 

 light, which had traversed a solution of bichromate of potassa, 

 sufHciently concentrated to absorb all tithonic rays. The 

 plants became green in about 2 J hours, so as to indicate not 

 only, that detithonised light was capable of producing the 

 green matter, but of doing so with remarkable activity. 

 Hence ^ the formation of chlorophyl is not due to Tithonicity. 



Nor is heat the active principle, for the maxima of heat which 

 has traversed flint-glass, do not correspond with the rays which 

 produce the chief action on etiolated plants. Chlorophyl is 

 therefore produced by the imponderable light, as distinguished 

 from all other known agents found in the sunbeam. 



2. On the movements of plants towards indigo light, 



(17.) Among the most interesting phenomena of plants, is 

 the apparent instinct of bending towards light. The charac- 

 ter of the movement may be seen with ease, by exposing a 

 crop of turnip seedlings near the light of an Argand lamp, pro- 

 vided with an opaque shade. If they be adjusted in such a 

 manner as to leave the leaflets slightly above the lower mar- 

 gin of the shade, the whole will be found inclined forwards in 

 two or four hours. It is this movement I propose to examine. 



(18.) All erect plants obtained in darkness, when exposed 

 to the solar spectrum, in distinct compartments, incline them- 

 selves forward towards the prism. It is, therefore, an efifect 

 which is produced in every variety of light ; even obscure 

 light can accomplish it ; therefore, in researches on this sub- 

 ject every precaution must be taken to darken the place of 



* See Dr Draper's paper in the Lond., Edin., and Dub. Phil. Mag. for 

 Dec. 1842. Tithonicity is the name of an un ponderable agent, supposed 

 to differ from light, by being invisible, and from heat, by not being con- 

 ducted by metals, and incapable of producing the expansion of bodies. 

 From this term, tithonometer and tithonic rays axe derived. 



