| OCR Text |
Show 110 CIRCUMNUTATION OF CO'£YLEDONS. CHAP. II. ciuecl· so that ellipses, more or less narrow, were described and the cotyledons may safely be saiu to have circ~mnutated. Nor could this fact Le accounted for by the mere increase in l~ngth of the coty_ledons through growth, for this by Itself would not mduce any lateral movement. Tha~ there was lateral movement in some instances, as with the cotyledons of the cabbage, was evident; for these, besid?s moving up and down, changed their course from nght to left 12 times in 14 h. 15 m. vVith Solanum lycopersicum the cotyledons, after falling in the forenoon, zigzagged from side to side between 12 and 4 P.M., and then commenced rising. The cotyledons of Lupinus hdeus, are so thick (about ·08 of an inch) and fleshy,* that they seemed little likely to move, and were th_erefore observed with especial interest; they certamly moved largely up and down, and as the line t;~cecl '~as zigzag there was some lateral ~ovement: Ih~ mne cotyledons of a seedling Pinus p2naster plaml y cucumnutated ; and the :figures described approached more nearly to irregular circles than to irregular ovals or ellipses, The sheath-like cotyledons of th~ Gm· minere circumnutate, that is, move to all sides, as plainly as do the hypocotyls or epicotyls of any dico· tyledonous plants. Lastly, the very young fronds of t1. Fern and of a Selaginella circumnutatcd. In a large majority of the cases '"hich wore care· fully observed, the cotyledons sink a little downwards in the forenoon and rise a little in the afternoon or evening. They thus stand rather more highly incli~erl during the night than during the mid-day, at whiCh *The cotyledons, though bri.ght green, resemble to a certain extent hypogean ones; see the in- · teresting discus~ion by Haberlandt ('Die Schutzeinrichtungen,' &c, 1877, p. 95), on the gradation~ in the Leguminosro between sub· nerial and subterraneau cotyle· dons. CuAP. II. CIRCUMNUTATION OF CO'£YLEDONS. 111 time they are expanded almost horizontally. The circumnutating movement is thus at least partially periodic, no doubt in connection, as we shall hereafter see, with the daily alternations of light and darkness. The cotyledons of several plants move up so much at night as to stand nearly or quite vertically; and in this latter case they come into close contact with one another. On the other hand, the coty lcdons of a few pla~ts sink almost or quite vertically down at night; and in this latter case they clasp the upper part of the hypocotyl. In the same genus Oxalis tho cotyledons of certain species stand vertically up, and .those of other species vertically down, at night. In all such cases the cotyledons may be said to sleep, for they act in the same manner as do the leaves of many sleeping plants. This is a movement for a special purpose, and will therefore be considered in a future chapter devoteu to this subject. . In order to gain some rude notion of the proportional number of cases in which the cotyledons of dicotyledonous plants (hypogean ones being of course excluded) changed their position in a conspicuous manner at night, one or more species in several genera were cursorily observed, besides those described ~n the last chapter. Altogether 153 genera, included m as many families as could be procured, were thus observed by us. The cotyledons were looked at in the middle of the day and again at night; and those were noted as sleeping which stood either vertically or at an angle of at least 60 ) above or beneath the horizon. Of such genera there were 26 ; and in 21 of ~hem the cotyledons of some of the species rose, and In only 6 sank at night; and some of these latter ~ases are rather doubtful from causes to be explained m the chapter on the sleep of cotyledons. When |
