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Show 440 MODIFIED o nwu:MNU'rA'l.'ION. OllAr. Ylll. branches which had been fastoneJ in various positions or turned upside clown. In our observations on tho cotyledons of seedlinrt plants, we often felt surpri. e at their persistent horizontal position during tho day, and were convinced before we had rea<L Prank's essay, thn.t some special explanation was necessary. De Vries has sbo\\'n• that the more or loss hori:wntal position of leaves is in most cases influenced by epinasty, by their 01rn weight, and by apogootropism. A young cotyledon or l af after bursting free is brought down into it proper position, as n.lrrady remarked, by opinasty, which, according to Do Vries, long continues to act on the midribs and petioles. vVoight can lHmlly be influential in the case of coty lo<lons, except in a few cases presently to be mentioned, but must be so \rith large and thick leaves. vVith respect to apogootropism, Do Vries maintains that it g nerally comes into plny, and of this fact we shall presently advan ce some indir ct evidence. But over these and other constant forces we believe that there is in many cases, but we <lo not say in all, a preponderant tomloncy in ] ,we. and cotyleJons to place themselves more or loss transversely with respect to the light. In tho cases above allnued to of soocllings exposed to a lateral bght with their hypocotyls secured, ~tis impossible that epinasty, weight and apogeotrop1 m~ either in opposition or combined, can be th~ C<tn~e of tho rising of one cotyledon, and of tho sinlnng ol the other, since tho forces in question act equally on both ; and since epinasty wei o·ht and a11ogootropism all act ' . . . f in a vertical plano, they cannot cause the tw1sttng 0 the petioles, which occurs in seedlings nncler the * 'A rbeiten des Bot. Instituts in Wiirzburg,' Heft. ii. 1872, PP· 22i:l- 277. CuAP. Vlll. DIAIIELIO'rHOP lSl\f. 441 above conditions of illumination. All these movements evidently depenJ in some manner on tho obliquity of the light, but ca.nnot be calleJ. heliotropic, as this implies bending towards tho light; whereas the cotyledon nearest to the light benLLs in an opposed direction or downwards, and both place themselves as nearly A$ possible at right angles to tho light. The movement, therefore, deserves a distinct name. As cotylodons and leaves are continually oscillating up and down, and yet retain all day long their proper position with their upper surfaces directed transversely to the light, and if displaced reassume this position, diaheliotropism must be considered as a modified form of circumnutation. This was often evident when the movements of cotyledons standing in front of a window were traced. We see something analogous in the case of sleeping leaves or cotyledons, which after oscillating up and down during the whole day, rise into a vertical position late in tho evening, and on the following morning sink down again into their horizontal or cliaheliotropic position, in direct opposition to heliotropism. This return into their diurnal position, which often requires an angular movement of 90°, is analogous to tho movement of loaves on <.lisplaced branches, whi_ch recover their former positions. It deserves n~twe. that any force such as a pogeotropism, will act '~tth different degrees of power* in the different positions of those leaves or cotyledons which oscillate largely up and down dm·ing tho day; and yet they recover their horizontal or diaholiotropic position. We may therefore conclude that diaheliotropic mo~ements cannot bo fully explained by the direct actwn of light, gravitation, weight, &c., any more • See former noto, in reference to Sacl1s' remarks on this suLjoot. |