Dr. Gardner on the Action of Light upon Vegetables. 7 



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, 

 sufficiently concentrated to absorb all tithonic rays. The plants 

 became green in about 2£ hours, so as to indicate not only, that 

 detithonized 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. 



II. 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 character of 

 the movement may be seen with ease, by exposing a crop of tur- 

 nip seedlings near the light of an Argand lamp, provided with 

 an opaque shade. If they be adjusted in such a manner as to 

 leave the leaflets slightly above the lower margin of the shade, 

 the whole will be found inclined forwards in two to 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 themselves 

 forwards towards the prism. It is therefore an effect which is 

 produced in every variety of light — even obscure light can ac- 

 complish it; therefore, in researches on this subject, every pre- 

 caution must be taken to darken the place of experiment. The 

 amount of bending frequently exceeds ninety degrees, and a 

 movement of the free extremity of the stem through one inch to 

 one inch and a half from the perpendicular, is not unusual in tur- 

 nip seedlings. 



(19.) If the young plants be exposed to a spectrum produced 

 as in Art. 13, in a box without compartments, after a time they 



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

 1842. Tithonicity is the name of an imponderable agent, supposed to differ from 

 light, by being invisible; and from heat, by not being conducted by metals and 

 incapable of producing the expansion of bodies. From this term tithonometer and 

 tithonic rays are derived. 



