Apheliotropic movements are comparatively rare in a well-marked degree, excepting with sub-aërial roots.  In the two cases investigated by us, the movement certainly consisted of modified circumnutation.

The position which leaves and cotyledons occupy during the day, namely, more or less transversely to the direction of the light, is due, according to Frank, to what we call diaheliotropism.  As all leaves and cotyledons are continually circumnutating, there can hardly be a doubt that diaheliotropism results from modified circumnutation.  From the fact of leaves and cotyledons frequently rising a little in the evening, it appears as if diaheliotropism had to conquer during the middle of the day a widely prevalent tendency to apogeotropism.

Lastly, the leaflets and cotyledons of some plants are known to be injured by too much light; and when the sun shines brightly on them, they move upwards or downwards, or twist laterally, so that they direct their edges towards the light, and thus they escape being injured.  These paraheliotropic movements certainly consisted in one case of modified circumnutation; and so it probably is in all cases, for the leaves of all the species described circumnutate in a conspicuous manner.  This movement has hitherto been observed only with leaflets provided with pulvini, in which the increased turgescence on opposite sides is not followed by growth; and we can understand why this should be so, as the movement is required only for a temporary purpose.  It would manifestly be dis-[page 565] advantageous for the leaf to be fixed by growth in its inclined position.  For it has to assume its former horizontal position, as soon as possible after the sun has ceased shining too brightly on it.

The extreme sensitiveness of certain seedlings to light, as shown in our ninth chapter, is highly remarkable.  The cotyledons of Phalaris became curved towards a distant lamp, which emitted so little light, that a pencil held vertically close to the plants, did not cast any shadow which the eye could perceive on a white card.  These cotyledons, therefore, were affected by a difference in the amount of light on their two sides, which the eye could not distinguish.  The degree of their curvature within a given time towards a lateral light did not correspond at all strictly with the amount of light which they received; the light not being at any time in excess.  They continued for nearly half an hour to bend towards a lateral light, after it had been extinguished.  They bend with remarkable precision towards it, and this depends on the illumination of one whole side, or on the obscuration of the whole opposite side.  The difference in the amount of light which plants at any time receive in comparison with what they have shortly before received, seems in all cases to be the chief exciting cause of those movements which are influenced by light.  Thus seedlings brought out of darkness bend towards a dim lateral light, sooner than others which had