OCR Text |
Show 502 MODIFIED CIROU:M:NUTATION. CHAP. X. 10.~0 P.M., during which time it zigzagged m1d described an olhpse of cons.i derable Rizo. .T ho hypocotyl of JJr(/",, s•'c a o1 e1 'ucea (Rce former Fig. 173) moved m n. straight IillO to the Ji n·J1t 1-1 r.: 1 r;: d tl f . . . n nn l D. D P.M., an 1cn rom the h ght, makmg in ils backward c~ursc a great rectangular bend, and then rcturnml for a short d.1 stancc towards the former somco of tl10 Jio:-·. Jtt ., 110 ol >scr.v a-twns we_re ma~o after. ~.10 P.M., but during tho night it re- ?ovcred Its ~crt1?al position. A hypocotyl of ( 'assi11 tora moved I? the c~onmg m a somewhat zigzag line towards the failing hght u~til 6 P.l\1., and was now bowed '20° from the JK'rpcnclicnlar; It then returned on its course, making before 10.30 P.M. fo~r great, nearly rectangular bonds aucl almost compl eting an clhp~o. Several other analoo·ous cases were casually observed, nnd mall of them tho apogcotropic movement could be seen to consiRt of modified circumnutation. .Apogeotropic Movements ~f}'ec ted by the aid of Joints or pulvini. -Movements of this kind ::tro well kuo"·n to occm in tho Gramincro, and are effected by means of the thi ckened bases of. their sheathing leaves ; the Rtom within b •ing in this part thmncr than elsewhere.* Accordi11g to the analogy of aU other pulvini, such joints ought to continuo circnnmutat,ing for a long period, after the adjoining parts have ccaRcd to grow. 1\'c therefore wishccl to asccrt::tin whether this \vas the cnse with tl10 Graminero; for if so, the upward cm vaturc of their stems, when extended horizontally or laid prostrate, would be explained in accordance with om vi w-namcly, that apogeotropism results from modified circumnutation. After th ese joil1ts have curved upwards, they arc fixud in their now po1>ition by increased growth along their lower siclcs. Lolittm p erenne.-A young stem, 7 inchcB in height, consisting of 3 internodes, with the flower-head not yet vrotrudud, was selected for observation. A long and very thin glass fila· mont was cemented horizontally to the stem close above the second joint, 3 inches above the grouncl. Tbis joint was subsequently proved to be in an active condition, as its lower side swelled much through tho action of apogcotropism (in the manner described by Do Vries) after tlw haulm l1ad been fastened down for 24 h. in a horizontal position. Tho pot was * Tlds structure has been recrntly d• ·scribcd by Do Vries in an jntcrcsting article, 'Ueber die Aufrichtung des gclngcrl~n Gctroido:; ' in 'Lrllldwirthsrlwfl· licho J ah~bi.icbcr,' 1880, p. 4/il. OuAP. X. APOG EOT~OrTSM. 003 80 placed that tho end of tho filament Rtoocl boncat,h tl1o 2-i11 c1t object glass of a microscope with n.n eye-pi ece micrometer, mch division of which oqnallccl 500 of an inch. 'l'hc cml or the filament was ropoatoclly obscl'Vcd cluriltg G h., aml WttS foiccn to l>u in constant movement; and it cros:-lcd G divisions of tlw micrometer (rio inch) in 2 h. OccasionaJiy it moved forwards by jerks, some of which were To~ inch in length, ancl then s lowly retreated a little, afterwards again jerking forw::tn1 s. 'l'hcFio oscillations were exactly like those clcscr.ibcd under Brassjca and Dionroa, but they occuncd only occasion ally. Wo mny therefore conclude that this moderately old joiHt was continually circumnutating on a small scalo. Alopecunts pmtensis.-A young pl::tnt, 11 i11Chcs in height., with the flower-head protruded, but with tho florotR not yet expand eel, had a glass filament fixed close above tlJO ~ ccond jojnt., at a height of only 2 inches above the ground. The lJttsal internode, 2 inches in length, was cemented to a stick to prevent any possibility of its circumnutating. Tho extremity of the filament, which projected about 50° above the horizon, was often obscrvc<l during 24 h. in the same manner as in tho last case. Whenever looked at, it was always in movement, and it crossed 30 divisjons of the micrometer (5 3 0 inch) in 3~ h.; but it sometimes moved at a quicker rate, for at one time it crosf:;ed 5 divisiom; in 1~ h. The pot bad to be moved occasionally, as tho end of tho filament travelled beyond the field of vision; but as far as we could judge it followed during the daytime a semicircular courRo · and it certainly travelled in two difl'ercnt dir ctions at ri c•ht angles to one another. It sometime;-; oscillated in tile s~~no manner as in tho last species, some of the jerkR forwar!lS being as much as rdoo of an inch. We may therefore conclude that t~1ejoints in this and tho last species of grass long continuo to Clrcumnutato; so that this movement would bo ready to he converted into an apogcotropic movement, whoncvc.r tho stem was placed in an inclined or horizontal position. Movements of the Flower-peduncles (if Oxatis carnosa, due to apogeotropism and othe1· forces.-Thc movements of tho main pe~uncle, and of the three or four Rub-peduncles which each mam peduncle of this pl:lnt bears, are extremely complex, and are determined by soveml distinct causes. ·whilst tho flo\\>crs are expanded, both kinds of peduncles circnmnutato about tho same spot, as we have ~con (Fig. !H) in tho fourth chapter. But soon after tho flowers have begun to ·wither tho sub- |