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Show 496 then MODIFIED CIRO UMNUTATION. Cl!AP. x. suddenly bog::m to circumnutato. A partially etiolated Fig. 183. !!Z:'ss· m~ and. somowl.1at old hypocotyl f:: P of a seedhng cabbage (2} inches in height) was so sensitive that when placed at an angle of only 23° from } r l the ~orp~ndicular, it became I vertical m 33 minutes. As 12°3' . · 1t could not have been strongly acted upon by apogeotropism in the above sUghtly inclined po. ition, we expected that it would have circumnntatecl, or at least have moved in a zigzag course. Accordingly, dots were malic every 3 minutes; but, when these were joined, the line wns nearly straight. After thi. hypocotyl had become up· right it still moved onw:trds for half an hour in tlte same general direction, but in a zigzag manner. Duri11g the succeeding 9 h. it eircnmnutated regularly, and described 3 large ellipses. In this case apogeotropism, although acting at a very unfavourable angle, quite o>crcame tbo ordinary cir· cumnutating movement. The hypocotyls of JJetct R0fi/J'am.88l!'- [\ VIJlgaris arc highly sensitive to a pogeotropism. One was placed so as to project 19° beneath tho horizon; it fell at first a very little c.sec Fig. 183), no doubt ?wmg to its weight; but as It was circumnutating the line was Beta vulgaris: apogeotropic movement ofhypocotyl from 19° beneath horizon to a vertical position, with subseq ueut circumnutation, traced on a vertical and on a horizontal glass-plate, f1·om il.28 A.l\1. Sept. 28th to 8.40 A.M. 29th. Figure rerluced toone-third of original ~calr. CaAP. X. APOG EO'rROPISM. 487 oblique. During the next 3 h. 8 m. it rose in a nearly straight line, passing through an_ angle of 109°, and then (at 12. B P.M.) stood upright. It contmued for 55 m. to move in the sarno general direction beyond the perpendicular, but in a zigzag COUI'Se. It returned also in a zigzag line, and then circumnutated regularly, describing three large ellipses during the remainder of the day. It should be observed that tho ellipses in this figure are exaggerated in size, relatively to tho length of the upward straight line, owing to the position of the vertical and horizontal glass-plates. Another and somewhat old hypocotyl was placed so as to stand at only 31° from the perpendicular, in which position apogeotropism acted on it with little force, and its course accordingly was slightly zigzag. 'fhe sheath-like cotyledons of Pllaz,,r,is Caltariensis arc extremely sensitive to apogeotropism. One was placed so as to project 40° beneath the horizon. Although it was rather old and 1· 3 inch in height, it became vertical in 4 h. 30 m., havi11g passed through an angle of 130° in a nearly straight line. It then suddenly began to circumnutate in the ordinary manner. The cotyledons of this plant, after the first leaf has begun to protrude, are but slightly apogcotropic, though they still continue to circumnutate. One at this stage of development was placed horizontally, and did not become upright even after 13 h., and itR course was slightly zigzag. So, again, a rather old hypocotyl of Cassia tora (It inch in height) required 28 h. to become upright, and its course was distinctly zigzag; whilst younger hypocotyls moved much more quickly and in a nearly straight line. When a horizontally placed stem or other organ rises in n zigzag line, we may infer from the many cases given in our previous ehapters, that we have a modified form of circumnutation; but when the course is straight, there is no evidence of ciroumnutation, and any one might maintain that this latter movement had been replaced by one of a wholly distinct kind. Tbis view seems the more probable when (as sometimes occurred with the hypocotyls of Brassica and Beta, the stems of Cucurbita, and the cotyledons of Phalaris) the part in question. after bending up in a straight course, suddenly begins to circumnutate to the full extent and in the usual manner. A fairly good instance of a sudden change of this kind-that is, f1~om a ~early straight upward movement to one of circumnutation- 18 shown in Fig. 183 · but more strikin()' instances were occasionally observed with~eta, Brassica, an°d Phalaris. We will now describe a few cases in which it may be 2 K |