YELLOW RAY PRODUCES LATERAL FLEXURE. 



the flexure towards the prism or the light. In this case we see the stems bend them- 

 selves exclusively towards the space illuminated by the green rays. A little later, when 

 the tendency of flexure towards the prism manifests itself in the stems situated in the 

 yellow rays, they cease to be bent exclusively in the longitudinal direction of the solar 

 spectrum, and follow obliquely the resultant of two rectangular tendencies which so- 

 licit them. The stems situated in the orange and green rays, adjacent to the yellow, 

 soon take on the same oblique lateral flexure ; it manifests itself then in those situated 

 in the red ; then, in the last place, in the green and adjacent blue. In the indigo, the 

 stems remain exclusively bent towards the prism. It is not until about an hour after 

 the stems situated in the yellow rays have commenced to bend laterally, that those sit- 

 uated in the violet, long before bent towards the prism, begin to take a lateral flexure, 

 which is in the opposite direction of that of the red, orange, yellow, blue, and green 

 ones. They bend then obliquely towards the stems situated in the indigo rays." 

 (Extracts from the Comptes Rendus, Juin, 1844.) 



395. MM. DUTROCHET and POUILLET found that, although the point towards which 

 lateral flexure takes place is commonly in the indigo, there are cases in which this 

 is departed from. Once they saw it in the violet, and, in another instance, they 

 met with two points, one, very feebly marked, in the violet, and the other, much more 

 strongly, in the green. While the movement towards the prism often extends a con- 

 siderable distance beyond the two visible extremities of the spectrum, the lateral mo- 

 tion is never seen in plants placed beyond those limits, nor is it manifested at all in 

 feeble lights. 



396. Regarding now the motions of young stems in the prismatic spectrum as taking 

 place in two directions, 1st. Towards the source of the light ; 2d. Towards the indigo 

 rays ; the following tables exhibit the relative activity of the different rays in produ- 

 cing these several effects : 



1st. Direct motion towards the light is brought about by the rays in the following 

 order : 



1. Violet. 



2. Indigo. 



3. Blue. 



4. Lavender. 



5. Yellow. 



6. Green. 



7. Orange. 



8. Red. 



2d. Lateral motion towards the indigo ray is brought about by the rays in the fol- 

 lowing order : 



1. Yellow. 



(Red. 



(Orange. , Greenish blue. 



' \ Greenish yellow. 



It has been suggested that the cause of the lateral motion is the indigo light reflected 

 by the plants which are in that ray ; for, being brightly illuminated by it, they send it 

 in all directions, each plant becoming thus a distant source of disturbance. It is known 

 that the amount of light necessary to produce motion is very small. The circumstance 

 that the lateral motion commences in the yellow, and not in the blue ray, which, from 



