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Show 314 MODIFIED CIRCUMNU'fATION. CuAr.n Raphanus for only~ single ni_ght. Thoro arc, however, some strong cxccptwns to tlus rule, as tho cotyledons of Gossypium, Anoda and I pomooa do not possess ~ulvini, yet continuo to move and to grow for a long time. We thought at first that when tho movement lasted~or only 2 or 3 nights, it could hardly be of any serviCe to tho plant, and hardly de erved to be called sleep; but as many quickly-growing leaves sleep for only a few nights, and as cotyledons arc rapidly developed and soon complete their growth, this doubt now seems to us not well-founded, more especially as those move· ments are in many instances so strongly pronounced. We may hero mention another point of similarity between sleeping leaves and cotyledons, namel!, that some of the latter (for instance, those of Cass1~ and Gi thago) are easily affected by the absence of hght; and they then either close, or if closed do not _open; whereas others (as with the cotyledons of Oxahs) a~o very little affected by light. In the next chapter 1t will be shown that the nyctitropic movements both of cotyledons and l aves consist of a modified form of circumnutation. As in the Leguminosro and Oxalid::c, the leaves and the cotyledons of the same species generally sleep, the idea at first naturally occurred to us, that the sleep of tho cotyledons was merely an early development of a habit proper to a more advanced stage of life. But no such explanation can be adm1· ttc d , a].t h oug h there seems to be some connectw· n, as m1·g · ht have been expected between the two se t s of cas C· · For the leaves or' many plants sleep, whilst their cotyledons do not do so-of which fact D· esmo dw · m gyran 8 offers. a good instance as likewise do three species of N ~leo· tiana observed' by us; also St.d a r lw m b"; 'J; ,0 zt·a , Abutz 0e11r Darwim"i, and Chenopod-ium album. On the oth 0HA)'. VI. SLEEP OF CO'fYLEDONS. 315 hand, the cotyledons of some plants sleep and not the leaves, as with the species of Beta, Brassica, Geranium, Apium, Solanum, and Mirabilis, named in our li. t. Still more striking is the fact that, in the same genu., the leaves of several or of all the species may sic p, but tho cotyledons of only some of them, as occurs with ~rrifolium, Lotus, Gossypium, and partially with Oxalis. Again, when both the cotyledons and tho leaves of the same plant sleep, their movements may be of a widely dissimilar nature: thus with Cassia the cotyledons rise vertically up at night, whilst their leaves sink down and twist round so as to turn th ir lower surfaces outwards. With seedlings of Owalis Valdivian~, having 2 or 3 well-developed leaves, it was a cunous spectacle to behold at nio-ht each leafl t folded in~ards and hanging perpendicularly downwards, whilst at the same time and on the same plant the cotyledons stood vertically upwards. These several facts, showing the independence of the nocturnal movements of the leaves and cotyledons on the same plant, and on plants belonging to the same. genus, l~ad to the belief that the cotyledons have acqmred their power of movement for some special purpose. Other facts lead to the same conclusion such as the presence of pulvini, by the aid of which the nocturnal movement is continued during some weeks. In Oxalis the cotyledons of some species move vertically. upwards, and of others vertically downwards at mght; but this great difference within the same natural genus is not so surprisino- as it ~ay at ~rst appear, seeing that the cotyledon~ of all ~ e. spemes are continually oscillating up and down ~ring the day, so that a small cause might determine we et~er th6y should rise or sink at night. Again, the P cuhar nocturnal movement of the left-hand coty-i:! " |