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Show 406 MODIFIED CIRCUMNU'rATION. C HAP. VII. made several little triangles at the rate of 43 m. for each. Mimosa pudica, similarly observed, described three little ellipses in 67 m.; and the apex of a leaflet crossed 5 -&0 of an inch in a socoml, or 0·12 inch in a minute. The leaflets of Averrboa made a countless number of little oscillations when the temperature was high and the sun shining. rrhe zigzag movement may in all cases be considered as an attempt to form small loops, which are drawn out by a prevailing movement in some one direction. The rapid gyrations of tho little lateral leaflets of Desmedium belong to tho same class of movements, somewhat exaggerated in ntpiJ.ity and amplitude. The jerking movements, with a small advance and still smaller retreat, apparently not oxn,ctly in the same line, of the hypocotyl of the cabbage and of the leaves of Dion::ca, as seen under tho microscope, all probably come under this same head. We may suspect that we here see the energy which is freed during the incessant chemical changes ju progress in the tissues, converted into motion. J?inally, it shonl!l be noted that leaflets and probal>ly some lcn.vcs, wbil t describing their ellipses, often rotate slightly on th ir axes; so that the plane of the loaf is directed first to one and th0n to another side. rfhis was plainly SCCll to be the case with the ]argo torminallcaflots of Des· modium, Erythrina and Amphicarpma, and is prolmuly common to all leaflets provided with a pulvinus. With respect to the periodicity of tho movements of sleeping leaves, Pfeffer* has so clearly shown that this depends on the daily alternations of Jio·ht an.d darkness, that nothing farther need bo said on thts * 'Die Periodischen Bowecrun"'en der Blattorganc,' 1875, P· 30, rt • 0 0 pass~m. CHAP. VII. SU:l\IMARY ON SLEEP OF LEAVES. 407 hffid, But wo may recall the behaviour of Mimosa in the North, w hero the sun docs not sot, and the complete inversion of the daily movements by artificial light and darkness. It has also been shown by ns, that although leaves subjected to darkness for a moderately long time continue to circumnutatc, yet the periodicity o~ their movements is soon greatly di:;~ urbed, or qmte annulled. The presence of light or 1ts absence cannot be supposed to bo the direct cause of the movements, for these are wonderfully diversified even with the leaflets of tho same leaf, although all have of course been similar! y exposed. ':rhe movements depend on innate causes, and are of an adaptive nature. The alternations of light and darkness me~ely give notice to the leaves that the period has amved for them to move in a certain manner. vV e may.infer from the fact of several plants (Trop::colum, ~up1~us, &c.) not sleeping unless they have been well Illummated during the day, that it is not the actual decrease of light in tho evening, but the contrast between the amount at this hour and during the early par~ of t~e day, which excites the leaves to modify theu ordmary modo of circumnutation. . As the loaves of most plants assume their proper dmrnal position in the morning, although light be exclud~d, and as the leaves of some plants continue to ~ove m the normal manner in darkness durin()' at ~st a whole day, we may conclude that the perfocli- ~It~ of .their movements is to a certain extent inented. The strength of such inheritance differs ' Pfeffer denies such inherit~ nce; he attributes(' Die Period r~r ~e~u~gen,' pp. 30-56) tl1~ thctty when prolonO'ed for ay or two in darkt~ess, to "Nacltwirkung,'' or the o.ftoreffects of light and <.larknc•ss. But we are unable to follovr his train of reasoning. '!'here does not seem to be anymore reasou for |