darwin and modern science-第104章

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is　legitimate　if　pollen　from　the　long…styled　flowers　reaches　the　stigma　of　the　short…styled　form　or　if　pollen　of　the　short…styled　flowers　is　brought　to　the　stigma　of　the　long…styled　flower；　that　is　the　organs　of　the　same　length　of　the　two　different　kinds　of　flower　react　on　one　another。　　Illegitimate　fertilisation　is　represented　by　the　two　kinds　of　self…fertilisation；　also　by　cross…fertilisation；　in　which　the　pollen　of　the　long…styled　form　reaches　the　stigma　of　the　same　type　of　flower　and；　similarly；　by　cross…pollination　in　the　case　of　the　short…styled　flowers。

The　applicability　of　the　terms　legitimate　and　illegitimate　depends；　on　the　one　hand；　upon　the　fact　that　insects　which　visit　the　different　forms　of　flowers　pollinate　them　in　the　manner　suggested；　the　pollen　of　the　short…　styled　flowers　adhere　to　that　part　of　the　insect's　body　which　touches　the　stigma　of　the　long…styled　flower　and　vice　versa。　　On　the　other　hand；　it　is　based　also　on　the　fact　that　experiment　shows　that　artificial　pollination　produces　a　very　different　result　according　as　this　is　legitimate　or　illegitimate；　only　the　legitimate　union　ensures　complete　fertility；　the　plants　thus　produced　being　stronger　than　those　which　are　produced　illegitimately。

If　we　take　100　as　the　number　of　flowers　which　produce　seeds　as　the　result　of　legitimate　fertilisation；　we　obtain　the　following　numbers　from　illegitimate　fertilisation：

Primula　officinalis　（P。　veris）　（Cowslip）　　。。。　　69　Primula　elatior　（Oxlip）　　。。。。。。。。。。。。。。。。。。。。　　27　Primula　acaulis　（P。　vulgaris）　（Primrose）　　。。。　　60

Further；　the　plants　produced　by　the　illegitimate　method　of　fertilisation　showed；　e。g。　in　P。　officinalis；　a　decrease　in　fertility　in　later　generations；　sterile　pollen　and　in　the　open　a　feebler　growth。　　（Under　very　favourable　conditions　（in　a　greenhouse）　the　fertility　of　the　plants　of　the　fourth　generation　increasesa　point；　which　in　view　of　various　theoretical　questions；　deserves　further　investigation。）　　They　behave　in　fact　precisely　in　the　same　way　as　hybrids　between　species　of　different　genera。　　This　result　is　important；　〃for　we　thus　learn　that　the　difficulty　in　sexually　uniting　two　organic　forms　and　the　sterility　of　their　offspring；　afford　no　sure　criterion　of　so…called　specific　distinctness〃　（〃Forms　of　Flowers〃；　page　242）：　　the　relative　or　absolute　sterility　of　the　illegitimate　unions　and　that　of　their　illegitimate　descendants　depend　exclusively　on　the　nature　of　the　sexual　elements　and　on　their　inability　to　combine　in　a　particular　manner。　　This　functional　difference　of　sexual　cells　is　characteristic　of　the　behaviour　of　hybrids　as　of　the　illegitimate　unions　of　heterostyled　plants。　　The　agreement　becomes　even　closer　if　we　regard　the　Primula　plants　bearing　different　forms　of　flowers　not　as　belonging　to　a　systematic　entity　or　〃species；〃　but　as　including　several　elementary　species。　　The　legitimately　produced　plants　are　thus　true　hybrids　（When　Darwin　wrote　in　reference　to　the　different　forms　of　heterostyled　plants；　〃which　all　belong　to　the　same　species　as　certainly　as　do　the　two　sexes　of　the　same　species〃　（〃Cross　and　Self　fertilisation〃；　page　466）；　he　adopted　the　term　species　in　a　comprehensive　sense。　　The　recent　researches　of　Bateson　and　Gregory　（〃On　the　inheritance　of　Heterostylism　in　Primula〃；　〃Proc。　Roy。　Soc。〃　Ser。　B；　Vol。　LXXVI。　1905；　page　581）　appear　to　me　also　to　support　the　view　that　the　results　of　illegitimate　crossing　of　heterostyled　Primulas　correspond　with　those　of　hybridisation。　　The　fact　that　legitimate　pollen　effects　fertilisation；　even　if　illegitimate　pollen　reaches　the　stigma　a　short　time　previously；　also　points　to　this　conclusion。　　Self…pollination　in　the　case　of　the　short…styled　form；　for　example；　is　not　excluded。　　In　spite　of　this；　the　numerical　proportion　of　the　two　forms　obtained　in　the　open　remains　approximately　the　same　as　when　the　pollination　was　exclusively　legitimate；　presumably　because　legitimate　pollen　is　prepotent。）；　with　which　their　behaviour　in　other　respects；　as　Darwin　showed；　presents　so　close　an　agreement。　　This　view　receives　support　also　from　the　fact　that　descendants　of　a　flower　fertilised　illegitimately　by　pollen　from　another　plant　with　the　same　form　of　flower　belong；　with　few　exceptions；　to　the　same　type　as　that　of　their　parents。　　The　two　forms　of　flower；　however；　behave　differently　in　this　respect。　　Among　162　seedlings　of　the　long…styled　illegitimately　pollinated　plants　of　Primula　officinalis；　including　five　generations；　there　were　156　long…styled　and　only　six　short…styled　forms；　while　as　the　result　of　legitimate　fertilisation　nearly　half　of　the　offspring　were　long…styled　and　half　short…styled。　　The　short…styled　illegitimately　pollinated　form　gave　five　long…styled　and　nine　short…styled；　the　cause　of　this　difference　requires　further　explanation。　　The　significance　of　heterostyly；　whether　or　not　we　now　regard　it　as　an　arrangement　for　the　normal　production　of　hybrids；　is　comprehensively　expressed　by　Darwin：　　〃We　may　feel　sure　that　plants　have　been　rendered　heterostyled　to　ensure　cross…fertilisation；　for　we　now　know　that　a　cross　between　the　distinct　individuals　of　the　same　species　is　highly　important　for　the　vigour　and　fertility　of　the　offspring。〃　　（〃Forms　of　Flowers〃；　page　258。）　　If　we　remember　how　important　the　interpretation　of　heterostyly　has　become　in　all　general　problems　as；　for　example；　those　connected　with　the　conditions　of　the　formation　of　hybrids；　a　fact　which　was　formerly　overlooked；　we　can　appreciate　how　Darwin　was　able　to　say　in　his　autobiography：　　〃I　do　not　think　anything　in　my　scientific　life　has　given　me　so　much　satisfaction　as　making　out　the　meaning　of　the　structure　of　these　plants。〃　　（〃Life　and　Letters〃；　Vol。　I。　page　91。）

