| OCR Text |
Show 428 MODIFIED CIRCUMNUTATION. CHAP. VIII. for the first 45 m. in a zigzag line; it then felt the full influence of the light, and tmvelled towards it for the next 2 h. 30m. in an almost straight line. The tracing has not been given, as it was almost identical with that of Apios under similar circumstances (Fig. 170). By noon it had bowed itself to its full extent; it then circumnutated about the same spot and described two ellipses; by 5 P.M. it had retreated consideral}ly from the light, through the action of apogeotropism. After some preliminary trials for ascertaining the right degree of obscurity, some seedlings were placed (Sept. 16th) before a north-cast window, and light was admitted through an ordinary linen and three muslin blinds. A pencil held close by tho pot now cast a very faint shadow on a white card, pointing from tho window. In the evening, at 4.30, and again at 6 P.M., somo of the blinds were removed. In Fig. 17 4 we see the course pursued under these circumstances by a rather old and not very sensitive cotyledon, 1· 9 inch in height, which became much bowed, but was never rectangularly bent towards the light. From 11 A.M., when the sky became rather duller, until 6.30 P.M., the ?Jigzagging was conspicuous, and evidently consisted of drawnout ellipses. After 6.30 P.M. and during the night, it retreated in a crooked line from the window. Another and younger seedling moved during the same time much more quickly and to a much greater distance, in an only slightly zigzag line towards the light; by 11 A.M. it was bent almost rectangularly in this direction, and now circumnutated about the same place. T1·opceolum m~fus.-Some very young seedlings, bearing only two leaves, and therefore not as yet arrived at tho climbing stage of growth, were first tried before a north-cast window without any blind. The epicotyls bowed themselves t~wa~ds the light so rapidly that in little more than 3 h. their_ tipS pointed rectangularly towards it. Tho lines traced were mther nearly straight or slightly zigzag; and in this latter case we see that a trace of circumnutation was retained oven under the influence of a moderately bright light. Twice whilst these epicotyls were bending towards the window, oots were mado every 5 or 6 minutes, in order to detect any .t:·ace of late~! movement, but there was hardly any; and the lmes forlll:ed Y their J'unction were nearly straight or only very slightly Zigzag, as in the other parts of the figur'e s. After the epiC· o tY I8 . had bowed themselves to the full extent towards the light, elhpses of considerable size were described in the usual manner. CU.lP. VIII. HELIOTROPISM. 429 After having seen how the epicotyls moved towards a moderately bright light, seedlings were placed at 7.48 A.M. (Sept. 7th) before a north-east window, covered by a towel, and shortly afterwards by an ordinary linen blind, but the epicotyls still moved towards the window. At 9.13 A.M. two additional muslin blinds were suspended, so that the seedlings received very little more light from the window than from the interior of the room. The sky varied in brightness, and the seedlings occasionally Fig. 175. .1ti'A.o'p.m r.p.;, . . v~.J f m maJus: heliotropic movement and cir<·umnutation of th" cpicotyl gol a yfoung seedling towards a dull lateral li O'ht traced on a horizontal ass rom 7 48 t l(l 4 o ' ori • 1 · A.M. 0 • 0 P.M. Figure reduced to one-half of the gJna scale. ~ived for a short time less light from the window than from e opposite side (as ascertained by the shadow cast) and then othn e bolif tdh e blm' d s was temporari·l y removed. In th'e evening be n ,8 were taken away, one by one. The course pursued lu:;PICotyl under these circumstances is shown in Fig. 175. tew ds the ~hole day, until 6.45 P .M., it plainly bowed itself Afte~ 6:e hgh_t; and the tip moved over a considerable space. · P.M. It moved backwards, or fl'Om the window;, till |
