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Show 430 MODIFIED CIRCUMNU'l'A'l'ION. CUAP. VIII. 10.40 P.M., when tho last dot was made. Here, then, wo have a distinct heliotropic movement, effected by llleans of six elongated figures (which if dots had ueun made every few minutes would have been more or lo. s elliptic) directetl towards Fig. 176. T1·opmolum majus: heliotropic movement aml circumnutation of an old internode towards a lateral light, traced on a horizontal glass from 8 A.M. NoY. 2nd to 10.20 A.M. Nov. 4th. Broken lines show the nocturnal course. tho bght, with the apex of oachsuccossi vo ellipse nearer to the window than tho previous one. Now,iftbe light had boon only a little brighter, the epieotyl would have bowed itself more to the light, as we may safely conclude from the previous trials; there would also have been less lateral movement, and the ellipses or other figures 'Youlcl have been drawn out into a strongly marked zigzag line, with probably one ortwo mall loops still formed. lf the light had been much brjghter, we should have had a slightly zjgzag line, or one quite straight, for there would have been more movement in the direction of the light, and muclllessfrom side to side. Sachs states that the oltler inter· nodes of this Troproolum are aphe· liotopic ; we therefore placed a plant, lU inches high, in a box, blackened within, but open on ono side in front of a north-east window without any blind. A filament was fixed to the third intcmodc from the summit on one plant, and to the fourth internode of another. These intemodes were either uot old enough, or the light wasnots~fficicntly bright, to induce aphello· tropism, for both plants bent slowly towar ds , ~·n s tc a d of fromf the window during four days. The course, durmg two da)S 0 the fi rst-montw. ned m. terno de , I.S gi.v en m· F 1·g · 176 '· and wo . sec that it either circumnutated on a small scale, or trav?Jled 10 ~ zigzag line towards the light. We have thought th~s c~e ~ feuble heliotropism in one of the older internodes of a Pan' CIIAP. VIII. HELIOTROPISM. 431 which, whilst young, is so extremely sensitive to light, worth giving. Cassia tora. -The cotyledons of this plant arc extremely sensitive to light, whilst tho hypocotyls are much less Fig. 177. sensitive than those of most other seedlings, as we had often observed with surprise. lt seemed therefore worth while to trace their movements. They were exposed to a lateral light before a north-east window, which was at first covered merely by a muslin blind, but as the sky grew brighter about 11 A.M., an additional linen blind was suspended. After 4 P.M. one blind and then the other was removed. The seedlings were protected on :':'!: o~o'pm 'ltft each side and above, but were open to the diffused light of the room in the rear. Up-right filaments were fixed to r.:· the hypocotyls of two seed- ~gs, which stood vertically ;[.:' mth e morning. The accom- f panying figure (Fig. 177) ! shows the course pursued by i one ~f them during two days; \! oa:m. a~'h but_1t should be particularly '\ )7oan 11.otft noticed that during the ~-4:1' second day the seedlings were Ram.'/1~11 \/ kept in darkness, and they Cassia tara: heliotropic movement and then circumnutated round ~ircu~nut_ation of a hypocotyl (1-t near! th mch In hctght) traced ou a horizontal Y e same small spaeo. glass fl'om t! A.M. to 10.10 P.M. Oct. ~n the first day (Oct. 7th) 7th. Also its circumnutation in e hypocotyl moved from darkness from 7 A.M. Oct. 8th to 7.'15 ~ A.?tl: to ~2.23 P:M., toward A.M. Oct. 9th. he light m a z1gzag line, then turned abruptly to tho loft and afterwards described a small ellipse. Another irregular |
