darwin and modern science-第125节

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eal　scientific　interest。　Kolreuter　mentioned　that　pollen　may　be　carried　from　one　flower　to　another　partly　by　wind　and　partly　by　insects。　　But　he　held　the　view；　and　that　was；　indeed；　the　natural　assumption；　that　self…　fertilisation　usually　occurs　in　a　flower；　in　other　words　that　the　pollen　of　a　flower　reaches　the　stigma　of　the　same　flower。　　He　demonstrated；　however；　certain　cases　in　which　cross…pollination　occurs；　that　is　in　which　the　pollen　of　another　flower　of　the　same　species　is　conveyed　to　the　stigma。　　He　was　familiar　with　the　phenomenon；　exhibited　by　numerous　flowers；　to　which　Sprengel　afterwards　applied　the　term　Dichogamy；　expressing　the　fact　that　the　anthers　and　stigmas　of　a　flower　often　ripen　at　different　times；　a　peculiarity　which　is　now　recognised　as　one　of　the　commonest　means　of　ensuring　cross…pollination。

With　far　greater　thoroughness　and　with　astonishing　power　of　observation　C。K。　Sprengel　（1750…1816）　investigated　the　conditions　of　pollination　of　flowers。　　Darwin　was　introduced　by　that　eminent　botanist　Robert　Brown　to　Sprengel's　then　but　little　appreciated　work；〃Das　entdeckte　Geheimniss　der　Natur　im　Bau　und　in　der　Befruchtung　der　Blumen〃　（Berlin；　1793）；　this　is　by　no　means　the　least　service　to　Botany　rendered　by　Robert　Brown。

Sprengel　proceeded　from　a　naive　teleological　point　of　view。　　He　firmly　believed　〃that　the　wise　Author　of　nature　had　not　created　a　single　hair　without　a　definite　purpose。〃　　He　succeeded　in　demonstrating　a　number　of　beautiful　adaptations　in　flowers　for　ensuring　pollination；　but　his　work　exercised　but　little　influence　on　his　contemporaries　and　indeed　for　a　long　time　after　his　death。　　It　was　through　Darwin　that　Sprengel's　work　first　achieved　a　well　deserved　though　belated　fame。　　Even　such　botanists　as　concerned　themselves　with　researches　into　the　biology　of　flowers　appear　to　have　formerly　attached　much　less　value　to　Sprengel's　work　than　it　has　received　since　Darwin's　time。　　In　illustration　of　this　we　may　quote　C。F。　Gartner　whose　name　is　rightly　held　in　the　highest　esteem　as　that　of　one　of　the　most　eminent　hybridologists。　　In　his　work　〃Versuche　und　Beobachtungen　uder　die　Befruchtungsorgane　der　vollkommeneren　Gewachse　und　uber　die　naturliche　und　kunstliche　Befruchtung　durch　den　eigenen　Pollen〃　he　also　deals　with　flower…pollination。　　He　recognised　the　action　of　the　wind；　but　he　believed；　in　spite　of　the　fact　that　he　both　knew　and　quoted　Kolreuter　and　Sprengel；　that　while　insects　assist　pollination；　they　do　so　only　occasionally；　and　he　held　that　insects　are　responsible　for　the　conveyance　of　pollen；　thorough　investigations　would　show　〃that　a　very　small　proportion　of　the　plants　included　in　this　category　require　this　assistance　in　their　native　habitat。〃　（Gartner；　〃Versucher　und　Beobachtungen。。。〃；　page　335；　Stuttgart；　1844。）　　In　the　majority　of　plants　self…pollination　occurs。

Seeing　that　even　investigators　who　had　worked　for　several　decades　at　fertilisation…phenomena　had　not　advanced　the　biology　of　flowers　beyond　the　initial　stage；　we　cannot　be　surprised　that　other　botanists　followed　to　even　a　less　extent　the　lines　laid　down　by　Kolreuter　and　Sprengel。　　This　was　in　part　the　result　of　Sprengel's　supernatural　teleology　and　in　part　due　to　the　fact　that　his　book　appeared　at　a　time　when　other　lines　of　inquiry　exerted　a　dominating　influence。

At　the　hands　of　Linnaeus　systematic　botany　reached　a　vigorous　development；　and　at　the　beginning　of　the　nineteenth　century　the　anatomy　and　physiology　of　plants　grew　from　small　beginnings　to　a　flourishing　branch　of　science。　　Those　who　concerned　themselves　with　flowers　endeavoured　to　investigate　their　development　and　structure　or　the　most　minute　phenomena　connected　with　fertilisation　and　the　formation　of　the　embryo。　　No　room　was　left　for　the　extension　of　the　biology　of　flowers　on　the　lines　marked　out　by　Kolreuter　and　Sprengel。　　Darwin　was　the　first　to　give　new　life　and　a　deeper　significance　to　this　subject；　chiefly　because　he　took　as　his　starting…point　the　above…mentioned　problems；　the　importance　of　which　is　at　once　admitted　by　all　naturalists。

The　further　development　of　floral　biology　by　Darwin　is　in　the　first　place　closely　connected　with　the　book　on　the　fertilisation　of　Orchids。　　It　is　noteworthy　that　the　title　includes　the　sentence；〃and　on　the　good　effects　of　intercrossing。〃

