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Show 46± SENSI'l'IVENESS 'fO LIGII'l'. CuAP. IX. light, continue to bend in the same direction for between a quarter and half an hour. In the two xpcrimcnts just given, the cotyledons moved backwards or from tho window shortly after being subjected to darkness ; and whilst tracing the circumnutation of various kinds of seedlings exposed to a lateral light, we repeatedly observed that late in the evening, as the light waned, they moved from it. This fact is shown in sumo of the diagrams given in the last chapter. Wo wished therefore to learn whether this was wholly duo to apoo·eotropism, or whether an organ after bending towarcls tho light tended from· any other cause to bend from it, as soon as the light failed. Accordingly, two pots of seedling Phalaris and one pot of seedling Brassica were exposed for 8 h. before a paraffin lamp, by which time the cotyledons of the former and the hypocotyls of the latter were bent rectangularly towarcls the light. The pots were now quickly laid horizontally, so that the upper part.s of the cotyledons and of the hypocotyls of 9 scedh~gs projected vertically upwards, as proved Ly a plumb-lme. In this position they could not be acted on by apogeotropism, and if they possessed any .t~ndcncy ~o straighten themselves or to bond in oppositiOn t~ ~heir former heliotropic curvature, this would be exhibited, for it would be opposed at first very slightly by apo~eo· tropism. They were k ept in the dark for 4 h., d~nng which time they were twice looked at ; but no ~mfor~ bending in opposition to their former ~elwt~·opiC curvature could be detected. vVe have sard umjorm bend.m g because they c·n cumnu t a t e d m· their new position,' and after 2 h. were m· e1 m· eu,, m· dif.f erent directions (between 4° and 11 °) from the perpend.I~ula~. Their directions were also changed after two additwna hours, and agai. n on t h e 1i!0 l lo w.m g mar ni·nOb' ' We may CHAP. IX. SEN~ITIVENESS '1'0 LIGHT. 465 therefore conclude that the bonding back of plants from a light, when this becomes obscure m· is Pxtinguished, is wholly due to apogeotropism.* In our various experiments we were often struck with the accuracy with which seedlings pointed to a light although of small size. To t est this, many seedlings of Phalaris, which had germinated in darkness in a very narrow box several feet in length, were placed in a darkened room near to and in front of a lamp having a small cylindrical wick. The cotyledons at the two ends and in the central part of the box, would ·therefore have to bend in widely differ ent directions in order to point to the light. After they had become rectangularly bent, a long white thread was stretched by two persons, close over and parallel, first to one and then to another cotyledon ; and the thread was found in almost every case actually to intersect the small circular wick of the now extinguished lamp. ~rhe deviation from accuracy never exceeded, as far as we could judge, a degree or two. 'l1his extreme accuracy seems at :first surprising, but is not r eally so, for an upri~ht cylindrical stem, whatever its position may ~e ~Ith respect to the light, would have exactly halt' Its Circumference illuminated and half in shadow; and as the difference in illuminatiou of the two sides is the exciting cause of heliotropism, a cylinder would ~aturally bend with much accuracy towards the light. T~e cotyledons, however, of Phalaris are not cylindncal, but oval in section ; and the longer axis was to the shorter axis (in the one which was measm·ed) as 100 to 70. Nevertheless, no difference could Le . "wI·t - appears from a refer enee N 1.esner ('Die UuJul1renJe lutatlon der Internodien ' p 7) tth l att H· Mu '1 1 er of ."l hurgau' f.u .u nd' a a stem wh1eh is bending heliotropically is at the same ti1110 striving, through apo""eotropi ~m to raise itself into a ve~tical p o~1~ tion, 2 H |