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Show 322 MODIFIED CIRCUMNUTATION. CHAP. VII. us (see also 'Flora,' Oct. 1st, 1873, p. 437) tbo.t those of S. napcea fall at night, but Fig. 1 2 6. to what angle be cannot tJ"40'a.m.BIJ'!I remember. Tho loaves of S. rhomb1jolia and retusa, on the other hand, rise up vertically, and are prossed against tho stem. We have therefore here within the same genus, directly opposite movements. Again, the lea vcs of S. rhombifolia are furnished with a pulvinus, formed of a mass of small cells destitute of chlorophyll, and with their longer axes perpendicular to the axis of the petiole. As measured along this latter line, those cells are only i th of tho length of thoso of tho petiole; but instead of being abruptly separated from the~ (as .is usual with tho pulvmus m most plants), they graduate into the larger cells of the petiole. On the ~ther hand, B. napa:a, accordmg to natalin doos not possess a pulvinus; and he informs us that a gradation may .be traced in the several specws of the genus between t~ese f}r.da rhombifolia: circumnutation and two states of the petJO]C. nyctitropic (or sleep) movements of ts a leaf on a young plant, 9. i_nches Sida rhombijoUa prese~ h high· filament fixed to m1dnb of another peculiarity,ofwhJ.C near!~ full-grown leaf, 2$ inches in we have seen no other Jll· length; movement traced under a sky- 'tl leaves that light. Apex of leaf .13 inches from stance WI 1 very the vertical glass, so dwgram not sleep : for those on greatly enlarged. young plants, though they . . • slee , as we observed rise somewhat m the evenmg, do not go to P CHAP. VII. SLEEP OF LEAVES. 323 on several occasions; whilst those on rather older plants sleep in a.con,.picuous manner. For instan('C, a Jcaf ( · 85 of an inch in length) on a very young see Uiug 2 inches high, stood at noon 9° above the horizon, and at 10 P.liL at 28°, so it had rison onlv 19°; another leaf (1 · 4 inch in length) on a seedling of 1 h~ same height, stood at the sn.mo two periods at 7° and 32°, n.ncl therefore had risen 25°. These leaves, which moved so little had a fairly well-developed pulvinns. .After n.n interval of som~ weeks, when the same seedlings were 2,1 and 3 inches in height, some of the young leaves stood up at night qnitc vertically, n.ncl others were highly inclined; and so it was with bushes which were fully grown and were flowering. The movement of a leaf was traced from 9.15 A.M. on May 28th to 8.30 A.M. on tho 30th. The temperature was too low (15°-16° C.), and the illumination hn.rdly sufficient; consequently the leaves did not become quito so highly inclill(x1 at night, as they had done previously n.nd nB they did subsequent!~ in t~e hot-house: but tbe movements did not appen.r otherwise disturbed. On tho first day the len.f sank till 5.15 P.M.; it then rose rapidly and greatly till 10.5 P.M., and only a little higher during tho 1·est of tho night (Fig. 126). Ea~ly on t~e next day (29th) it foil in a slightly zigzag line rapidly until 9 A.M., by which time it had reached nearly the same place as on the previous morning. During tho remainder o~ the day it fell slowly, and zigzagge llaterally. The evening rise began after 4 P.llf. in the same manner as before, and on tha second morning it a(Yain fell rapidly. The asccndin(Y and descending lines do not coincide, as may be Reen in the din~mm. On the 30th a now tracing was made (not hero given) on n. rather enlarged scale, as the apex of tbe leaf now stood 9 inches from the vertical glass. In ordm· to ob crvc more carefully the course pursued at the time when the diurnal fall changes into the nocturnal rise, dots were made every half-hour betwocn ~ P.M. and 10.30 P.M. This l'endcrccl tho lateral ziCYza(Y(Yin(Y ~ovement during the evening more conspicuous tha~ in o th~ diagr~m given, but it was of tho same natmo as there shown. The Impression forced on our minds was tl!at the leaf was e~pending superfluous movement, so that the great nocturnal rJSe might not occur at too en.rly an hour. Abutilon lJarwin'ii (Malvaccro).-1'ho leaves on somo very ~oung plants st~od almost horizontally during the dn.y, mHl ung down vertically at night. Very fino plants kept in t~ y 2 I , .r ,H.. ' I i~ 3I' :·1 |