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Show 560 SUMMARY AND CHAP. XII. tating movements-those modified through external agencies. The so-called sleep or nyctitropic movements of leaves arc determined by the daily alternations of light and darkness. It is not the darkness which excites them to move, but the difference in the amount of light which they receive dnrino· the day and night ; for with several species, if the loaves have not been brightly illuminated during the day, they do not sleep at night. rrhey inherit, however, some tendency to move at the proper poriOfls, independently of any change in the amount of light. The movements are in some cases extraonlinarily complex, but as a full summary has been given in the chapter devoted to this subject, we will here say but little on this head. Leaves and cotyledons assume their nocturnal position by two means, by the aiel of pulvini and without such ai<l. In the former case tho movement continues as long as the leaf or cotyledon remains in full health; whilst in the latter case it continues only whilst the part is growing. Cot.yle<lons appear to sleep in a larger proportiona.l number of species than do leaves. In some species, the leaves sleep and. not the cotyledons; in others, the cotyledons and not the leaves; or both may sleep, and yet assume widely different positions at night. Although the nyctitropic movements of leaves. and cotyledons are wonderfully diversified, and sometimes differ much in the species of the same genus, yet the blade is always placed in such a position at ~1ight, that its upper surface is exposed as little as. p~ss1ble to ~ull radiation. We cannot doubt that. th1s IS the obJeC~ gained by these movements; and It has been prove that leaves exposed to a clear sky, with their blades compelled to remain horizontal, suffered much more from the cold than others which were allowed to assume CHAP. XII. CONCLUDING REMARKS. 561 their proper vertical position Some cu · .[! . . · nous 1acts have been given under th1s head, showing that hori-zontally extended leaves suffered more at uio-ht h the au. , wh ' h . 1 d b ' w en lC Is not coo e by radiation was pre t d 1 . 1 . b , ven e from free y cucu atlng oneath their lower surfaces . and so it was, when ~he leaves were allowed to go t~ sleep on branc.h es wh hwh h. ad been rendered moti on 1e ss. In some sReCies t e petwles rise up greatly at night, and the pmnro close together. The whole plant is thus rendered more com pact, and a much smallc . surface is exposed to radiation. '1 11hat the various nyctitropic movements of leaves result from modified circumnutation has, we think, been clearly shown. In the simplest cases a leaf describes a sing!e large ellipse during the 24 h. ; and the movement 1s so ananged that the blade stands vertically during the night, and reassumes its former p~sitio~ on the ~ollowi~g morning. The course pursued d1ffer.s from ordm~ry ?ucumnutation only in its greater tllllpl!tude, and In Its greater rapidity late in the evemng and early on the following mornino-. Unless this movement is admitted to be one of bcircumnutation, such leaves do not circumnutate at all, and this woulu be a monstro~s anomaly. In other cases, leaves and coty1euons descnbe several vertical ellipses clurinothe 24 h. ; and in the evening one of them is increased g.reatly in amplitude until the blade stands vertieally e.1ther upw~rds or downwards. In this position it continues. to cucumnu~ate until tho following moruing, when It reassumes Its former position. These movements, when a pulvinus is present, are often complicate~ by the rotation of the leaf or leaflet ; an<l such rotation on a small scale occurs during onlinary circumnutation. The many diagrams showinO' the move-ments f 1 · 5 o s eepmg and non-sleeping leaves and coty- 2 0 |