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Show 438 MODIFIED CIRCUMNU'l'A'rlON. CIIAP. Ylll. movement of circumnutation which gives rise to, or is converted into, heliotropism and apheliotropism. On this view we need not assume against all analogy that a lateral light entirely stops circumnutation; it mcrclr excites the plant to modify its movement for a tim~ in a beneficial manner. The existence of every pos· sible gradation, between a straio·ht eourso towards a latera] light and a course consisting of a series of loop or ellipses, becomes perfectly intcJ]igiblo. .Finally, tho conversion of circumnutation into heliotropism or apheliotropism, is closely analogous to what takes place with sleeping plants, which durino· tho daytime describe one or more ellipses, often moving in zigzag lines and making little loops; for when they begin in the evening to go to sleep, they likewise expend all their energy in rendering their course rectilinear an(l rapid. In the case of sleep-movements, the exciting or regulating cause is a difference in the intensity of tho light, coming from aLove, at different periods of the twenty-four hours; 'rvhilst with heliotropie aud apheliotropic movements, it is a difference in the intensity of tho light on the two sides of the plant. Transversal-heliotropismus (of Pranlc *) or Diaheliotropism.- The cause of leaves placing themselves more or less transversely to the light, with their upper surfaces directed towards it, has been of late the subject of much controversy. We do not here refer to the object of the movement, which no doubt is that their upper surfaces may be fully illumin~ted, but the means by which this position is gm~ed. Hardly a better or more simple instance can be gmn "' 'Dio natiirlinho Wagercchto Hicl1tun.~ von Pflfmzontheilen,' 1870 f'CHl nlsn someintorcsting articles by the same author, " Zur F rnge u"' b er Trn"n sversal-Go.o -und, Heliotropismus," '13ot. Zertung, 1873, p. 17 et seq. CnAr. VIII. DIAHELIOTROPISM. 439 of diaheliotropism than that offered by many seedlings, the cotyledons of which are extended horizontally. When they first burst from their seed-coats they are in con tact and stand in various positions, often vertically upwards; they soon diverge, and this is effecte1l by epinasty, which, as we have seen, is a modified form of circumnutation. After they have diverged to their full extent, they retain nearly the same position, though brightly illuminated all day long from above, with their lower surfaces close to the ground and thus much shaded. ~rhcre is therefore a great contrast in the degree of illumination of their upper and lower surfaces, and if they were heliotropie they would bend quickly upwards. It must not, however, be supposed that such cotyledons are immovably fixed in a horizontal position. vVhen seedlings are exposed before a window, their hypocotyls, which are highly heliotropic, bend quickly towards it, and the upper surfaces of their coty lcd.ons still remain exposed at right angles to the light; but if the hypocotyl is secured so that it cannot bend, the cotyledons themselves change their position. If the two are placed in the line of the entering light, the one furthest from it rises up and that nearest to it often si~ks down; if 1)laced transversely to the light, they twrst a little laterally; so that in every case they endeavo~r to place their upper surfaces at right angles to the light. So it notoriously is with the leaves on plants nailed a~ainst a wall or oTown in front of a • v ' b < wmdow. A moderate amount of light suffices to in- (~Uce such movements; all that is necessary is that the h~ht ~hould s~cadily strike the plants in an oblique drrect10n. With respect to the above twisting movement of cotyledons, Frank has given many and much more striking instances in the case of the leaves on |