| OCR Text |
Show 18 CIRCUMNUTATION OF SEEDLINGS. 0JTA1'. I. begun to fall, and a dot was made on a fresh glass. The move- . ment was traced until 5.30 P.M. as shown in (Fig. 8), which is given, because the course followed was much more irregular Fig. 8. \ B1·assica olel'(lcea: conjoint. circumnutation of the hypocotyl and cotyledons during 8 hours. Figure here reduced to onethird of the original scale, as traced on a vertical glass. than on the two previous occasions. During these 8 hours the bead changed its course greatly 10 times. The upward movement of the cotyledon during the afternoon and early part of the night is here plainly shown. As the filaments were fixed in the three last cases to one of the cotyledons, and as the hypocotyl was left free, tho tracings show the movement of both oro·ans conjoined ; and we now wished to ascertain whether both circumnutateJ. Filaments were tl1erefore fixed horizontally to two hypocotyls close beneath the petioles of their cotyledons. These seedlings had stood for two days in the same position before a north-east window. In tho morning, up to about 11 A.M., they moved in 1.ig1.ag lines towards the light; and at night they again became almost upright through apogeotropism. After about 11 A.M. they moved a little back from the light, often crossing and recrossing their former path in zigzag lines. The sky Ol! this day varied much in brightness, and these observations merely proved that tho hypocotyls were continually moving in a manner resembling circumnutation. On a previous day which was uniformly cloudy, a hypocotyl was firmly secured to a little stick, and a filament was fixed to the larger of the two cotyledons, and its movement was traced on a vertical glass. It fell greatly from 8.52 A.M., when the first clot was made, till 10.55 A.M.; it then rose greatly until 12.17P.M. Afterwards it fell a little and made a loop, but by 2.22 P.M. it had risen a little and continued rising till 9.23 P.M., when it made another loop, and at 10.30 P.M. was again rising. These observations show that tho cotyledons move CnAP. I. BRASSICA. 19 v~rtically up and down all day long, and as there was some slight lateral movement, they circumnutated. The cabbage was one of the first plants, tho seedlings of which were observed by us, and we did not then know how far Fig. 9. /..f ~,/ the circumnutation of the different parts was affected by light. Young seedlings were therefore kept in complete darkness except for a minute or two during each observation, when they wore illuminated by a small wax taper held almost vertically above thorn. During the first day tho hypocotyl of one changed its course 13 times (see Fig. 9); and it deserves notice that the longer axes of tho figures described often ru·oss one another at right or nearly right angles. Another seedling was observed in the same manner, but it was much older, for it had formed a true leaf a quarter of an inch in length, and tho hypocotyl was 1~- inch in height. The figure traced was a very complex one, though the ~ovement was not so great m extent as in the last case. Emssica olamcea: circnmnutation of The hypocotyl of another hn~ocotyl, in darkness, traced on n seedling of the same ago was honzontal glass, by means of a fila-secured to a little stick, and ment .with a bead fixed across its a filament having- been fixed summit, between 9.15 A.M. anrl ~ ~.~0 A •• M. on the following morn-to tho midrib of one of the mg. F1gure here reduced to one-ootyledoos, the movement of half of original scale. tho bead was traced during· 14 h 15m (see F' 10) . d l It h · · Ig. m ar mess. s . ould be noted that the chief movement of tho cotyledons namely, up and do~n, ':ould be shown on a horizontal glass~ plato only by the lines m the direction of the midrib (that iR, c 2 |
