a history of science-4-第25节

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d　that　in　no　single　case　has　the　exact　nature　of　the　metabolisms　wrought　by　these　organs　been　fully　made　out。　　Each　is　in　its　way　a　chemical　laboratory　indispensable　to　the　right　conduct　of　the　organism；　but　the　precise　nature　of　its　operations　remains　inscrutable。　The　vast　importance　of　the　operations　of　these　intermediate　organs　is　unquestioned。

A　consideration　of　the　functions　of　that　other　set　of　organs　known　collectively　as　the　nervous　system　is　reserved　for　a　later　chapter。



VI。　THEORIES　OF　ORGANIC　EVOLUTION

GOETHE　AND　THE　METAMORPHOSIS　OF　PARTS

When　Coleridge　said　of　Humphry　Davy　that　he　might　have　been　the　greatest　poet　of　his　time　had　he　not　chosen　rather　to　be　the　greatest　chemist；　it　is　possible　that　the　enthusiasm　of　the　friend　outweighed　the　caution　of　the　critic。　　But　however　that　may　be；　it　is　beyond　dispute　that　the　man　who　actually　was　the　greatest　poet　of　that　time　might　easily　have　taken　the　very　highest　rank　as　a　scientist　had　not　the　muse　distracted　his　attention。　　Indeed；　despite　these　distractions；　Johann　Wolfgang　von　Goethe　achieved　successes　in　the　field　of　pure　science　that　would　insure　permanent　recognition　for　his　name　had　he　never　written　a　stanza　of　poetry。　Such　is　the　versatility　that　marks　the　highest　genius。

It　was　in　1790　that　Goethe　published　the　work　that　laid　the　foundations　of　his　scientific　reputationthe　work　on　the　Metamorphoses　of　Plants；　in　which　he　advanced　the　novel　doctrine　that　all　parts　of　the　flower　are　modified　or　metamorphosed　leaves。

〃Every　one　who　observes　the　growth　of　plants；　even　superficially；〃　wrote　Goethe；　〃will　notice　that　certain　external　parts　of　them　become　transformed　at　times　and　go　over　into　the　forms　of　the　contiguous　parts；　now　completely；　now　to　a　greater　or　less　degree。　　Thus；　for　example；　the　single　flower　is　transformed　into　a　double　one　when；　instead　of　stamens；　petals　are　developed；　which　are　either　exactly　like　the　other　petals　of　the　corolla　in　form；　and　color　or　else　still　bear　visible　signs　of　their　origin。

〃When　we　observe　that　it　is　possible　for　a　plant　in　this　way　to　take　a　step　backward；　we　shall　give　so　much　the　more　heed　to　the　regular　course　of　nature　and　learn　the　laws　of　transformation　according　to　which　she　produces　one　part　through　another；　and　displays　the　most　varying　forms　through　the　modification　of　one　single　organ。

〃Let　us　first　direct　our　attention　to　the　plant　at　the　moment　when　it　develops　out　of　the　seed…kernel。　The　first　organs　of　its　upward　growth　are　known　by　the　name　of　cotyledons；　they　have　also　been　called　seed…leaves。

〃They　often　appear　shapeless；　filled　with　new　matter；　and　are　just　as　thick　as　they　are　broad。　　Their　vessels　are　unrecognizable　and　are　hardly　to　be　distinguished　from　the　mass　of　the　whole；　they　bear　almost　no　resemblance　to　a　leaf；　and　we　could　easily　be　misled　into　regarding　them　as　special　organs。　　Occasionally；　however；　they　appear　as　real　leaves；　their　vessels　are　capable　of　the　most　minute　development；　their　similarity　to　the　following　leaves　does　not　permit　us　to　take　them　for　special　organs；　but　we　recognize　them　instead　to　be　the　first　leaves　of　the　stalk。

〃The　cotyledons　are　mostly　double；　and　there　is　an　observation　to　be　made　here　which　will　appear　still　more　important　as　we　proceedthat　is；　that　the　leaves　of　the　first　node　are　often　paired；　even　when　the　following　leaves　of　the　stalk　stand　alternately　upon　it。　Here　we　see　an　approximation　and　a　joining　of　parts　which　nature　afterwards　separates　and　places　at　a　distance　from　one　another。　It　is　still　more　remarkable　when　the　cotyledons　take　the　form　of　many　little　leaves　gathered　about　an　axis；　and　the　stalk　which　grows　gradually　from　their　midst　produces　the　following　leaves　arranged　around　it　singly　in　a　whorl。　This　may　be　observed　very　exactly　in　the　growth　of　the　pinus　species。　Here　a　corolla　of　needles　forms　at　the　same　time　a　calyx；　and　we　shall　have　occasion　to　remember　the　present　case　in　connection　with　similar　phenomena　later。

〃On　the　other　hand；　we　observe　that　even　the　cotyledons　which　are　most　like　a　leaf　when　compared　with　the　following　leaves　of　the　stalk　are　always　more　undeveloped　or　less　developed。　This　is　chiefly　noticeable　in　their　margin　which　is　extremely　simple　and　shows　few　traces　of　indentation。

