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Show 282 MODIFIED CIRCUMNU'fATION. CrrAP. VI. pound leaves, forwards, t~at is, towa~cls the a pox of t~1e leaf, or backwards, that Is, towards Its baso; or, agam, they may rotate on their own axes .without moving either upwards or downwards. But In almost every case the plane of the blade is so placed as to stand nearly or quite vertically at night. 'fherefore the apex, or the base, or either lateral edo· , may be directed towards the zenith. Moreover, the upper surface of each leaf, and more especially of each leaftot, is often brought into close contact with that of tho opposite one; and this is sometimes effected by singularly complicated movements. This f<:tet suggests that the upper surface requires more protection than the lower one. For instance, the terminal leaflet in Trifolium, after turning up at night so as to stand vertically, often continues to bend over until the upper surface is directed downwards whilst the lower surface is fully exposed to the sky ; and an arched. roof is th1~s formed over the two lateral leaflets, whwh have theu upper surfaces pressed closely together. Here wo h~ve the unusual case of one of the leaflets not standmg vertically, or almost vertically, at night. . . Consid ring that leaves in assuming theu nycti· tropic positions often move through an angl~ of 90°; that the movement is rapid in the evenmg; that in some cases as we shall sec in the next chapter, it is extrao;dinarily complicated; that with certain seedlings, old enough to bear true lc~ves, the cotyledons move vertically upwards at mght, whilst at the same time the leaflets move ver· tically downwards; and that in the same genus the leaves· or cotyledons of some species move upwards, whilst those of other species move down· wards ·-from these and other such facts, it is hardly possibl' e to doubt that plants must clen.v e som e CHAP. VI. SLEEP MOVEMENTS. 283 great advantage from such remarkaLle powers of movement. The nyctitropic movements of leaves and cotyledons are effected in two ways,* firstly, by means of pulvini which become, as Pfeffer has shown, alternately more turgescent on opposite sides; and secondly, by increased growth along one side of the petiole or midrib, and then on the opposite side, as was first proved by Batalin.t But as it bas been shown by De Vries + that in these latter cases increased growth is preceded by the inereased turg scence of the cells, the difference between the above two means of movement is much diminished, and consists chiefly in the turgescence of the cells of a fully developed pulvinus, not being followed by growth. When the move~ ents of leaves or cotyledons, furnished with a pulvmus and destitute of one, are comparell., they are seen to be closely similar, and are apparently effected for the same purpose. Therefore with our object in view it does not appear advisable ;o separate the above tw~ sets of cases into two distinct classes. There is, however, one important distinction between them, namely, t~at movements effected by growth on the alternate Sides, are confined to young growing leaves, whilst those effected by means of a pulvinus last for a long time. We have already seen well-marked instances of this latter fact with cotyledons, and so it is with leaves as hash een ob s erved by Pfeffer and by ourselves. T' he long endurance of the nyctitropic movements when e:ecte~ by the aid of pulvini indicates, in addition to t e evidence already advanced, the functional import- ; This distinction was first fD~e~ o~t (~cording to Pfeifer, d cnodtschen BewegunO'en er 'Blattorga.ne/ 187[;, p. l6d\y Dassen in 1837. t ' Flora,' 187R, p. 433. t 'Bot. Zeituug,' 1879, Dec. 19th, p. 830. !,,,1,:,:;,,; ;, :: ~; ,, .. 1' , .. .,,. i"l |