darwin and modern science-第103章

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id　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　Myanthus　barbatus；　was　regarded　by　Darwin　as　a　third　form：　　this　was　afterwards　recognised　by　Rolfe　（Rolfe；　R。A。　〃On　the　sexual　forms　of　Catasetum　with　special　reference　to　the　researches　of　Darwin　and　others；〃　〃Journ。　Linn。　Soc。〃　Vol。　XXVII。　（Botany）；　1891；　pages　206…225。）　as　the　male　flower　of　another　species；　Catasetum　barbatum　Link；　an　identification　in　accordance　with　the　discovery　made　by　Cruger　in　Trinidad　that　it　always　remains　sterile。

Darwin　had　noticed　that　the　flowers　of　Catasetum　do　not　secrete　nectar；　and　he　conjectured　that　in　place　of　it　the　insects　gnaw　a　tissue　in　the　cavity　of　the　labellum　which　has　a　〃slightly　sweet；　pleasant　and　nutritious　taste。〃　　This　conjecture　as　well　as　other　conclusions　drawn　by　Darwin　from　Catasetum　have　been　confirmed　by　Crugerassuredly　the　best　proof　of　the　acumen　with　which　the　wonderful　floral　structure　of　this　〃most　remarkable　of　the　Orchids〃　was　interpretated　far　from　its　native　habitat。

As　is　shown　by　what　we　have　said　about　Catasetum；　other　problems　in　addition　to　those　concerned　with　fertilisation　are　dealt　with　in　the　Orchid　book。　　This　is　especially　the　case　in　regard　to　flower　morphology。　　The　scope　of　flower　morphology　cannot　be　more　clearly　and　better　expressed　than　by　these　words：　　〃He　will　see　how　curiously　a　flower　may　be　moulded　out　of　many　separate　organshow　perfect　the　cohesion　of　primordially　distinct　parts　may　become；how　organs　may　be　used　for　purposes　widely　different　from　their　proper　function；how　other　organs　may　be　entirely　suppressed；　or　leave　mere　useless　emblems　of　their　former　existence。〃　　（〃Fertilisation　of　Orchids〃；　page　289。）

In　attempting；　from　this　point　of　view；　to　refer　the　floral　structure　of　orchids　to　their　original　form；　Darwin　employed　a　much　more　thorough　method　than　that　of　Robert　Brown　and　others。　　The　result　of　this　was　the　production　of　a　considerable　literature；　especially　in　France；　along　the　lines　suggested　by　Darwin's　work。　　This　is　the　so…called　anatomical　method；　which　seeks　to　draw　conclusions　as　to　the　morphology　of　the　flower　from　the　course　of　the　vascular　bundles　in　the　several　parts。　　（He　wrote　in　one　of　his　letters；　〃。。。the　destiny　of　the　whole　human　race　is　as　nothing　to　the　course　of　vessels　of　orchids〃　（〃More　Letters〃；　Vol。　II。　page　275。）　　Although　the　interpretation　of　the　orchid　flower　given　by　Darwin　has　not　proved　satisfactory　in　one　particular　pointthe　composition　of　the　labellumthe　general　results　have　received　universal　assent；　namely　〃that　all　Orchids　owe　what　they　have　in　common　to　descent　from　some　monocotyledonous　plant；　which；　like　so　many　other　plants　of　the　same　division；　possessed　fifteen　organs　arranged　alternately　three　within　three　in　five　whorls。〃　　（〃Fertilisation　of　Orchids〃　（1st　edition）；　page　307。）　　The　alterations　which　their　original　form　has　undergone　have　persisted　so　far　as　they　were　found　to　be　of　use。

We　see　also　that　the　remarkable　adaptations　of　which　we　have　given　some　examples　are　directed　towards　cross…fertilisation。　　In　only　a　few　of　the　orchids　investigated　by　Darwinother　similar　cases　have　since　been　describedwas　self…fertilisation　found　to　occur　regularly　or　usually。　　The　former　is　the　case　in　the　Bee　Ophrys　（Ophrys　apifera）；　the　mechanism　of　which　greatly　surprised　Darwin。　　He　once　remarked　to　a　friend　that　one　of　the　things　that　made　him　wish　to　live　a　few　thousand　years　was　his　desire　to　see　the　extinction　of　the　Bee　Ophrys；　an　end　to　which　he　believed　its　self…fertilising　habit　was　leading。　　（〃Life　and　Letters〃；　Vol。　III。　page　276　（footnote）。）　　But；　he　wrote；　〃the　safest　conclusion；　as　it　seems　to　me；　is；　that　under　certain　unknown　circumstances；　and　perhaps　at　very　long　intervals　of　time；　one　individual　of　the　Bee　Ophrys　is　crossed　by　another。〃　（〃Fertilisation　of　Orchids〃　page　71。）

