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Show 400 MODIFIED CIRCUMNUTA'l'ION. CHAP, VII. than the upper surfaces; but this depends merely on each leaflet becoming folded. at night so as to be able to sink vertically downwards. The torsion or rotation of leaves and leaflets, which occurs in so many cases, apparently always serves to bring their upper surfaces into close approximation with one another, or with other parts of the plant, for their mutual protection. vVe see this best in such cases as those of Arachis, Mimosa albida, and Marsilea, in which all the leaflets form together at night a single vertical packet. If with Mimosa pudica the opposite leaflets bad merely moved upwards, their upper surfaces would have eome into contact and been well protected ; but as it is, they all successively move towards the apex of the leaf; and thus not only their upper surfaces are pro· tected, but the successive pairs become imbricated and mutually protect one another as well as the petioles. ~rhis imbrication of the leaflets of sleeping plants is a common phenomenon. The nyctitropic movement of the blade is gene· rally effected by the curvature of tho upp?rmo~t part of the petiole, which has often been modified mto a pulvinus; or the whole petiole, when short, may be thus modified. But the blade itself sometimes curves or moves of which fact Bauhinia offers a striking instance,' as the two halves rise up and come into close contact at night. Or the blade and the upper part of the petiole may both move. Morco~er, the petiole as a whole commonly either rises or smks tit night. This movement is sometimes large: thus the petioles of Cassia pubescens stand only a lI' ttle . ab ore the horizon during the day, and at night nse up almost, or quite, perpendicularly. The petioles of tho Youn()'er leaves of Desmodium gyrans a1 s o n·s e u} J ver.· ticall0y at night. On the other han d, w1. t h 1~ Jlll)hl· CnAr. VII. SUMMARY ON SLEEP OF LEA YES. 401 oarprea, the. petioles of some leaves sank down as much as 57° at night; with Arachis they sank 39'), and then stood at right angles to the stem. Generally, when the rising or sinking of several petioles on the same plant was measured, the amount differed greatly. This is largely determined by the age of the leaf: for instance, the petiole of a moderately old leaf of Desmodium g~rans rose only 4G0 , whilst the young ones rose up vertically; that of a young leaf of Cass£a ftoribunda rose 41°, whilst that of an older leaf rose only 12°. It is a more singular fact that the age of the plant sometimes influences greatly the amount of movement; thus with some young seedlings of a Ban~ hinia the petioles rose at night 30° and 34°, whereas those on these same plants, when grown to a height of 2 or 3 feet, hardly moved at all. The position of the leaves on the plant as determined by the light, seems also to influence the amount of movement of the petiole; for no other cause was apparent" why the petioles of some leaves of liielilotus officinalis rose as much as 59°, and others only 7° and go at night. . . In t~e case of many plants, the petioles move at mght. m one direction and the leaflets in a directly opposite one. Thus, in three genera of Phaseolere the lea~ets move~ vertically down,vards at night, and the ~boles ros~ m .two of them, whilst in the third they . nk. Species m the same genus often differ widely Ill the movements of their petioles. Even on the same plant of Lupinus pubescens some of tb e petioles rose 30° ~thers only 6°, and others sank 4° at nio·ht. Th~ eafiets of Cassia Barclayana moved so little at night t~at they could not be said to sleep, yet the petioles 0 some young leaves rose as much as 34°. These several facts apparently indicate that the movements 2 D |
