1-^6 ANATOMY AKD PHYSIOLOGY OF 



on one side of a plant, the lattei^ becomes more or less diverted 

 out of its normal direction. 



It is an every-day's experience that this is the case in a high 

 degree with young and still soft stems, since in plants which re- 

 ceive light on one side only, the stems curve strongly towards the 

 side whence the light comes. In plants which are veiy sensitive 

 to light, like germinating Cruciferse, I found that the influence ot 

 light might wholly overcome the effect of gravity, for when I sus- 

 pended some of these in a horizontal direction, in a blackened box, 

 closed on all sides and at the top, and lighted through the open 

 lower end hy a mirror, the plarits were compelled to turn their 

 stems vertically downwards. 



This curvature is produced in unequal degrees by the differently 

 coloured rays of the spectrum, and this independently of their 

 illuminating power, being caused most of all by the blue rays ; 

 and, according to Payer (" Comptes rendus'' xvii), in general 

 only by that part of the spectrum lying between F and H, a 

 statement, which, however, would undergo some modification 

 through the experiments of Dutrochet (which, it is true, were not 

 made with the aid of a heliostat, but with red glass, and conse- 

 quently were imperfect), for according to these experiments, even 

 the red rays produced curvature, although in slight degree (^'Ann, 

 des. So. nat 2 Sir/' xx. S29). 



It is proved that this curvature is produced by the illuminated 

 side, and that the convex side only mechanically follows its cur- 

 vature, by the fact that when the concave side is removed from 

 the convex side by a longitudinal incision, it curves more strongly 

 than before, and the convex side springs back into the upright 

 position (Dutrochet, " Memoires" ii. 74). 



Ohserv. The fact last stated completely refates De Oandolle's explana- 

 tion, which appears, at first sight, to give a very simple cause for the cur- 

 vature of the stem towards light falling on it. De Oandolle thought 

 C'Mim. d. 1 8ooiU% dPArcueU;' 1809, ii. 104:) that in accordance with the 

 common experience of plants which receive hut little hght growing very 

 mxich longitudinally ; plants which received hght only on one side would 

 grow much longer on the dark side than on the lighted side, and thus 

 woxild curve towards the source of Hght. 



We find a similar contrast in the dependence of stem and root 

 upon light, to that of the movements produced in these parts by 

 gravity, for the root turns away from the light ; a phenomenon 

 which was first discovered by Dutrochet in germinating plants of 

 YisGum album, was subsequently demonstrated more extensively 

 by Payer Q'Gomptes rendus/' xvii), Durand, and Dutrochet 

 Q'Ann. des. So. nat 8 S^r/' v. 65), by experiments, chiefly on the 

 roots of Cruciferse and Compositse: and of which proof may fre- 

 quently be obtained in hot-houses, from the aerial roots of Oacfus 

 grandifiorus and other plants. The only cases in which this re- 



