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Show 54G , UMl\IAR Y AND OIIAP. X.ll. CHAPTER XII. SUliTllfAUY AND C ONCLUDING R~<:AfAHKS. Nature of the circumnuto.ting movement- History of a germinating seeu- Tho radicle first protrudes and drcumnutatcs-lts tip highly sensitivo-Emorgenco of the hypocotyl oro~ the cpicu.tyl from tho ground under the form of ~n urch- Its cncumnutat~on and that of tho cotylcdons-'fho eodhng throws up a len f-ben~mg stem-The circumuutation of all tho parts or organ s-M.odt~C<l circumnutation-Epinasty and hyponasty-Mo v e~1o nts of .chmbmg l.ants - Nyctitropic movements-Mo\'Oments excttcd by hght and ~ravitation _Localised son itivoness - Resemblance .between tho movements of plants and animals-The tip of the rudtele nets like a brain. IT may be useful to the reader if we brioily s~m up the chief conclusions, which, as far as we can JU~ge, have been fairly well established by tho observat10~1s O'iven in this volume. All the parts or organs m ~very plant whilst they continue ~o. grow, and. some parts which are provided with pulvm1 after .they ~~v.e ceased to grow, are continually circumnutatmg. I.hls movement commences even before the young seedlmg has broken through the ground. '11he natl~re of the movement and its causes, as far as ascert.amed, have been briefly described in the Intro<luctwn. . Why every part of a plant w h1.1 s t 1·t 1· s grow·m g , <a nd •m some cases after growth has cease d , s h ou ld 11av.o tts cells rendered more turO'escent and its cell-wa.lls more extcnsile first on onb e si<le and t h en °11 a· n oth· er ' thulds inducinO' circumnutation, is not known. It w~ul appear ba s if the changes 1· n tl1 e co lls required penocs of rest. CHAP. XII. CONCLUDING REl\IARKS. 547 In somefc ~~s, as with the hypocotyls of Brassica, the l~aves o 1~mea an<l the joints of the Graminero, the mrcumnutatmg movement when viewed under the microscope is seen to consist of innumerable small oscillations. The part under observation suddenly jerks forwards for a length of ·002 to ·001 of an inch and then slowly retreats for a part of this distance: after a few seconds it again j erks forwards, but with many i~termissions. Th? :·ctreating movement apparently IS d~e to t~e elast1c1ty of the resisting tissues. How far th1s oscillatory movement is general we do not know, as not many circumnutating plants were observed by us under tho microscope ; but no such movement could be detected in the case of Drosera with a ~-inch object-glass which we usecl. The phenomenon Is a remarkable one. The whole hypocotyl of a cabbage or the whole loaf of a Dionrea could not jerk forwards unless a very lar()'c nu~ber of cells on one side were simultaneously affected. Are \ve to suppose that these cells steadily be ·ome more and more turgescent o~ one .side, until the part suddenly yields and bends, mducmg what mav be called a microscopically min~te earthquake ir{ the plant; or do the cells on one stele suddenly become turo·escent in an intermittent manner; each forward m~vement thus caus.ed being opposed by the elasticity of the tissues? . Cucumnutation is of paramount importance in the hfe of every plant; for it is throuo·h its modification that many highly beneiicial or ne~essary movements have been acquired. When 1i o·ht strikes one side of a.pl~nt, or light changes into darkness, or when graVItatw.n acts on a displaced part, the plant is enabled In some unknown manner to increase tho always varying turgescence of tho cells on one side . so that tl.te ordinary circumnutating movement l~ 2 .N 2 |