If；　on　the　one　hand；　we　remember　how　much　more　sure　self…fertilisation　would　be　than　cross…fertilisation；　and；　on　the　other　hand；　if　we　call　to　mind　the　numerous　contrivances　for　cross…fertilisation；　the　conclusion　is　naturally　reached　that　〃it　is　an　astonishing　fact　that　self…fertilisation　should　not　have　been　an　habitual　occurrence。　　It　apparently　demonstrates　to　us　that　there　must　be　something　injurious　in　the　process。　　Nature　thus　tells　us；　in　the　most　emphatic　manner；　that　she　abhors　perpetual　self…　fertilisation。。。For　may　we　not　further　infer　as　probable；　in　accordance　with　the　belief　of　the　vast　majority　of　the　breeders　of　our　domestic　productions；　that　marriage　between　near　relations　is　likewise　in　some　way　injurious；　that　some　unknown　great　good　is　derived　from　the　union　of　individuals　which　have　been　kept　distinct　for　many　generations？〃　　（Ibid。；　page　359。）

This　view　was　supported　by　observations　on　plants　of　other　families；　e。g。　Papilionaceae；　it　could；　however；　in　the　absence　of　experimental　proof；　be　regarded　only　as　a　〃working　hypothesis。〃

All　adaptations　to　cross…pollination　might　also　be　of　use　simply　because　they　made　pollination　possible　when　for　any　reason　self…pollination　had　become　difficult　or　impossible。　　Cross…pollination　would；　therefore；　be　of　use；　not　as　such；　but　merely　as　a　means　of　pollination　in　general；　it　would　to　some　extent　serve　as　a　remedy　for　a　method　unsuitable　in　itself；　such　as　a　modification　standing　in　the　way　of　self…pollination；　and　on　the　other　hand　as　a　means　of　increasing　the　chance　of　pollination　in　the　case　of　flowers　in　which　self…pollination　was　possible；　but　which　might；　in　accidental　circumstances；　be　prevented。　　It　was；　therefore；　very　important　to　obtain　experimental　proof　of　the　conclusion　to　which　Darwin　was　led　by　the　belief　of　the　majority　of　breeders　and　by　the　evidence　of　the　widespread　occurrence　of　cross…pollination　and　of　the　remarkable　adaptations　thereto。

This　was　supplied　by　the　researches　which　are　described　in　the　two　other　works　named　above。　　The　researches　on　which　the　conclusions　rest　had；　in　part　at　least；　been　previously　published　in　separate　papers：　　this　is　the　case　as　regards　the　heterostyled　plants。　　The　discoveries　which　Darwin　made　in　the　course　of　his　investigations　of　these　plants　belong　to　the　most　brilliant　in　biological　science。

The　case　of　Primula　is　now　well　known。　　C。K。　Sprengel　and　others　were　familiar　with　the　remarkable　fact　that　different　individuals　of　the　European　species　of　Primula　bear　differently　constructed　flowers；　some　plants　possess　flowers　in　which　the　styles　project　beyond　the　stamens　attached　to　the　corolla…tube　（long…styled　form）；　while　in　others　the　stamens　are　inserted　above　the　stigma　which　is　borne　on　a　short　style　（short…styled　form）。　　It　has　been　shown　by　Breitenbach　that　both　forms　of　flower　may　occur　on　the　same　plant；　though　this　happens　very　rarely。　　An　analogous　case　is　occasionally　met　with　in　hybrids；　which　bear　flowers　of　different　colour　on　the　same　plant　（e。g。　Dianthus　caryophyllus）。　　Darwin　showed　that　the　external　differences　are　correlated　with　others　in　the　structure　of　the　stigma　and　in　the　nature　of　the　pollen。　　The　long…styled　flowers　have　a　spherical　stigma　provided　with　large　stigmatic　papillae；　the　pollen　grains　are　oblong　and　smaller　than　those　of　the　short…styled　flowers。　　The　number　of　the　seeds　produced　is　smaller　and　the　ovules　larger；　probably　also　fewer　in　number。　　The　short…styled　flowers　have　a　smooth　compressed　stigma　and　a　corolla　of　somewhat　different　form；　they　produce　a　greater　number　of　seeds。

These　different　forms　of　flowers　were　regarded　as　merely　a　case　of　variation；　until　Darwin　showed　〃that　these　heterostyled　plants　are　adapted　for　reciprocal　fertilisation；　so　that　the　two　or　three　forms；　though　all　are　hermaphrodites；　are　related　to　one　another　almost　like　the　males　and　females　of　ordinary　unisexual　animals。〃　　（〃Forms　of　Flowers〃　（1st　edition）；　page　2。）　　We　have　here　an　example　of　hermaphrodite　flowers　which　are　sexually　different。　　There　are　essential　differences　in　the　manner　in　which　fertilisation　occurs。　　This　may　be　effected　in　four　different　ways；　there　are　two　legitimate　and　two　illegitimate　types　of　fertilisation。　　The　fertilisation　is　legitimate　if　pollen　from　the　long…styled　flowers　reaches　the　stigma　of　the　short…styled　form　or