〃A　few　or　many　of　the　next　following　leaves　are　often　already　present　in　the　seed；　and　lie　enclosed　between　the　cotyledons；　in　their　folded　state　they　are　known　by　the　name　of　plumules。　Their　form；　as　compared　with　the　cotyledons　and　the　following　leaves；　varies　in　different　plants。　　Their　chief　point　of　variance；　however；　from　the　cotyledons　is　that　they　are　flat；　delicate；　and　formed　like　real　leaves　generally。　They　are　wholly　green；　rest　on　a　visible　node；　and　can　no　longer　deny　their　relationship　to　the　following　leaves　of　the　stalk；　to　which；　however；　they　are　usually　still　inferior；　in　so　far　as　that　their　margin　is　not　completely　developed。

〃The　further　development；　however；　goes　on　ceaselessly　in　the　leaf；　from　node　to　node；　its　midrib　is　elongated；　and　more　or　less　additional　ribs　stretch　out　from　this　towards　the　sides。　The　leaves　now　appear　notched；　deeply　indented；　or　composed　of　several　small　leaves；　in　which　last　case　they　seem　to　form　complete　little　branches。　　The　date…palm　furnishes　a　striking　example　of　such　a　successive　transformation　of　the　simplest　leaf　form。　A　midrib　is　elongated　through　a　succession　of　several　leaves；　the　single　fan…shaped　leaf　becomes　torn　and　diverted；　and　a　very　complicated　leaf　is　developed；　which　rivals　a　branch　in　form。

〃The　transition　to　inflorescence　takes　place　more　or　less　rapidly。　In　the　latter　case　we　usually　observe　that　the　leaves　of　the　stalk　loose　their　different　external　divisions；　and；　on　the　other　hand；　spread　out　more　or　less　in　their　lower　parts　where　they　are　attached　to　the　stalk。　If　the　transition　takes　place　rapidly；　the　stalk；　suddenly　become　thinner　and　more　elongated　since　the　node　of　the　last…developed　leaf；　shoots　up　and　collects　several　leaves　around　an　axis　at　its　end。

〃That　the　petals　of　the　calyx　are　precisely　the　same　organs　which　have　hitherto　appeared　as　leaves　on　the　stalk；　but　now　stand　grouped　about　a　common　centre　in　an　often　very　different　form；　can；　as　it　seems　to　me；　be　most　clearly　demonstrated。　　Already　in　connection　with　the　cotyledons　above；　we　noticed　a　similar　working　of　nature。　The　first　species；　while　they　are　developing　out　of　the　seed…kernel；　display　a　radiate　crown　of　unmistakable　needles；　and　in　the　first　childhood　of　these　plants　we　see　already　indicated　that　force　of　nature　whereby　when　they　are　older　their　flowering　and　fruit…giving　state　will　be　produced。

〃We　see　this　force　of　nature；　which　collects　several　leaves　around　an　axis；　produce　a　still　closer　union　and　make　these　approximated；　modified　leaves　still　more　unrecognizable　by　joining　them　together　either　wholly　or　partially。　　The　bell…shaped　or　so…called　one…petalled　calices　represent　these　cloudy　connected　leaves；　which；　being　more　or　less　indented　from　above；　or　divided；　plainly　show　their　origin。

〃We　can　observe　the　transition　from　the　calyx　to　the　corolla　in　more　than　one　instance；　for；　although　the　color　of　the　calyx　is　still　usually　green；　and　like　the　color　of　the　leaves　of　the　stalk；　it　nevertheless　often　varies　in　one　or　another　of　its　partsat　the　tips；　the　margins；　the　back；　or　even；　the　inward　sidewhile　the　outer　still　remains　on　green。

〃The　relationship　of　the　corolla　to　the　leaves　of　the　stalk　is　shown　in　more　than　one　way；　since　on　the　stalks　of　some　plants　appear　leaves　which　are　already　more　or　less　colored　long　before　they　approach　inflorescence；　others　are　fully　colored　when　near　inflorescence。　　Nature　also　goes　over　at　once　to　the　corolla；　sometimes　by　skipping　over　the　organs　of　the　calyx；　and　in　such　a　case　we　likewise　have　an　opportunity　to　observe　that　leaves　of　the　stalk　become　transformed　into　petals。　Thus　on　the　stalk　of　tulips；　for　instance；　there　sometimes　appears　an　almost　completely　developed　and　colored　petal。　Even　more　remarkable　is　the　case　when　such　a　leaf；　half　green　and　half　of　it　belonging　to　the　stalk；　remains　attached　to　the　latter；　while　another　colored　part　is　raised　with　the　corolla；　and　the　leaf　is　thus　torn　in　two。

〃The　relationship　between　the　petals　and　stamens　is　very　close。　In　some　instances　nature　makes　the　transition　regulare。g。；　among　the　Canna　and　several　plants　of　the　same　family。　　A　true；　little…modified　petal　is　drawn　together　on　its　upper　margin；　and　produces　a　pollen　sac；　while　the　rest　of　the　petal　takes　the　place　of　the　stamen。　In　double　flowers　we　can　observe　this　transition　in　all　its　stages。　In　several　kinds　of　roses；　within　the　fully　developed　and　colored　petals　there　appear　other　ones　which　are　drawn　together　in　the　middle　or　on　the　side。　　This　drawing　together　is　produced　by　a　small　weal；　which　appears　as　a　more　or　less　complete　pollen　sac；　and　in　the　same　proportion　the　leaf　approaches　the　simple　form　of　a　stamen。

〃The　pistil　in　many　cases　looks　almost　like　a　stamen　without　anthers；　and　the　relationship　between　the　formation　of　the　two　is　much　closer　than　between　the　other　parts。　　In　retrograde　fashion　nature　often　produces　cases　where　the　style　and　stigma　（Narben