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Show 442 MODIFIED CIRCUMNUTATION. CHAP. VIII. than can the nyctitropic movements of cotyledons and leaves. In the latter case they place themselves so ~hat their ~pper . surfaces may radiate at night as httle as poss1ble Into open space, with the upper surfaces of the opposite leaflets often in contact. 1'hesc movements, which are sometimes extremely complex, are regulated, though not directly caused, by the alternations of light and darkness. In the case of diaheliotropi sm, cotyledons and leaves place themselves so that their upper surfaces may be exposed to the light, and this movement is· regulated, though not directly caused, by the direction whence the Eght proceeds. In both cases the movement consists of circumnutation modified by innate or constitutional causes, in the same manner as with climbing plants, the circumnutation of which is increased in amplitude and rendered more circular, or again with very young cotyledons and leaves which are thus brought down into a horizontal position by epinast y. vVe have hitherto referred only to those leaves and cotyledons which occupy a permanently horizontal position; but many stand more or less obliquely, and some few upright. The cause of these differences of position is not known; but in accordanee with ·Wiesner's ·views, hereafter to be given, it is probable that some loaves and cotyledons would suffer. if they were fully illuminated by standing at right angles to the light. vV e have seen in the second and fourth chapters that those cotyledons and leaves which do not alter their positions at night sufficiently to be said to sleep, commonly rise a little in the evening and fall agam on the next morning, so that they stand dur~ng the night at a rather higher inclination than dunn~ ~be middle of the day. It is incredible that a nsmg movement of 2° or 3° or even of 10° or 20°, can be of ' CHAP. VUI. DIAHELIOTROPISM. . 443 any service to the plant, so as to have been specially acquired. It must be the result of some periodical change in the conditions to which they are subjected, and there can hardly be a doubt that this is the daily alternations of light and darkness. De Vries states in the paper before referred to, that most petioles and midribs are apogeotropic ;* and apogeotropism would account for the above rising movement, which is common to so many widely distinct species, if we suppose it to be conquered by diaheliotropism during the middle of the day, as long as it is of importance to the plant that its cotyledons nnd leaves should be fully exposed to the light. The exact hour in the afternoon at which they begin to bend slightly upwards, and the extent of the movement, will depend on their degree of sensitiveness to gravitation and on their power of resisting its action during the middlo of the clay, as well as on the amplitude of their ordinary circumnutating movements; and as these qualities differ much in different species, we might expect that the hour in the afternoon at which they begin to rise would differ much in different species, as is the case. Some other ~gency, however, besides apogeotropism, must come mto play, either directly or indirectly, in this upward ~ovement. Thus a young bean (Vicia faba), growing m. a small pot, was placed in front of a window in a klinostat; and at night the leaves rose a little, although • Acrording to Frank (' Die nat. Wagerechte Richtung von Pflanzentheilen.' 1870, p. 46) the ~!-leaves of many plants kept 10 darkness, rise up and ev~n hecome vertical; and so it is in some ~8 with shoots. (See Rauwen~ m' .: A1·chivcs Necrlandaises,' · : xn. P· 32.) These movemeuts md,cate apogootropism ; but when organs have been long kept in the dark, the amount of water and of mineral matter which they contain is so much altered, and their regular growth is so much disturbed, fhat it is perhaps rash to infer from their movement:; what would occur under normal conditions. (See Godlewski, 'Bot. Zeitung,' Feb. 14th, 1879.) |