100 INDIGO RAY CONTAINS THE ACTIVE FORCE. 



larity by a lateral light recover their original position in the dark, but in the case of 

 the roots the flexure remains. 



390. Several observers have occupied themselves with determining the particular 

 spectrum-ray which directs these singular movements. In 1817, Dr. FOGG ion at- 

 tempted the solution of the problem in the case of the stems, by comparing the flexures 

 obtained in the violet and red rays. His experiments were, however, roughly made ; 

 for, not possessing a heliostat, or any means of rendering his spectrum stationary, he 

 resorted to the tedious and uncertain process of following the movements of the ray in 

 the dark chamber by the hand. He came to the conclusion that the violet was three 

 times as powerful as the red ray. 



391. The first correct solution of this problem was obtained in ]843 by Dr. GARD- 

 NER, and less perfectly, about the same time, by M. PAYER, whose memoir, presented 

 to the French Academy, was reported upon in June, 1844, by a committee, consisting 

 of MM. MIRBEL, BECQUEREL, POUILLET, and DUTROCHET. This commission repeated 

 the experiments of Dr. GARDNER, and verified them, and corrected those of M. PAYER. 



392. "On exposing a crop of young turnips which had been raised in the dark to a 

 solar spectrum fixed by a heliostat, Dr. GARDNER discovered that the young stems bent 

 in two different directions : 1st. They bent towards the prism, or the light, and that un- 

 der the influence of all the coloured rays; 2d. They bent towards the space which is 

 illuminated by the indigo rays, so that the plants illuminated by the red, orange, yellow, 

 green, and blue rays, on one side, incline towards the space illuminated by the indigo 

 rays ; while, on the other side, the plants illuminated by the violet incline towards the 

 same indigo region, the crop taking the appearance of a field of corn blown by two 

 opposite winds. The plants which are illuminated by the indigo rays offer but one 

 of these tendencies to flexure, that which directs them towards the prism or towards 

 the light." " From these experiments, he concludes that the force which produces the 

 bending of stems laterally in the solar spectrum resides in the indigo ray. For these 

 experiments etiolated plants are preferable to green ones : they are much more easy 

 to bend. It requires but one or two hours for the phenomena to manifest themselves. 

 They are equally observed in making use of the light of the moon." 



393. " The first movement which is observed in vegetable stems is that of their flex- 

 ure towards the prism, that is to say, towards the light. This movement always com- 

 mences to manifest itself in the stems submitted to the violet rays. To this we have 

 never seen an exception. The motion of flexure towards the prism is then exhibited 

 by the steins iu the. indigo and blue, and ordinarily, at the same time, in the lavender 

 rays, which form the almost invisible continuation of the spectrum beyond the violet. 

 Then the steins situated in the yellow and green rays commence to bend ; then follow 

 those in the orange. The stems which are situated in the red bend the last." 



394. " We have hitherto said nothing of the lateral flexure of the stems discovered 

 by Dr. GARDNER, a phenomenon which does not commence to manifest itself until half 

 an hour, or even an hour, after the flexure of the stems towards the prism in the violet 

 rays. This lateral flexure or bending in the direction of the length of the solar spec- 

 trum commences always in the stems situated in the yellow, and there it often precedes 



