darwin and modern science-第124节

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to　conceive　this　occurring　in　plants　which　have　no　power　of　circumnutatingand；　as　already　pointed　out；　physiologists　do　as　a　fact　neglect　circumnutation　as　a　factor　in　the　evolution　of　movements。　Whatever　may　be　the　fate　of　Darwin's　theory　of　circumnutation　there　is　no　doubt　that　the　research　he　carried　out　in　support　of；　and　by　the　light　of；　this　hypothesis　has　had　a　powerful　influence　in　guiding　the　modern　theories　of　the　behaviour　of　plants。　　Pfeffer　（〃The　Physiology　of　Plants〃；　Eng。　Tr。　III。　page　11。）；　who　more　than　any　one　man　has　impressed　on　the　world　a　rational　view　of　the　reactions　of　plants；　has　acknowledged　in　generous　words　the　great　value　of　Darwin's　work　in　the　same　direction。　　The　older　view　was　that；　for　instance；　curvature　towards　the　light　is　the　direct　mechanical　result　of　the　difference　of　illumination　on　the　lighted　and　shaded　surfaces　of　the　plant。　　This　has　been　proved　to　be　an　incorrect　explanation　of　the　fact；　and　Darwin　by　his　work　on　the　transmission　of　stimuli　has　greatly　contributed　to　the　current　belief　that　stimuli　act　indirectly。　　Thus　we　now　believe　that　in　a　root　and　a　stem　the　mechanism　for　the　perception　of　gravitation　is　identical；　but　the　resulting　movements　are　different　because　the　motor…irritabilities　are　dissimilar　in　the　two　cases。　　We　must　come　back；　in　fact；　to　Darwin's　comparison　of　plants　to　animals。　　In　both　there　is　perceptive　machinery　by　which　they　are　made　delicately　alive　to　their　environment；　in　both　the　existing　survivors　are　those　whose　internal　constitution　has　enabled　them　to　respond　in　a　beneficial　way　to　the　disturbance　originating　in　their　sense…organs。


XX。　　THE　BIOLOGY　OF　FLOWERS。

By　K。　GOEBEL；　Ph。D。　Professor　of　Botany　in　the　University　of　Munich。

There　is　scarcely　any　subject　to　which　Darwin　devoted　so　much　time　and　work　as　to　his　researches　into　the　biology　of　flowers；　or；　in　other　words；　to　the　consideration　of　the　question　to　what　extent　the　structural　and　physiological　characters　of　flowers　are　correlated　with　their　function　of　producing　fruits　and　seeds。　　We　know　from　his　own　words　what　fascination　these　studies　possessed　for　him。　　We　repeatedly　find；　for　example；　in　his　letters　expressions　such　as　this：〃Nothing　in　my　life　has　ever　interested　me　more　than　the　fertilisation　of　such　plants　as　Primula　and　Lythrum；　or　again　Anacamptis　or　Listera。〃　　（〃More　Letters　of　Charles　Darwin〃；　Vol。　II。　page　419。）

Expressions　of　this　kind　coming　from　a　man　whose　theories　exerted　an　epoch…　making　influence；　would　be　unintelligible　if　his　researches　into　the　biology　of　flowers　had　been　concerned　only　with　records　of　isolated　facts；　however　interesting　these　might　be。　　We　may　at　once　take　it　for　granted　that　the　investigations　were　undertaken　with　the　view　of　following　up　important　problems　of　general　interest；　problems　which　are　briefly　dealt　with　in　this　essay。

Darwin　published　the　results　of　his　researches　in　several　papers　and　in　three　larger　works；　（i）　〃On　the　various　contrivances　by　which　British　and　Foreign　Orchids　are　fertilised　by　insects〃　（First　edition；　London；　1862；　second　edition；　1877；　popular　edition；　1904。）　　（ii）　　〃The　effects　of　Cross　and　Self　fertilisation　in　the　vegetable　kingdom〃　（First　edition；　1876；　second　edition；　1878）。　　（iii）　　〃The　different　forms　of　Flowers　on　plants　of　the　same　species〃　（First　edition；　1877；　second　edition；　1880）。

Although　the　influence　of　his　work　is　considered　later；　we　may　here　point　out　that　it　was　almost　without　a　parallel；　not　only　does　it　include　a　mass　of　purely　scientific　observations；　but　it　awakened　interest　in　very　wide　circles；　as　is　shown　by　the　fact　that　we　find　the　results　of　Darwin's　investigations　in　floral　biology　universally　quoted　in　school　books；　they　are　even　willingly　accepted　by　those　who；　as　regards　other　questions；　are　opposed　to　Darwin's　views。

The　works　which　we　have　mentioned　are；　however；　not　only　of　special　interest　because　of　the　facts　they　contribute；　but　because　of　the　MANNER　in　which　the　facts　are　expressed。　　A　superficial　reader　seeking　merely　for　catch…words　will；　for　instance；　probably　find　the　book　on　cross　and　self…　fertilisation　rather　dry　because　of　the　numerous　details　which　it　contains：　　it　is；　indeed；　not　easy　to　compress　into　a　few　words　the　general　conclusions　of　this　volume。　　But　on　closer　examination；　we　cannot　be　sufficiently　grateful　to　the　author　for　the　exactness　and　objectivity　with　which　he　enables　us　to　participate　in　the　scheme　of　his　researches。　　He　never　tries　to　persuade　us；　but　only　to　convince　us　that　his　conclusions　are　based　on　facts；　he　always　gives　prominence　to　such　facts　as　appear　to　be　in　opposition　to　his　opinions；a　feature　of　his　work　in　accordance　with　a　maxim　which　he　laid　down：〃It　is　a　golden　rule；　which　I　try　to　follow；　to　put　every　fact　which　is　opposed　to　one's　preconceived　opinion　in　the　strongest　light。〃　　（〃More　Letters〃；　Vol。　II。　page　324。）

