102 BENDING OF ROOTS FROM THE LIGHT. 



its position, might be supposed to be under the most powerful influence, as being 

 nearest to the source of disturbance, admits of a ready explanation ; for in the yellow 

 ray the tendency to move towards the prism is so small, that a slight lateral force is 

 sufficient to turn the stems entirely in any direction, while in the blue, the tendency 

 to the prism being very great, the lateral motion is correspondingly less apparent. 



397. The indigo ray thus directing the movements of stems, Dr. GARDNER suggests 

 whether this may not be one of the causes which bring about the upright growth of 

 trees. Obviously, upon these principles, the blue colour of the sky, acting like the blue 

 portion of the spectrum, ought to exert an influence on steins, bringing them into a 

 perpendicular position ; but that this does not account for the phenomenon entirely 

 is evident from the circumstance that, although stems may be made to grow in total 

 darkness, they are nevertheless upright. 



398. The roots of many plants, as has been said, recede from the light. When 

 white mustard is made to grow upon the surface of water, the seeds being supported 

 on pieces of cork, the roots bend from the light and the stems towards it. If this 

 experiment be made in the spectrum, after the course of a few hours it will be found 

 that this motion of retrocession commences in the violet ray, then goes on in the indigo 

 and blue, then in the lavender. Thus the first movement of the roots occurs in the 

 same ray as that of the stems, and the order for the other rays appears to be nearly 

 the same. 



399. A lateral flexure for the roots is also observed. It commences in the yellow, 

 then in the orange, then in "the green rays which are next to the orange, then in the 

 green and blue. It is directed towards the red space. This flexure being combined 

 with that of retrocession from the light which crosses it at right angles, the points of 

 the roots are curved in the resultant of these tendencies. 



400. " Thus the retrocession from the light by the roots commences in the yellow 

 rays, as is the case for the flexure of the stems to the light, and for their lateral flexure ; 

 but all the movements of the roots are the inverse of those of the stems. For the 

 stems there is flexure towards the light and flexure towards the space illuminated by 

 the indigo rays, while for the roots there is flexure to retreat from the light and flexure 

 to retreat from the indigo space. The latter mode of flexure is not presented to us in 

 roots submitted to the red rays, nor in those which are in the violet, and this probably 

 because in those two rays the tendency for lateral movement is to the feeblest extent. 

 Even in the case of stems, when the light is not sufficiently intense, lateral flexure does 

 not manifest itself in those same red and violet rays." (Comptes Rendus.} 



401. In estimating the position of the ray of maximum action in these experiments, 

 it should be recollected that there is a difference between its apparent and true place, 

 that difference being greater according as the ray is more refrangible. This arises 

 from the circumstance already pointed out, that at the violet end of the spectrum the 

 colours are widely spread out (183). The increased dispersion which these rays un- 

 dergo of course affects the position of the maximum point, and renders the force of 

 the ray less than it ought to be. If the prism gave the same concentration to the in- 

 digo and violet region which it gives to the red and orange, the relative activity of 



