darwin and modern science-第120节

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sent　forms　come　to　be　what　they　are　includes　a　knowledge　of　why　they　survived。　　They　forget　that　the　SUMMATION　of　variations　on　which　divergence　depends　is　under　the　rule　of　the　environment　considered　as　a　selective　force。　　They　forget　that　the　scientific　study　of　the　interdependence　of　organisms　is　only　possible　through　a　knowledge　of　the　machinery　of　the　units。　　And　that；　therefore；　the　investigation　of　such　widely　interesting　subjects　as　extinction　and　distribution　must　include　a　knowledge　of　function。　　It　is　only　those　who　follow　this　line　of　work　who　get　to　see　the　importance　of　minute　points　of　structure　and　understand　as　my　father　did　even　in　1842；　as　shown　in　his　sketch　of　the　〃Origin〃　（Now　being　prepared　for　publication。）；　that　every　grain　of　sand　counts　for　something　in　the　balance。　　Much　that　is　confidently　stated　about　the　uselessness　of　different　organs　would　never　have　been　written　if　the　naturalist　spirit　were　commoner　nowadays。　　This　spirit　is　strikingly　shown　in　my　father's　work　on　the　movements　of　plants。　　The　circumstance　that　botanists　had　not；　as　a　class；　realised　the　interest　of　the　subject　accounts　for　the　fact　that　he　was　able　to　gather　such　a　rich　harvest　of　results　from　such　a　familiar　object　as　a　twining　plant。　　The　subject　had　been　investigated　by　H。　von　Mohl；　Palm；　and　Dutrochet；　but　they　failed　not　only　to　master　the　problem　but　（which　here　concerns　us）　to　give　the　absorbing　interest　of　Darwin's　book　to　what　they　discovered。

His　work　on　climbing　plants　was　his　first　sustained　piece　of　work　on　the　physiology　of　movement；　and　he　remarks　in　1864：　　〃This　has　been　new　sort　of　work　for　me。〃　　（〃Life　and　Letters〃；　III。　page　315。　　He　had；　however；　made　a　beginning　on　the　movements　of　Drosera。）　　He　goes　on　to　remark　with　something　of　surprise；　〃I　have　been　pleased　to　find　what　a　capital　guide　for　observations　a　full　conviction　of　the　change　of　species　is。〃

It　was　this　point　of　view　that　enabled　him　to　develop　a　broad　conception　of　the　power　of　climbing　as　an　adaptation　by　means　of　which　plants　are　enabled　to　reach　the　light。　　Instead　of　being　compelled　to　construct　a　stem　of　sufficient　strength　to　stand　alone；　they　succeed　in　the　struggle　by　making　use　of　other　plants　as　supports。　　He　showed　that　the　great　class　of　tendril…　and　root…climbers　which　do　not　depend　on　twining　round　a　pole；　like　a　scarlet…runner；　but　on　attaching　themselves　as　they　grow　upwards；　effect　an　economy。　　Thus　a　Phaseolus　has　to　manufacture　a　stem　three　feet　in　length　to　reach　a　height　of　two　feet　above　the　ground；　whereas　a　pea　〃which　had　ascended　to　the　same　height　by　the　aid　of　its　tendrils；　was　but　little　longer　than　the　height　reached。〃　　（〃Climbing　Plants〃　（2nd　edition　1875）；　page　193。）

Thus　he　was　led　on　to　the　belief　that　TWINING　is　the　more　ancient　form　of　climbing；　and　that　tendril…climbers　have　been　developed　from　twiners。　　In　accordance　with　this　view　we　find　LEAF…CLIMBERS；　which　may　be　looked　on　as　incipient　tendril…bearers；　occurring　in　the　same　genera　with　simple　twiners。　　（Loc。　cit。　page　195。）　　He　called　attention　to　the　case　of　Maurandia　semperflorens　in　which　the　young　flower…stalks　revolve　spontaneously　and　are　sensitive　to　a　touch；　but　neither　of　these　qualities　is　of　any　perceptible　value　to　the　species。　　This　forced　him　to　believe　that　in　other　young　plants　the　rudiments　of　the　faculty　needed　for　twining　would　be　founda　prophecy　which　he　made　good　in　his　〃Power　of　Movement〃　many　years　later。

In　〃Climbing　Plants〃　he　did　little　more　than　point　out　the　remarkable　fact　that　the　habit　of　climbing　is　widely　scattered　through　the　vegetable　kingdom。　　Thus　climbers　are　to　be　found　in　35　out　of　the　59　Phanerogamic　Alliances　of　Lindley；　so　that　〃the　conclusion　is　forced　on　our　minds　that　the　capacity　of　revolving　（If　a　twining　plant；　e。g。　a　hop；　is　observed　before　it　has　begun　to　ascend　a　pole；　it　will　be　noticed　that；　owing　to　the　curvature　of　the　stem；　the　tip　is　not　vertical　but　hangs　over　in　a　roughly　horizontal　position。　　If　such　a　shoot　is　watched　it　will　be　found　that　if；　for　instance；　it　points　to　the　north　at　a　given　hour；　it　will　be　found　after　a　short　interval　pointing　north…east；　then　east；　and　after　about　two　hours　it　will　once　more　be　looking　northward。　　The　curvature　of　the　stem　depends　on　one　side　growing　quicker　than　the　opposite　side；　and　the　revolving　movement；　i。e。　circumnutation；　depends　on　the　region　of　quickest　growth　creeping　gradually　round　the　stem　from　south　through　west　to　south　again。　　Other　plants；　e。g。　Phaseolus；　revolve　in　the　opposite　direction。）；　on　which　most　climbers　depend；　is　inherent；　though　undeveloped；　in　almost　every　plant　in　the　vegetable　kingdom。〃　　（〃Climbing　Plants〃；　page　205。）