The　remarkable　conditions　represented　in　plants　with　three　kinds　of　flowers；　such　as　Lythrum　and　Oxalis；　agree　in　essentials　with　those　in　Primula。　　These　cannot　be　considered　in　detail　here；　it　need　only　be　noted　that　the　investigation　of　these　cases　was　still　more　laborious。　　In　order　to　establish　the　relative　fertility　of　the　different　unions　in　Lythrum　salicaria　223　different　fertilisations　were　made；　each　flower　being　deprived　of　its　male　organs　and　then　dusted　with　the　appropriate　pollen。

In　the　book　containing　the　account　of　heterostyled　plants　other　species　are　dealt　with　which；　in　addition　to　flowers　opening　normally　（chasmogamous）；　also　possess　flowers　which　remain　closed　but　are　capable　of　producing　fruit。　　These　cleistogamous　flowers　afford　a　striking　example　of　habitual　self…pollination；　and　H。　von　Mohl　drew　special　attention　to　them　as　such　shortly　after　the　appearance　of　Darwin's　Orchid　book。　　If　it　were　only　a　question　of　producing　seed　in　the　simplest　way；　cleistogamous　flowers　would　be　the　most　conveniently　constructed。　　The　corolla　and　frequently　other　parts　of　the　flower　are　reduced；　the　development　of　the　seed　may；　therefore；　be　accomplished　with　a　smaller　expenditure　of　building　material　than　in　chasmogamous　flowers；　there　is　also　no　loss　of　pollen；　and　thus　a　smaller　amount　suffices　for　fertilisation。