The　purpose　of　the　book　is　clearly　stated　in　the　introduction：〃The　object　of　the　following　work　is　to　show　that　the　contrivances　by　which　Orchids　are　fertilised；　are　as　varied　and　almost　as　perfect　as　any　of　the　most　beautiful　adaptations　in　the　animal　kingdom；　and；　secondly；　to　show　that　these　contrivances　have　for　their　main　object　the　fertilisation　of　each　flower　by　the　pollen　of　another　flower。〃　　（〃Fertilisation　of　Orchids〃；　page　1。）　　Orchids　constituted　a　particularly　suitable　family　for　such　researches。　　Their　flowers　exhibit　a　striking　wealth　of　forms；　the　question；　therefore；　whether　the　great　variety　in　floral　structure　bears　any　relation　to　fertilisation　（In　the　older　botanical　literature　the　word　fertilisation　is　usually　employed　in　cases　where　POLLINATION　is　really　in　question：　　as　Darwin　used　it　in　this　sense　it　is　so　used　here。）　must　in　this　case　possess　special　interest。

Darwin　succeeded　in　showing　that　in　most　of　the　orchids　examined　self…　fertilisation　is　either　an　impossibility；　or；　under　natural　conditions；　occurs　only　exceptionally。　　On　the　other　hand　these　plants　present　a　series　of　extraordinarily　beautiful　and　remarkable　adaptations　which　ensure　the　transference　of　pollen　by　insects　from　one　flower　to　another。　　It　is　impossible　to　describe　adequately　in　a　few　words　the　wealth　of　facts　contained　in　the　Orchid　book。　　A　few　examples　may；　however；　be　quoted　in　illustration　of　the　delicacy　of　the　observations　and　of　the　perspicuity　employed　in　interpreting　the　facts。

The　majority　of　orchids　differ　from　other　seed　plants　（with　the　exception　of　the　Asclepiads）　in　having　no　dust…like　pollen。　　The　pollen；　or　more　correctly；　the　pollen…tetrads；　remain　fastened　together　as　club…shaped　pollinia　usually　borne　on　a　slender　pedicel。　　At　the　base　of　the　pedicel　is　a　small　viscid　disc　by　which　the　pollinium　is　attached　to　the　head　or　proboscis　of　one　of　the　insects　which　visit　the　flower。　　Darwin　demonstrated　that　in　Orchis　and　other　flowers　the　pedicel　of　the　pollinium；　after　its　removal　from　the　anther；　undergoes　a　curving　movement。　　If　the　pollinium　was　originally　vertical；　after　a　time　it　assumed　a　horizontal　position。　　In　the　latter　position；　if　the　insect　visited　another　flower；　the　pollinium　would　exactly　hit　the　sticky　stigmatic　surface　and　thus　effect　fertilisation。　　The　relation　between　the　behaviour　of　the　viscid　disc　and　the　secretion　of　nectar　by　the　flower　is　especially　remarkable。　　The　flowers　possess　a　spur　which　in　some　species　（e。g。　Gymnadenia　conopsea；　Platanthera　bifolia；　etc。）　contains　honey　（nectar）；　which　serves　as　an　attractive　bait　for　insects；　but　in　others　（e。g。　our　native　species　of　Orchis）　the　spur　is　empty。　　Darwin　held　the　opinion；　confirmed　by　later　investigations；　that　in　the　case　of　flowers　without　honey　the　insects　must　penetrate　the　wall　of　the　nectarless　spurs　in　order　to　obtain　a　nectar…like　substance。　　The　glands　behave　differently　in　the　nectar…bearing　and　in　the　nectarless　flowers。　　In　the　former　they　are　so　sticky　that　they　at　once　adhere　to　the　body　of　the　insect；　in　the　nectarless　flowers　firm　adherence　only　occurs　after　the　viscid　disc　has　hardened。　　It　is；　therefore；　adaptively　of　value　that　the　insects　should　be　detained　longer　in　the　nectarless　flowers　（by　having　to　bore　into　the　spur）；than　in　flowers　in　which　the　nectar　is　freely　exposed。　　〃If　this　relation；　on　the　one　hand；　between　the　viscid　matter　requiring　some　little　time　to　set　hard；　and　the　nectar　being　so　lodged　that　moths　are　delayed　in　getting　it；　and；　on　the　other　hand；　between　the　viscid　matter　being　at　first　as　viscid　as　ever　it　will　become；　and　the　nectar　lying　all　ready　for　rapid　suction；　be　accidental；　it　is　a　fortunate　accident　for　the　plant。　　If　not　accidental；　and　I　cannot　believe　it　to　be　accidental；　what　a　singular　case　of　adaptation！〃　　（〃Fertilisation　of　Orchids〃　（1st　edition）；　page　53。）

Among　exotic　orchids　Catasetum　is　particularly　remarkable。　　One　and　the　same　species　bears　different　forms　of　flowers。　　The　species　known　as　Catasetum　tridentatum　has　pollinia　with　very　large　viscid　discs；　on　touching　one　of　the　two　filaments　（antennae）　which　occur　on　the　gynostemium　of　the　flower　the　pollinia　are　shot　out　to　a　fairly　long　distance　（as　far　as　1　metre）　and　in　such　manner　that　they　alight　on　the　back　of　the　insect；　where　they　are　held。　　The　antennae　have；　moreover；　acquired　an　importance；　from　the　point　of　view　of　the　physiology　of　stimulation；　as　stimulus…　perceiving　organs。　　Darwin　had　shown　that　it　is　only　a　touch　on　the　antennae　that　causes　the　explosion；　while　contact；　blows；　wounding；　etc。　on　other　places　produce　no　effect。　　This　form　of　flower　proved　to　be　the　male。　The　second　form；　formerly　regarded　as　a　distinct　species　and　named　Monachanthus　viridis；　is　shown　to　be　the　female　flower。　　The　anthers　have　only　rudimentary　pollinia　and　do　not　open；　there　are　no　antennae；　but　on　the　other　hand　numerous　seeds　are　produced。　　Another　type　of　flower；　known　as　Myanthu