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Show 478 TRANSMITTED EFFECTS OF LIGHT. CuAr. IX. E~per~ments similnr to tho foregoing ones were tried, nncl wo Will give the. results as .l>rioAy as possible. 'l'ltoy arc somewhat less . conchlSivo thnu m the cnso of Phalaris, auJ. this may p_oss1~ly be ac~?untod for by i.ho sensitive zone varying in extenSion, m a spocws so long 9nltiv11tcd nncl v11riablc as the common Oat. Cotyledons a little under throc-qnartcrs of an inch in height were selected for tri11l : six lta.d their summits protected from light by tin-foil caps, ·25 i110h in depth, and two others bv caps · 3 inch in depth. Of those 8 cotylcJ.ons, five romaine~[ upi·ight during 8 homs of exposure, although their lower parts wore fully exposed to tho light all tho time; two were very slightly, ancl one considomhly, bowed towards it. CapN only· 2 or ·22inch in dop th wore placed ovm· 4 other coty lcclon. , and now only one remained upright, one was slightly, and two considerably bowed to tho lio·ht. In this and tho following cases all the free seedlings in tho same pots became greatly bowed to the light. Our 11ext trial was mado with short lengths of thin a])(] fairly transparent quills; for glass-tnbcs of snfiicient diameter to go over tho coi.yledons wouldlw.vc been too hcrwy. Firstly, the summits of 13 cotyledons were enclosed in unpo,intecl quills, and of tho. e 11 became grently and 2 slightly bo1vcd to the lio·ht; so that tho more act of enclosmo did not prevent tho lower part from becoming bowed. Secondly, the snmmits of 11 cotyloc1ons wore onclo odin gui ll i:i ·3 inch in length, prtintcd so as to be impormea.ble to light; ol' thos , 7 did not become at all inclined. towards the light, but 3 of them were slightly bent more or lo s tr:.tmvcrsoly with respect to the line of light, and those might perhaps have been altogether excluded; one alone was slightly bowed towards the light. Painted quills, ·25 inch in length, wore placed overthe smnmits of 4 other cotyledons; of those, one alone remained upright, n second w:.ts slightly bowed, and the two others as much bowed to the light as the free seedlings in the same pots. These two latter cases, considering that the caps wore · 25 in length, rtre inexplicable. Lastly, the summits of H cotyledons wore coatocl with flexible and highly transparent gold-beatori:>' skin, and all becan:e a .. much bowed to the light as the free seo<llings. The summJtso! 9 other cotyledons were similarly coated with gold-ueaters' ~km, which was then painted to a depth of between ·25 and ·3 ~nch, so as to be impermeable to light; of these 5 rcm<~ined uprJgh~ and 4 were well bowed to tho light, almost or qUito as well a CuAP. IX. TRANSMITTED EFFECTS OF I.,IGJIT. 479 the free seedlings. Those latter fom cases, as well as tho two in the last paragraph, offor a strong exception to tho rule that the illumination of the upper part determines the curvature of the lower part. Nevertheless, 5 of those 8 cotyledons remain 1 quite up~ight, alth?ugh their lower hal vos were fully illuminat.~~l all tl~e time; a~d It wo~ld almost be a prodigy to find five free seedlings sta?dmg vertiCally al'tor an exposure for several ltonrs to a lateral hght . . The cotyledons of A vena, like those of Phalaris, wben growing m soft, damp, fine sand, leave an open ere. centric funow on tho shaded side, after bencli~1g to a lateral lio-ht; and they h como bowed beneath the surface at a dop.lh to which, as we know light cannot penetrate. Tho arcs of tho chords of tho burio(l bowed portions formed in two cases angles of 20° and 21 o with the perpendicular. Tho open funows on tho shaded sido wore in four cases, · 008, · 016, · 024, and · 024 of an i11ch in breadth. ' 1J1'assica olemcea (Common Rocl).-It will hero be shown that the upper half of the hypocoty l of tho cabbage, w lton illuminai()d by~ lateral light, determines tho curvn.turo of tho lower half. It ~s necessary to oxperimontiso on young scedliltgs about half anmch or rather less in height, for when grown to an inch and. upwards the bas~l part ?Oaf:i~R to bond. W o fir ·t tried painting tho hypocotyls w1th Ind1an mlc, or cutting off their summitf:i for various lengths; but those experiments arc not worth giving, though they .confirm, as far as they can be trusted, tho result.. of.the followmg ones. Those were made by folding gol<l-beators' sk~n .one~ ro~nd the upper halves of young hyl)Ocotyls, and pamtmg 1t thickly with Indian ink or with black grease. As a control experiment, tho same transparent skin, loft unpainted, wns folded round tho upper ltalvos of 12 hypocotyls; and those all became greatly curved to tho light, oxcOJJiing one, which waR only m?dorately cnrvod. 'Cwonty other yourw bypocotyls had the skm round their upper halves llaintocl, whilst their lower halves were loft quite uncovered. Those soocllings wore then e~po~ed, generally for between 7 a11d 8 h., in a box blackened Withm and open in front, either boforo a south-west window or a paraffin lamp. This expo. uro was amply su11iciont, as was 8.how~ by the strongly-marked. heliotropism of all the free seedbugs m. tho same pots; nevorthclcFs, some were left exposed to tho light for a much lo11gor tiino. Of tlte 20 hypocotylR thus trcatou, 14 remained quit0 11pright, and G became slio·hily bowed to the light; but 2 of those latter cases wore not I~ally |