In　the　〃Origin〃　（Edition　I。　page　427；　Edition　VI。　page　374。）　Darwin　speaks　of　the　〃apparent　paradox；　that　the　very　same　characters　are　analogical　when　one　class　or　order　is　compared　with　another；　but　give　true　affinities　when　the　members　of　the　same　class　or　order　are　compared　one　with　another。〃　　In　this　way　we　might　perhaps　say　that　the　climbing　of　an　ivy　and　a　hop　are　analogical；　the　resemblance　depending　on　the　adaptive　result　rather　than　on　community　of　blood；　whereas　the　relation　between　a　leaf…climber　and　a　true　tendril…bearer　reveals　descent。　　This　particular　resemblance　was　one　in　which　my　father　took　especial　delight。　　He　has　described　an　interesting　case　occurring　in　the　Fumariaceae。　　（〃Climbing　Plants〃；　page　195。）　　〃The　terminal　leaflets　of　the　leaf…climbing　Fumaria　officinalis　are　not　smaller　than　the　other　leaflets；　those　of　the　leaf…climbing　Adlumia　cirrhosa　are　greatly　reduced；　those　of　Corydalis　claviculata　（a　plant　which　may　be　indifferently　called　a　leaf…climber　or　a　tendril…bearer）　are　either　reduced　to　microscopical　dimensions　or　have　their　blades　wholly　aborted；　so　that　this　plant　is　actually　in　a　state　of　transition；　and　finally　in　the　Dicentra　the　tendrils　are　perfectly　characterized。〃

It　is　a　remarkable　fact　that　the　quality　which；　broadly　speaking；　forms　the　basis　of　the　climbing　habit　（namely　revolving　nutation；　otherwise　known　as　circumnutation）　subserves　two　distinct　ends。　　One　of　these　is　the　finding　of　a　support；　and　this　is　common　to　twiners　and　tendrils。　　Here　the　value　ends　as　far　as　tendril…climbers　are　concerned；　but　in　twiners　Darwin　believed　that　the　act　of　climbing　round　a　support　is　a　continuation　of　the　revolving　movement　（circumnutation）。　　If　we　imagine　a　man　swinging　a　rope　round　his　head　and　if　we　suppose　the　rope　to　strike　a　vertical　post；　the　free　end　will　twine　round　it。　　This　may　serve　as　a　rough　model　of　twining　as　explained　in　the　〃Movements　and　Habits　of　Climbing　Plants〃。　　It　is　on　these　pointsthe　nature　of　revolving　nutation　and　the　mechanism　of　twiningthat　modern　physiologists　differ　from　Darwin。　　（See　the　discussion　in　Pfeffer's　〃The　Physiology　of　Plants〃　Eng。　Tr。　（Oxford；　1906）；　III。　page　34；　where　the　literature　is　given。　　Also　Jost；　〃Vorlesungen　uber　Pflanzenphysiologie〃；　page　562；　Jena；　1904。）

Their　criticism　originated　in　observations　made　on　a　revolving　shoot　which　is　removed　from　the　action　of　gravity　by　keeping　the　plant　slowly　rotating　about　a　horizontal　axis　by　means　of　the　instrument　known　as　a　klinostat。　　Under　these　conditions　circumnutation　becomes　irregular　or　ceases　altogether。　　When　the　same　experiment　is　made　with　a　plant　which　has　twined　spirally　up　a　stick；　the　process　of　climbing　is　checked　and　the　last　few　turns　become　loosened　or　actually　untwisted。　　From　this　it　has　been　argued　that　Darwin　was　wrong　in　his　description　of　circumnutation　as　an　automatic　change　in　the　region　of　quickest　growth。　　When　the　free　end　of　a　revolving　shoot　points　towards　the　north　there　is　no　doubt　that　the　south　side　has　been　elongating　more　than　the　north；　after　a　time　it　is　plain　from　the　shoot　hanging　over　to　the　east　that　the　west　side　of　the　plant　has　grown　most；　and　so　on。　　This　rhythmic　change　of　the　position　of　the　region　of　greatest　growth　Darwin　ascribes　to　an　unknown　internal　regulating　power。　　Some　modern　physiologists；　however；　attempt　to　explain　the　revolving　movement　as　due　to　a　particular　form　of　sensitiveness　to　gravitation　which　it　is　not　necessary　to　discuss　in　detail　in　this　place。　　It　is　sufficient　for　my　purpose　to　point　out　that　Darwin's　explanation　of　circumnutation　is　not　universally　accepted。　　Personally　I　believe　that　circumnutation　is　automaticis　primarily　due　to　internal　stimuli。　　It　is　however　in　some　way　connected　with　gravitational　sensitiveness；　since　the　movement　normally　occurs　round　a　vertical　line。　　It　is　not　unnatural　that；　when　the　plant　has　no　external　stimulus　by　which　the　vertical　can　be　recognised；　the　revolving　movement　should　be　upset。

Very　much　the　same　may　be　said　of　the　act　of　twining；　namely　that　most　physiologists　refuse　to　accept　Darwin's　view　（above　referred　to）　that　twining　is　the　direct　result　of　circumnutation。　　Everyone　must　allow　that　the　two　phenomena　are　in　some　way　connected；　since　a　plant　which　circumnutates　clockwise；　i。e。　with　the　sun；　twines　in　the　same　direction；　and　vice　versa。　　It　must　also　be　granted　that　geotropism　has　a　bearing　on　the　problem；　since　all　plants　twine　upwards；　and　cannot　twine　along　a　horizontal　support。　　But　how　these　two　factors　are　combined；　and　whether　any　（