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Show [)64 SUMMARY AND CHAP. XII. Heliotropism prevails widely throughout the vegetable kingdom, but whenever, from th changed habits of life of any pla.nt, such movements become injurious or us less, the tendency is easily eliminated, as we see with climbing and insectivorous plants. Apholiotropic movements are comparatively rare in a well-marked degree, excepting with sub-aerial roots. In tho two cases investigated by us, tho movement certainly consisted of modified circumnutation. The position which loaves and cotylouons occupy during the day, namely, more or less transversely to the direction of the light, is due, accoruing to Frank, to what we call diaheliotropism. As all loaves <tn(l cotyledons are continually circumnutating, there can hardly be a doubt that diaheliotropism results from modified circumnutation. From tho fact of leaves and cotyledons frequently rising a little in tho evening., it appears as if diaholiotropism had to conquer dunng the middle of the day a wiuely prevalent tendency to a pogeotropism. Lastly, the leaflets and cotyl dons of some plants are known to be injured by too much light; and when the sun shines brightly on them, they move up,~ards or downwards, or twist laterally, so that they ducct their edges towards the light, and thus they escape being injured. rrhese paraheliotropic mov~ments certainly consisted in one case of modified cucumnuta· tion; and so it l>robably is in all cases, for ~he leaves of all the species describeJ circumnutate m a con· spicuous manner. rrhis movement has hithort~ ~e~n observed only with leaflets provided with pulvml, 1? which the increased. turo·oscence on opposite sides 18 not followed by o-rowth ; band we can unders~and why this should be sob as the movement I·S reqm red odn'l y f or a tem Porary P'u I'IJose . It would manifestly be lS· CHAP. XII. CONCLUDING REMARKS. 565 adv.a ntageous. .f or the loaf. to bo fixed by th · · ~ grow m Its inc~med positi.o?. For It has to assume its former horizontal positiOn, as soon as possible aft th d h. . , er o sun has cease s 1mng too brightly on it. . The extreme. sensi ti ~eness of certain seedlings to hght, as shown In our ninth chapter, is highly remarkable. The cotyledons of Phalaris became curved towards a distant lamp, which omitted so little ]' o·ht that a pencil held vertically close to the plants:bdici not cast any shadow which the eye could percoi on a white card. These coty leclons, therefore we:e affect~d by a ~ifference in he amount of light 0~ their two sides, whwh the eye could not distinguish. Tho degree of their curvature within a given time towards a lateral light d~d not c?rrespond at all strictly with the amount of hght whwh they received; the lio-ht not being at any time in excess. They continued bfor nearly half an hour to bend towards a lateral lio-ht after it had been extinguished. They bend ~ith rem~rkab~e p.recision towards it, and this depends on the Illummatwn of one whole side, or on the obscuration of the whole opposite side. The difference in the amo1mt of light which plants at any time receive in co~parison w~th what they have shortly before received, seems In all cases to be the chief excitino· cause of those movements which are influenced byb light. Th~s seedlings brought out of darkness bend towards a.d1m lateral light, sooner than others which had preVIously been exposed to daylight. We have soon several analogous cases with the nyctitropic movements of leaves. A striking instance was observed in the case of the periodic movements of the cotyledons of a 0a ss·i a,. m· the morn·m g a pot was placed. in an obscure part of a room, and all th cotyledons rose up closed; another pot had stool in the sunlight, and. |