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Show 338 l\IODIFIED CIRCUMNU'fATION. CHAP. VII. leq,st, and did not close at all during the hottest dFLyR. Another trial was made with the same bush on A ngust 2nd and 6th (the soil appearing at this latter date extremely dry): for it waR exposed out of doors during the whole <lay to the wmd, but the len.Oets showed no signs of closing. Tho Chi.lian form t~1cre~ore differs widely from the one at \Viirzlmrg, m not cl?smg 1t~ leaflets when suffering from the -.;vant of water; ancl 1t can hve for a surprisingly long time without water. 'l'ropreolwn wtjus (?)(cultivated var.) (Tropmolero).- Seveml plants in pots stood in the gr e~1hou P, and tl:o blades of the leaves which faced the front-llghts were dunng the d~ty highly inclined and at night verticnJ; ':'hilst. the loaves on the back of the pots, though of course 1llummatcd. through the roof, did not become vertical at night. vV c thought, n.t first, that this difference in their positions was in some manner clue to heliotropism, for the leaves arc highly heliotropic. .'!.'he true explanation, howcYcr, is that unloRs they arc well Illuminated durin()' at least a part of the day they do not sloop at night; and a llttle difference in the degree of ~llnmina~ion determines whether or not they shall become vcrhcal at mght. We have observed no other so wcll-ruark d a case as this, of the influence of previous illumination on nyctitropic ~ovem.ents. The leaves present also another peculiarity in thell' habit of rising or awakinO' in tho morning, b ing more strongly fixed or inherited than that of sinking or sleeping at night. The m?vements are caused by the bending of an upper part of the petwlc, between t and 1 inch in length; but the part close to the blade: for about i of an inch in length, docs not b~nd aud. alway~ remains at right angles to tho b~u.uo. The. bendmg ~Jort1~n do~: not present any external or mtornal d1ffo.rcnce m st~~ctu from the rest of the petiole. We will now g1vo tho expeumouts on which the above conclu. ions are founded. . A large pot with several plants was broug ht on the mormncor of Sept. 3rd out of the green house and placed before a north~e~~t wiudow in the same position as before with respect to tho llgl' as far as' that was possible. On the front of the P1 a nts ' 24 leaves 1 . were marked with thread, some of . w h 1· c h h ad their blades Jon- 45 o zontal but the greater number were m· e 1m· e d at about . ' beneat' h the horizon; at m.g ht all t h. ese, "'1·.t b.o ut exceptwn, 4 th they became vertical. Early on tho followmg mormng ~ ) reassumed their former pos1.t 1. 0ns, an d a t m·g h t a.g am became 15 nd vertical. On the 5th the shutters were opened at G. A.M., a CHAP. VII. SLEEP OF LEAVES. 339 by 8.18 .A.M., after the loaves had boon muminated for 2 h. 3m., and had acquired their diurnal position, they were placed in a dark cupboard. They were looked at twice dUl'ing the day and thrice in the evening, the last time at 10.30 P.M., and not one had become vertical. .At H A.M. on the following morrung (Gth) they still retained the same diurnal position, awl were now replaced before the north-cast window. At night all tho leaves which had faced the light had their petioles curved and their blades vertical; whereas none of the leaves on tho back of the plants, although they had been moderately illuminated lJy the diffused light of the room, were vertical. 'l'hoy w ·re now at night placed in the same dark cupboard; at 9 A. :If. on tho next morrung (7th) all those which had been asleep had roas. umed their cliumal position. The pot was then placed for 3 h. in tho s1m. ·hinc, so as to stimulate the plant ; at noon they were }>lac d before the same north-east window, and at night the leaves slept in the usual manner and awoke on the following morning. At noon on thisday(~th) the plants, after having been left before the northe~ t window for 5 b. 45 m. and thus illuminated (though not brightly, .as the sky was cloudy during the whole time), wero replaced m the dark cupboard, and at 3 P.M. the positiou of the lea.ves was very little, if at all, alter d, so that they arc not qruckly affected by darkness; but by 10.15 P.l\1. all the leaves ~hie~ h~ faced the north-ea. t sky d uriug the 5 h. 45 m. of Iil1llllliJation stood vertical, whereas tholie on the back of the plant retained their diurnal position. On tho following morning (9th) the leaves awoke as on the two former occasions in the dark, and they were kept in the dark during tho whole day; at night avery few of them became vertical, and this was the one in: t~nce in which we observed any inherited tendency or habit in his plant to sleep at the proper time. That it was real leep ;as shown by these same leaves rea. suming their diurnal po i-daion on the following morning (lOth) wbil t still kept in the rk. k T;e pot was then (9.45 A.M. lOth) replaced, after having been ~ph for 36 h. in darkness, before the north-east window · and at :g t tho blades of all the leaves (excepting a fow on the' back of e plants) became conspicuously vertical. !t6 .. 45A.M. (11th) after the plants had boon illuminated on the ~~~de as before during only 25m., tho pot was turned round, intc . the leaves which had faced the light now faced the rlor of the room, and not one of these went to sleep at night; z 2 ,., I ' r· I I r I |