13U RELATION BETWEEN Chap. VIU 



and in many others, it was interesting to notice how 

 gradually the stems began to circumnutate as the 

 light waned in the evening. We have therefore many 

 kinds of gradations from a movement towards the light, 

 which must be considered as one of circumnutation 

 very slightly modified and still consisting of ellipses 

 or circles, — though a movement more or less strongly 

 zigzag, with loops or ellipses occasionally formed, — to 

 a nearly straight, or even quite straight, heliotropic 

 course. 



A plant, when exposed to a lateral light, though 

 this may be bright, commonly moves at first in a 

 zigzag line, or even directly from the light ; and 

 this no doubt is due to its circumnutating at the 

 time in a direction either opposite to the source of 

 the light, or more or less transversely to it. As soon, 

 however, as the direction of the circumnutating move- 

 ment nearly coincides with that of the entering light, 

 the plant bends in a straight course towards the light, 

 if this is bright. The course appears to be rendered 

 more aud more rapid and rectilinear, in accordance with 

 the degree of brightness of the light — firstly, by the 

 longer axes of the elliptical figures, which the plant 

 continues to describe as long as the light remains very 

 dim, being directed more or less accurately towards 

 its source, and by each successive ellipse being de- 

 scribed nearer to the light. Secondly, if the light 

 is only somewhat dimmed, by the acceleration and 

 increase of the movement towards it, and by the 

 retardation or arrestment of that from the light, some 

 lateral movement being still retained, for the light 

 will interfere less with a movement at right angles 

 to its direction, than with one in its own direction.* 



" In Ma paper, ' Ufber ortho- tlieile' (' Arbeiten des Bot. Inst 

 trope und pUigiotrope Pflanzeu- in Wurzburg,' Band ii. lleft U 



