OCR Text |
Show 420 MODIFIED CillCUMNUTATION. C11Ar. VIII. rate of bending towards it, and tho accuracy with which they point towards its source, &c., will be given. Afterwards it ~ill bo shown-and. t.his seems to us a point of much Interest-that sens1t1voness to light is sometimes confined to a small part of the plant; and that this part :vhon stimuhtte~ ~y light, transmits an influence to d1stant parts, exCltmg them to bend. Iieliotropism. --When a plant which is strongly heliotropic (and species differ much in this respect) is exposed to a bright lateral light, it bonds quickly towards it, and the course pursued by tho stem is quite or nearly straight. But if tho light i.s mu~h dimmed, or occasionally interrupted, or admitted m Fig. 1G8. only a slightly oblique direction, the course pursued is more or less zigzag; and. as wo have seen and shall again see, such zigzag move· ment results from tho elongation or drawing out of the ellipses, loops, &c., which the plant would have de· scribed ' if it had been illumina.te d from above. On several occaswns we were much struck with this fact, whilst observing the circumnuta· tion of highly sensitive seedli.ngs, which were unintentionally Illn· minated rather obliquely, or only at successive intervals of time. Bata vulgwis: circumnn· tation of hypocotyl, ucfiectcd by the light being slightly lateral, traced ou a horizontal glass from 8.30 A.M. to 5.30 P.M. Directiou of the lighted taper by which it was illuminated, showu by a line joining For instance two young seedlings of the first and penultimate B t l ,. . w~re placed in the middle dots. Figure reduced to e a vu gan.s · dows and one-third of the original of a room w1th north-cast wm ''. scale. were kept covered up, except dur~g each observation which lasted for only a minute or two; but.\ 0 result was thnt tl1eir hypocotyls bowed themselves to .the sJce, whence some light occasionally en t· erccl , m· rIn es• which were CHAP. VIII. IIELIOTROPISl\1. 421 only slightly zigzag. Although not a sjnglo ellipse was even approximately formed, we inferred from tho zigzag lines-and, as it proved, correctly-that their hypocotyls were circumnutating, for on the following clay these same seedlings were placed in a completely darkened room, and were observed each time by tho aid of a small wax taper held almost directly above them, and their movements Fig. 169. were traced on a horizontal glass above; and now their hypocotyls clearly circum-nutated (Fig. 168, and Fig. 39, formerly given, p. 52); yet they moved a short distance towards the side where tho taper was held up. If we look at these diagrams, and suppose that the taper had been held more on one side, and that tho hypocotyls, still circumnutating, had bent themselves within the same time much more towards the light, long zigzag lines would obviously have been the result. Again, two seedlings of Solanum lycopersicum were illuminated from above, but accidentally a little more light entered on one than on any other side, and their hypocotyls became slightly bowed towards the brighter side ; they moved in a zigzao-line and described in their course two li ttl~ ~risngles, as seen in Fig. 37 (p. 50), and m another tracing not given. Tho sheath-like cotyledons of Zea mays behaved, under / n.~ly similar circumstanceR, in a nearly B."a.~n. . . Bllllllar manner, as described in our first Avena sattva: heh?tropw chapter (p 64) £ tl b d th movement and cJrcnmd . · ' or 1ey owe omsolves nutation of sheath-like u~g the whole day towards one side, cotyledon (lt inch in making, however, in their course some height) traced on horiconspicuous flexures Before w 1 zontal glass from 8 A.:M. how gr tl d' · . 0 mew to 10.25 P.M. Oct.llith. ea Y or mary mrcumnutation was modified by a lateral light, some seedling oats, with mthor old ~d therefor~ not hig.hly sensitive cotyledons, wore placed in nt of a no1th-oast wmdow, towards which they bent all day in ~s~ngl~ zigzag courso. On the following day they continued 1~ nd m tho same direction (Fig. 169), but zigzagged much · The sky, however, became between 12.40 and 2.35 P.M. |