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Show 364 MODIFIED CIRCUMNUTATION. C HAP. VII. sequent appearance, may be attributed to reversion to more or less distant progenitors.* No one supposes that the rapid movements of tho lateral leaflets of D. gyrans are of any use to tho plant; and why they should behave in this manner is quite unknown. We imagined that their power of movement might stand in some relation with their rudimentary condition, and therefore observed the almost rudimentary leaflets of M·imosa albida vel sensitiva (of which a drawing will hereafter be given, Fig.l59); but they exhibited no extraordinary movements, and at night they went to sleep like the full-sized leafl ets. There iR, however, this remarkable difference in the two cases; in Desmodium the pulvinus of the rudimentary leaflets has not been reduced in length, in correspondence with the reduction of tho blade, to the same extent as has occurred in the Mimo. a; and it is on the length and degree of curvature of the pulvinus that tho amount of movement of the blade depends. Thus, tho average length of the pulvinus in the large terminal leaflets of Dosmodium is 3 mm., whilst that of the rudimentary leaflets is 2·86mm.; so that they differ only a little in length. But in diameter they difl:'er much, that of the pulvinus of the little leaflets being only 0 · 3 mm. to 0 · 4 mm.; whilst that of the terminal leaflets is 1· 33 mm. If we now turn to tho Mimosa, we find that tho average length of the pulvinus of the almost rudimentary leaflets is only 0 · 466 mm., or ra thor more than a quarter of the length of the pulvinus of the full- sizedlcaflcts, namely, 1· 66 mm. In this small reduction in length of the pulvinus of tho rudimentary leaEiets of Desmoclium, we apparently have tho proximate cause of their great and rapid circumnutating movement, in contrast with that of the almost rudimentary leaflets of the Mimosa. The small size and weight of the blade, and tho little resistance opposed by the airto its movement, no doubt also como into play; for we have seen that these leaflets if immersed in water, when the resistance would be much greater, wore prevented from jerking forwards. Why, during the reduction of the lateral leaflets of Desmodium, or during their reappearance -if they owe their origin to reversion-the pulvinus shouJJ have been so much less affected than the blade, whilst with tho * Desmodium vespe1·tilionis is ?loscly allied to D. gyrans, and 1t seems only occasionally to bear rudimentary lateral leaflets . . Du: chartre, 'Elements de Botamque, 1867, p. 353. CnAP. VII. SLEEP OF LEAVES. Mimosa the pulvinus has boon greatly reduced, we do not know. Nevertheless, it deserves notice that tho reduction of the leaflets in these two genera has apparently been effected by a different process and for a difl'erent end; for with tho Mimosa the reduction of the inner and basal leaflets was necessary from the want of space; but no such necessity exists with Desmodium, and the reduction of its lateral leaflets seems to have been due to the principle of compensation, in consequence of the great size of the terminal leaflet. Uraria (Tribe 6) and Ce1dro8ema (Tribe 8).-The leaflets of Uraria layopus and the leaves of a Controsema from Brazil both sink vertically down at night. In the latter plant the petiole at the same time rose 16!0 • Amphicwpcea monoica (Tribe 8).-The leaflets sink down vertically at night, and the petioles likewise fall considerably. Fig. 151. 'l :t1p.m.JIJ 1~ \~ ..4m~i~rpma mon.oica: circumnutation :mu nyctitropic movement of leaf t urmg 4h~ h.; 1ts apex 9 inches from the vertical glass. Figure reduced. 1o7 oion-e1-t8 !oJr dC .o f orig'm a l sea l e. PI an t J' ll umm. ated from above; temp. ~~tiole, which was carefully observed, stood during the day fell ~~ve the horizon and at night 32° below it; it therefore ]eafl • A filament was fixed transversely across the terminal et of a fine young leaf (2±- inches in length includin~ the |