6 Dr. Gardner on the Action of Yellow 



15. From the experiments I conclude that the centre of the 

 yellow ray is the point of maximum effect in the production of 

 chlorophyll and that the action diminishes on either side to the 

 termination of the mean red and blue. 



1 6. In this stage of the subject an interesting question sug- 

 gests itself; is the active agent light ? some form of chemical 

 ray ? or heat ? To discover whether it was due to tithonicity*; 

 I placed a crop of turnep- seedlings in a box, illuminated ex- 

 clusively with light which had traversed a solution of bichro- 

 mate of potassa, sufficiently concentrated to absorb all tithonic 

 rays. The plants became green in about c 2\ hours, so as to 

 indicate, not only that the detithonized rays were capable of 

 producing green matter, but of doing so with remarkable acti- 

 vity. Hence the formation of chlorophyl is not due to titho- 

 nicity. 



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



has traversed flint glass do not correspond with the rays which 



produce the principal action on etiolated plants. Chlorophyl 



is therefore produced by the imponderable light, as distinguished 



from all other known agents found in the sunbeam. 



Part II. On the Movements of Plants towards indigo light. 



] 7. Among the most interesting phaenomena 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 turnep- 

 seedlings near the light of an Argand lamp provided with an * 

 opake shade. If they be adjusted in such a manner as to have 

 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 them- 

 selves forward towards the prism. It is therefore an effect 

 which is produced in every variety of light, even obscure light 

 produces it; therefore, in researches on this subject, every 

 precaution must be taken to darken the place of experiment. 

 The amount of bending frequently exceeds 90°, and a move- 

 ment of the free extremity of the stem through 1 to 1^ inch 

 from the perpendicular is not unusual in turnep-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 

 will be found inclined diagonally towards a common axis; 

 those in the red, orange, yellow and green bending towards 

 the indigo, and the plants of the violet and lavender moving 

 to meet them. When a large spectrum of fourteen inches was 



* See Dr. Draper's paper in this Magazine, December 1842, p, 457. 



