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Show '412 MODIFIED CIRCUMNUTATION. CIIAP. YII. in ~aining quickly its proper nocturnal position by a direct c<mrse. In ::;evoral other cases, for instance, when a loaf after describing during the day one or more fairly regular ellipses, zigzags much in the evening, it appears as if energy was being expended, so that the great evening rise or fall might coin· cide with the period of the day proper for this movement. The most complex of all the movements performed by sleeping plants, is that when leaves or leaflets, after describing in tho daytime several vertically directed ellipses, rotate greatly on their axes in the evening, by which twisting movement they occupy a wholly different position at night to what they do during the day. For instance, tho terminal leaflets of Cassia not only move vertically dowmvard.s in the evening, but twist round, so that their lower smfaces face outwards. Such movemouts arc wholly, or almost wholly, confined to l aflets provided with a pulvinus. But this torsion is not a new kind of movement introdueed solely for tho purpose of sleep; for it has been shown that some leaflets whilst describing their ordinary ellipses during the daytime rotate slightly, causing their blaaes to face first to one side and then to another. Although we can see how the slight periodical movemonts of leaves in a vertical plane could be easily converted into the greater yet simple nyctitropic movements, we do not at present know by what graduated steps the more comp!e~ movements, effected by the torsion of the pulnm, have been acquired. A probable explanation. co~Jid be given in each case only after a close invest1gatwn of the movements in all the allied forms. From the facts and considerations now advanced we may conclude that nyctitropism, or the sleep of leaYeS CrrAP. VII. MODIFIED CIRCU:MNUTATION. 413 and cotyledons, is merely a modification of their ordinary circumnutating movement, regulated in its period and amplitude by the alternations of light and darkness. The object gained is the protection of the upper surfaces of the leaves from radiation at night, often combined with the mutual protection of the several parts by their close approximation. In such cases as those of the leaflets of Cassia-of the terminal leaflets of !Ielilotus-of all the leaflets of Arachis, Marsilea, &c.-we have ordinary circumnutation modified to the extreme extent known to us in any of the several great classes of modified circumnutation. On this view of the origin of nyctitropism we can understand how it is that a few plants, widely distributed throughout the Vascular series, have been able to acquire the habit of placi~g the blades of their leaves vertically at night, that 1s, of sleeping,--a fact otherwise inexplicable. The leaves of some plants move during the day in amanner, which has improperly been called diurnal s~eep; for.when the sun shines brightly on them, they direct the1r edges towards it. ~J..1o such cases we shall recur in the following chapter on Heliotropism. It has been shown that the leaflets of one form of Porlieria hygrometrica keep closed during the day, as ~ng as the plant is scantily supplied with water, in e same manner as when asleep; aud this apparently serves to check evaporation. ~rhero is only one other ~nalo~ous case known to us, namely, that of certain ~rammeru, which fold inwards the sides of their narrow deaves, when these are exposed to tho sun and to a hry atmosphere, as described by Duval-J ouve. * vVo ave also observed the same phenomenon in Elymus are~UJreus. *'Annal. des Sc. Nat. (Bot.),' 1875, tom. i. pp. ~2G-329. |