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Show 422 MODIFIED CIRCUMNUTA'l'ION. CuAP. nn. ?vereas~ with extraordinaril! dark thunder-clouds, and it was mterestmg to note how plamly tho cotyledons circumnutat 1 dnring this interval. ec The foregoing observations arc of some Fig. 170 · value, from having been made when we were I ~n .S not attending to heliotropism; and they led ] ] us to experiment on several kinds of sced-t~ lings, by exposing them to a dim laternlli rrht ~ M so as to observe tJ1c gradations betwec~ ~ ~ ordinary ch·cumnutation and heliotropism. E .~fl Seedlings in pots were placed in front of, j ~;:..., and about a yn,rcl from, a north-cast window · "B -:5 on each side aHCl over the pots black bourd~ «:00 ~ ~ were placed; in the rear the pots were open ~ ~ to tl1c djffuRc<-1 ligl1t of tho room, "·hich ~ rr. had a second north-east and a north-west I ]n ~ window. By hanging up one or more blinds :::: o before the window where the seedlings Rtood, :;; ~ it was easy to dim tho light, RO that very o .-< .S 2 little more onterec1 on this Ride than on tho J 'o )::l opposite one, which received the diffused ~ ..:l light of tho room. Late in the evening the :=:g blinds wore succm;sively removed, and as the ~00 8 8 plants ha,d been subjecte d during- the da y to 8.. ~ a very obscure light, th ey continued to l1cnd ~, ..... ..c ~ towards tbo winclo·w later in tho evening than 'o ~ would othonviso have occurred. Most of tho r:l ~ seedlings were selected because they "lroro ~ § known to be highly sensitive to light, and ~ ·E some because they were but little sensitive, a _g or had become so from having grown old. ·S. ~ ~ Tho movements were traced in the usual ] ~ ,_: manner on a horizontal glass cover; :1 fine ~ ~ .2 glass filament with little triangles of pupcr ..c .;;.~Jl having boon cemented in an upright position ;,; ~~ ~ to tho hypocotyls. Whenever tl10 stem or ;:! bfl 0 l tl ~ ~] hypocotyl bccmne much bowc l towarb·c s 1 1 10 :; -- ~ light, the latter part of its course ac o I;,~ ~ be traced on a vertical glass, parallel to tho l...:.< o window, and at right angles to the horizontal ~ glass cover . .Apios graveolens.-Tho hypocotyl bonds in a few hours rectnn· CHAP. VIII. IIELIOTROPISl\I. 42~ gularly towards a bright latoml light. In order to ascertain how straight a course it would pursue when f~1.irly well illuminated on one side, seedlings were first placed before a , outh-woRt window on a cloudy and rainy morning; and tho movement of two hypocotyls were traced for 3 h., during which time they became greatly bowed towards the light. One of th ose tmcingR is given on p. 422 (Fig. 170), and the course may bo seen to be almost straight. But tho amount of Ught on this occasion wn.s superfluous, for two seedlings wore placed before a north-cast window, protected by an ordinary linen and two muslin blinds, yet their hypocotyls moved towards this rather dim light in only slightly zigzag lines; but after 4 P.M., as tho light wanecl, the lines became distinctly zigzag. One of these secdlin~s, moreover, described in the afternoon an ellipse of considomble size, with its longer axis directed towards tho window. We now determined that the light should bo made dim enough, so we began by exposing seveml seedlings before a north-east window, protected by one linen blind, three muslin blinds, and a towel. But so little light entered that a pencil cnst no perceptible shadow on a white card, and the hypocotyls did not bend at all towards tho window. During this time, from 8.15 to 10.50 A.M., the hypocotyls zigzagged or circumnutated near the snme spot, as may be seen at A, in Fig. 171. The towel, therefore, was removed at 10.50 A.M., and replaced by two muslin blinds, and now tho light passed through one ordinary linen and four muslin blinds. When a pencil was held upright on a card close to the seedlings, it cast n. shadow (pointing from the window) which could only just be di·sati nguished. Yet this very slio-ht excess of lio·ht on b b ~ne s1.e sufficed to cause the hypocotyls of all the seedlings m:mediately to begin bending in zigzag lines towards tho IVl.n~ow. The course of one is shown at A (Fig. 171): after movmg towards the window from 10.50 A.M. to 12.48 P.M. it ~nt from the window, and then returned in a nearly pamllol hne; that is, it almost completed between 12.48 and 2 P.:II • :h narrow ellipse. Late in the evening, as the light waned, e hypocotyl ceased to bend towards the window, and circum~ utated on a small scale round the same spot; during the night tihmoved considerably backwards that is became more uprig·ht ro ug hthe actw· n of apogeotrop' ism A' t B we have a tracing' ofth · ' ... •h e movements of another seedling from the hour (10.50 A.llf.) 11 en the towel was removed; and it is in all essential respects |