The　result　of　this　method　of　presentation　is　that　the　works　mentioned　above　represent　a　collection　of　most　valuable　documents　even　for　those　who　feel　impelled　to　draw　from　the　data　other　conclusions　than　those　of　the　author。　Each　investigation　is　the　outcome　of　a　definite　question；　a　〃preconceived　opinion；〃　which　is　either　supported　by　the　facts　or　must　be　abandoned。　　〃How　odd　it　is　that　anyone　should　not　see　that　all　observation　must　be　for　or　against　some　view　if　it　is　to　be　of　any　service！〃　　（Ibid。　Vol。　I。　page　195。）

The　points　of　view　which　Darwin　had　before　him　were　principally　the　following。　　In　the　first　place　the　proof　that　a　large　number　of　the　peculiarities　in　the　structure　of　flowers　are　not　useless；　but　of　the　greatest　significance　in　pollination　must　be　of　considerable　importance　for　the　interpretation　of　adaptations；　〃The　use　of　each　trifling　detail　of　structure　is　far　from　a　barren　search　to　those　who　believe　in　natural　selection。〃　　（〃Fertilisation　of　Orchids〃　（1st　edition）；　page　351；　（2nd　edition　1904）　page　286。）　　Further；　if　these　structural　relations　are　shown　to　be　useful；　they　may　have　been　acquired　because　from　the　many　variations　which　have　occurred　along　different　lines；　those　have　been　preserved　by　natural　selection　〃which　are　beneficial　to　the　organism　under　the　complex　and　ever…varying　conditions　of　life。〃　　（Ibid。　page　351。）　　But　in　the　case　of　flowers　there　is　not　only　the　question　of　adaptation　to　fertilisation　to　be　considered。　　Darwin；　indeed；　soon　formed　the　opinion　which　he　has　expressed　in　the　following　sentence；〃From　my　own　observations　on　plants；　guided　to　a　certain　extent　by　the　experience　of　the　breeders　of　animals；　I　became　convinced　many　years　ago　that　it　is　a　general　law　of　nature　that　flowers　are　adapted　to　be　crossed；　at　least　occasionally；　by　pollen　from　a　distinct　plant。〃　　（〃Cross　and　Self　fertilisation〃　（1st　edition）；　page　6。）

The　experience　of　animal　breeders　pointed　to　the　conclusion　that　continual　in…breeding　is　injurious。　　If　this　is　correct；　it　raises　the　question　whether　the　same　conclusion　holds　for　plants。　　As　most　flowers　are　hermaphrodite；　plants　afford　much　more　favourable　material　than　animals　for　an　experimental　solution　of　the　question；　what　results　follow　from　the　union　of　nearly　related　sexual　cells　as　compared　with　those　obtained　by　the　introduction　of　new　blood。　　The　answer　to　this　question　must；　moreover；　possess　the　greatest　significance　for　the　correct　understanding　of　sexual　reproduction　in　general。

We　see；　therefore；　that　the　problems　which　Darwin　had　before　him　in　his　researches　into　the　biology　of　flowers　were　of　the　greatest　importance；　and　at　the　same　time　that　the　point　of　view　from　which　he　attacked　the　problems　was　essentially　a　teleological　one。

We　may　next　inquire　in　what　condition　he　found　the　biology　of　flowers　at　the　time　of　his　first　researches；　which　were　undertaken　about　the　year　1838。　　In　his　autobiography　he　writes；〃During　the　summer　of　1839；　and；　I　believe；　during　the　previous　summer；　I　was　led　to　attend　to　the　cross…　fertilisation　of　flowers　by　the　aid　of　insects；　from　having　come　to　the　conclusion　in　my　speculations　on　the　origin　of　species；　that　crossing　played　an　important　part　in　keeping　specific　forms　constant。〃　　（〃The　Life　and　Letters　of　Charles　Darwin〃；　Vol。　I。　page　90；　London；　1888。）　　In　1841　he　became　acquainted　with　Sprengel's　work：　　his　researches　into　the　biology　of　flowers　were　thus　continued　for　about　forty　years。

It　is　obvious　that　there　could　only　be　a　biology　of　flowers　after　it　had　been　demonstrated　that　the　formation　of　seeds　and　fruit　in　the　flower　is　dependent　on　pollination　and　subsequent　fertilisation。　　This　proof　was　supplied　at　the　end　of　the　seventeenth　century　by　R。J。　Camerarius　（1665…　1721）。　　He　showed　that　normally　seeds　and　fruits　are　developed　only　when　the　pollen　reaches　the　stigma。　　The　manner　in　which　this　happens　was　first　thoroughly　investigated　by　J。G。　Kolreuter　（1733…1806　（Kolreuter；　〃Vorlaufige　Nachricht　von　einigen　das　Geschlecht　der　Planzen　betreffenden　Versuchen　und　Beobachtungen〃；　Leipzig；　1761；　with　three　supplements；　1763…　66。　　Also；　〃Mem。　de　l'acad。　St　Petersbourg〃；　Vol。　XV。　1809。））；　the　same　observer　to　whom　we　owe　the　earliest　experiments　in　hybridisation　of　real　scientific　interest。　Kolreuter　mentioned　that　pollen　may　be　carried　from　one　flower　to　another　p