by the apex of each successive elongated figure or ellipse standing nearer to the light than the previous one.  In the case of Cassia (Fig. 177) the comparison of the movement of the hypocotyl, when exposed to a dim lateral light and to darkness, is very instructive; as is that between the ordinary circumnutating movement of a seedling Brassica (Figs. 172, 173), or that of Phalaris (Figs. 49, 174), and their heliotropic movement towards a window protected by blinds.  In both these cases, [page 436] 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 movement 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 and 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 described 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 his paper, ‘Ueber orthotrope und plagiotrope Pflanzentheile’ (’Arbeiten des Bot.  Inst. in Würzburg,’ Band ii.  Heft ii. [[page 437]] 1879), Sachs has discussed the manner in which geotropism and heliotropism are affected by differences in the angles at which the organs of plants stand with respect to the direction of the incident force. [page 437]

The result is that the course is rendered more or less zigzag and unequal in rate.  Lastly, when the light is very bright all lateral movement is lost; and the whole energy of the plant is expended in rendering the circumnutating movement rectilinear and rapid in one direction alone, namely, towards the light.