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Show 310 MODIFIED CIRCUM:NUTA'l'ION. CHAP. VI. upwards, and at night, when the bases of the blade~ are in contact, the two petioles together form a vertical ring surrounding the plumule. The cotyledons continued to act iu nearly tho same manner for 8 or 10 days from the period of germination; but the petioles had by this time become straight and had increased much in length. After from 12 to 14 clays the first simple true leaf was formed, and during the ensuing fortnight a remarkable movement was repeatedly observed. At I. (Fig. 125) we have a sketch, made in the middle of the day, of a seedling about a fortnight old. The two cotyledons, of which Rc is the right, and Lc the left one, stand directly opposite one another, :Fig. 125. Rc F I. )f. F~Gr F III. T1·ijolium strictttm: diumal and nocturnal positions of the two cotylcu~ns and of the first leaf. I. Seedling Yiewed obliquely from above, dunng the day: Rc, right cotyledon; Lc, left cotyledon; F, .first true leaf. II. A rather younger seedling, viewed at night: Rc, n ght cotylc.don raised, but its po$itiou not otherwise changed; ],~, l eft cotyledo~ raised and laterally twisted· F. fil'st leaf raised and twi sted so as to lace the left twi ·ted cotyledo~. 'III. Same seedling viewed at nigh~ from the opposite side. The back of the first leaf, F, is here shown mstead of the front, as in II. and the first true leaf (F) projects at right angles to them. At night (see II. and III.) the right cotyledon (Rc) is greatly raised, but is not otherwise changed in position. The left cotyledon (Lc) is likewise raised, but it is also twisted, so that its blade, instead of exactly facing the opposite one, now stands at nearly right angles to it This nocturnal twisting movement is effected not by means of the pulvinus, but by the twJ·s t'J Dg of the whole length of the petiole as could be seen by the curved hne of its upper concave surfa~e. At tho same time the true leaf (I!') rises up, so as to stand vertically, or it even passe~ the vertical and is inclined a little inwards. It also twists a llttl~ J:>y which means the upper surface of its blade fronts,. and almost comes into contact with, the upper surface of the twJste CHAP. VI. SLEEP OF COTYLEDON'. 311 left cotyledon. This seems to be the object gained by these singular movements. Altogether 20 seedlings were examined on successive nights, and in 19 of them it was the left cotyledon alone which became twisted, with the true leaf always so twisted that its upper surface approached closely and fronted that of the left cotyledon. In only one instance was tho right cotyledon twisted, with the true leaf twisted towards it; but this seedling was in an abnormal condition, as the left cotyledon did not rise up properly at night. This whole case is remarkable, as with the cotyledons of no other plant have we seen any nocturnal movement except vertically upwards or downwards. It is the more remarkable, because we shall meet with an analogous case in the leaves of the allied genus Mclilotus, in which the terminal leaflet rotates at night so as to present one edge to the zenith and at the same time bends to one side, so that its upper surface comes into contact with that of one of the two now vertical lateral leaflets. Concluding Remarks on the Nyctitropic Movements of Gotyledons.-11he sleep of cotyledons (thouo-h this is a subject which has been little attended to), ~eems to be a more common phenomenon than that of leaves. We obser~ed t~e position of the cotyledons during the day and mght m 153 genera, widely distributed throuo-hout the dicotyledonous series, but otherwise selec%ed almost by hazard ; and one or more species in 26 of these genera placed their cotyledons at night so aR to stand vertically or almost vertically, having generally moved through an angle of at least 60°. If we lay. on one side the Leguminosre, the cotyledons of whiC~ are particularly liable to sleep, 140 gen ra remam; and out of these, the cotyledons of at least one species in 19 genera slept. Now if we were to select by hazard 140 genera, excluding the Leguminosre, and observed their leaves at night, assuredly not nearly 80 s m. any as 19 would be found to include sleepino- b pec1es. \Ve here refer exclusively to the plants observed by ourselves. |