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Show 55± SUl\11\iARY A~D CHAP. XII. pletoly confluent, ancl they Lroak through tho ground ~~s an arch; afterwards the petiol 'S of tho successively formed early lcav s arc arched, and thoy arc thus enabled to br n,k throngh tho bas of tho confluent petioles of tho coty luclons. In tho case of 1\fcgarrhir.a it is the plumule which breaks as an <trch through th~ tube formed by the conflu nco of tho cotyledon· petioles. With matnre plants, tho f1owt~r-stoms and the leaves of some few species, ancl tho rachis of several ferns, as they omOTgo separately from the ground, al'C likewis archccl. The fact of so many different oro·<ms in plants of many kintls breaking throngh tho gronncl under the form of an arch, shows that this must be in some manner highly important to them. According w Haberlandt, the tel1flcr growing apex is thus saved from abrasion, and this is probauly tho true explana· tion. But as Loth 1 gs of the arch grow, their power of breakino- through tho groun<l \rj]J be much in· creased as long as tho tip rem<.tins 'ritLin the seed· coats and has a point of support. In tho case of monocotyledons the plnmu]o or cotyleLlon is rarely arched, as far as we have s en ; Lnt this is tho case with the leaf-like cotyl don of tile onion; and the crown of the arch is here strengthened by a special protuberance. In the Graminom tbo summit of the straight, sheath-like cotyledon is developed into a hard sharp crest, which evidently serves for breaking through the earth. With dicotyledons tbe arching of the opicotyl or hypocotyl often appears as if it merely resulted from the mann or in which the parts arc })acked within tho sccrl; but it is doubtf11l whether this js the whole of tho truth in any case, and it cer· tainl y was not so in several cases, in which tho arch· ing was seen to commence after tho parts had wholly CHAP. XII. CONCLUDING REMARKS. 555 escaped from the seed-coats. As the archin()' occunod in whatever position tho soocls were placed, it is 110 doubt due to temporarily increased growth of tho nature of epinasty or hyponasty along one side of tho part. As this habit of tho hypocotyJ to arch itself appears to be universal, it is probably of very ancient origin. It is therefore not surprising that it should be inherited, at least to some extent, by plants havinrr hypogoan cotyledons, in which tho hypocotyl is onl~ slightly developed and never protrudes above the ground, and in which the arching is of course now quite useless. This tendency explains, as we havo seen, the curvature of the hypocotyl (and tho consequent movement of the radicle) which was :first observed by Sachs, and which we have often had to refer to as Sachs' curvature. The several foregoing archcu organs are continually circumnutating, or end avouring to circumnutate, even before they break through th ground. As soon as any part of the arch protrudes from the seed-coats it is acted upon by apogeotropism, and both the legs bend upwards as quickly as the surrounding earth will permit, until the arch stands vertically. By continued growth it then forcibly breaks through the ground; but as it is continually striving to ciroumnutate this will aid its emergence in some slight degree, for we know that a circumnutating hypocotyl can push away ~amp sand on all sides. As soon as the fa,intest ray of hght reaches a seedling, h liotropism will gui<le it through any crack in the soil, or through an ntangled ~ass of overlying vegetation ; for apogeotropism by 1tself can direot the seedling only blindly up,m.rds. Hence probably it is that sensitiveness to lio·ht resides . b ~n the tip of the cotyledons of the Graminem, and in |