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Show I 402 MODIFIED CIRCUMNUTATION. CHAP. III. of the petioles are not performed for any special purpose ; though a conclusion of this kind is generally rash. vVhen the leaflets sink vertically down at night and the petioles rise, as often occurs, it is certain that the upward movement of the latter docs not aid the leaflets in placing themselves in their proper position at night, for they have to move through a greater angular space than would otherwise have been necessary. Notwithstanding what has just been said, it may be strongly suspected that in some cases the rising of the petioles, when considerable, does beneficially serre the plant by greatly reducing the surface exposed to radiation at night. If the reader will compare the tv..-o drawings (:B ig. 155, p. 371) of Cassia pubescens, copied from photographs, he will see that tho diameter of the plant at night is about one-third of what it is by day, and therefore the surface exposed to radiation is nearly nine times less. A similar eonclusion may be deduced from the drawings (Fig. 149, p. 358) of a branch awake and asleep of Desmodium gyrans. So it was in a very striking manner with young plants of Bauhinia, and with Oxalis Ortegesii. W o are led to an analoO'ous conclusion with respect b . to the movements of the secondary petioles of certam pinnate leaves. The pinnro of Mimosa pudica con· verge at night; and thus the imbricated and closed leaflets on each separate pinna are all brought close toO'ether into a single bundle and mutually protect b ' l one another with a somewhat smaller surface expose< to radiation.' vVith Albizzia lophantha tho pinnre ?lose together in the same manner. Although the pmme of Acacia Farnesiana do not converge much, tb.ey sink downwards. Those of Neptunia oleracea lilcelrl e Cn.AP. VII. SUMMARY ON SLEEP OF LEA YES. 403 move downwards, as well as backwards, towards the base of the leaf, whilst tho main petiole rises. With Schrankia, again, the pinnm are depressed at night. Now in these three latter cases, though the pinnm do not mutually protect one another at night, yet after having sunk down they expose, as does a dependent sleeping leaf, mnch less surface to the zenith and to radiation than if they had remained horizontal. Any one who had never observed continuously a sleeping plant, would naturally suppose that the leaves moved only in the evening wh n going to sleep, and in the morning when awaking; but he would be quittl mistaken, for we have found no exception to the rule that leaves which sleep continue to move during the whole twenty-four hours; they move, however, more quickly when going to sleep and when awaking than at other times. That they are not stationary durino· the day is shown by all the diagrams given, and b; the many more which were traced. It is troublesome to observe the movements of leaves in the middle of the night, but this was done in a few cases · and tracings were made during tho early part of the ~ight of the m_ovoments, in the case of Oxalis, Amphicarproa, two speCI~s of Erythrina, a Cassia, Passifiora, Euphorbia and Mars1lea; and the leaves after they had gone to sleep, were found to be in constant movement. When, however, opposite leaflets come into close contact with on~ another or with the stem at night, they are, as we be~1eve, mechanically prevented from moving, but this pomt was not sufficiently investigated. d W. hen the movements of sloepinob· leaves are traced . un~g twenty-four hours, the ascending and descend-mg. hnes do not coincide, except occasionally and Ly accident for a short space; so that with many plants a 2 D 2 |