Almost　all　these　plants；　as　Darwin　pointed　out；　have　also　chasmogamous　flowers　which　render　cross…fertilisation　possible。　　His　view　that　cleistogamous　flowers　are　derived　from　originally　chasmogamous　flowers　has　been　confirmed　by　more　recent　researches。　　Conditions　of　nutrition　in　the　broader　sense　are　the　factors　which　determine　whether　chasmogamous　or　cleistogamous　flowers　are　produced；　assuming；　of　course；　that　the　plants　in　question　have　the　power　of　developing　both　forms　of　flower。　　The　former　may　fail　to　appear　for　some　time；　but　are　eventually　developed　under　favourable　conditions　of　nourishment。　　The　belief　of　many　authors　that　there　are　plants　with　only　cleistogamous　flowers　cannot　therefore　be　accepted　as　authoritative　without　thorough　experimental　proof；　as　we　are　concerned　with　extra…european　plants　for　which　it　is　often　difficult　to　provide　appropriate　conditions　in　cultivation。

Darwin　sees　in　cleistogamous　flowers　an　adaptation　to　a　good　supply　of　seeds　with　a　small　expenditure　of　material；　while　chasmogamous　flowers　of　the　same　species　are　usually　cross…fertilised　and　〃their　offspring　will　thus　be　invigorated；　as　we　may　infer　from　a　wide…spread　analogy。〃　　（〃Forms　of　Flowers〃　（1st　edition）；　page　341。）　　Direct　proof　in　support　of　this　has　hitherto　been　supplied　in　a　few　cases　only；　we　shall　often　find　that　the　example　set　by　Darwin　in　solving　such　problems　as　these　by　laborious　experiment　has　unfortunately　been　little　imitated。

Another　chapter　of　this　book　treats　of　the　distribution　of　the　sexes　in　polygamous；　dioecious；　and　gyno…dioecious　plants　（the　last　term；　now　in　common　use；　we　owe　to　Darwin）。　　It　contains　a　number　of　important　facts　and　discussions　and　has　inspired　the　experimental　researches　of　Correns　and　others。

The　most　important　of　Darwin's　work　on　floral　biology　is；　however；　that　on　cross　and　self…fertilisation；　chiefly　because　it　states　the　results　of　experimental　investigations　extending　over　many　years。　　Only　such　experiments；　as　we　have　pointed　out；　could　determine　whether　cross…　fertilisation　is　in　itself　beneficial；　and　self…fertilisation　on　the　other　hand　injurious；　a　conclusion　which　a　merely　comparative　examination　of　pollination…mechanisms　renders　in　the　highest　degree　probable。　　Later　floral　biologists　have　unfortunately　almost　entirely　confined　themselves　to　observations　on　floral　mechanisms。　　But　there　is　little　more　to　be　gained　by　this　kind　of　work　than　an　assumption　long　ago　made　by　C。K。　Sprengel　that　〃very　many　flowers　have　the　sexes　separate　and　probably　at　least　as　many　hermaphrodite　flowers　are　dichogamous；　it　would　thus　appear　that　Nature　was　unwilling　that　any　flower　should　be　fertilised　by　its　own　pollen。〃

It　was　an　accidental　observation　which　inspired　Darwin's　experiments　on　the　effect　of　cross　and　self…fertilisation。　　Plants　of　Linaria　vulgaris　were　grown　in　two　adjacent　beds；　in　the　one　were　plants　produced　by　cross…　fertilisation；　that　is；　from　seeds　obtained　after　fertilisation　by　pollen　of　another　plant　of　the　same　species；　in　the　other　grew　plants　produced　by　self…fertilisation；　that　is　from　seed　produced　as　the　result　of　pollination　of　the　same　flower。　　The　first　were　obviously　superior　to　the　latter。

Darwin　was　surprised　by　this　observation；　as　he　had　expected　a　prejudicial　influence　of　self…fertilisation　to　manifest　itself　after　a　series　of　generations：　　〃I　always　supposed　until　lately　that　no　evil　effects　would　be　visible　until　after　several　generations　of　self…fertilisation；　but　now　I　see　that　one　generation　sometimes　suffices　and　the　existence　of　dimorphic　plants　and　all　the　wonderful　contrivances　of　orchids　are　quite　intelligible　to　me。〃　　（〃More　Letters〃；　Vol。　II。　page　373。）

The　observations　on　Linaria　and　the　investigations　of　the　results　of　legitimate　and　illegitimate　fertilisation　in　heterostyled　plants　were　apparently　the　beginning　of　a　long　series　of　experiments。　　These　were　concerned　with　plants　of　different　families　and　led　to　results　which　are　of　fundamental　importance　for　a　true　explanation　of　sexual　reproduction。

The　experiments　were　so　arranged　that　plants　were　shielded　from　insect…　visits　by　a　net。　　Some　flowers　were　then　pollinated　with　their　own　pollen；　others　with　pollen　from　another　plant　of　the　same　species。　　The　seeds　we