darwin and modern science-第121节

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　cannot　twine　along　a　horizontal　support。　　But　how　these　two　factors　are　combined；　and　whether　any　（and　if　so　what）　other　factors　contribute；　we　cannot　say。　　If　we　give　up　Darwin's　explanation；　we　must　at　the　same　time　say　with　Pfeffer　that　〃the　causes　of　twining　are。。。unknown。〃　　（〃The　Physiology　of　Plants〃；　Eng。　Tr。　（Oxford；　1906）；　III。　page　37。）

Let　us　leave　this　difficult　question　and　consider　some　other　points　made　out　in　the　progress　of　the　work　on　climbing　plants。　　One　result　of　what　he　called　his　〃niggling〃　（〃Life　and　Letters〃；　III。　page　312。）　work　on　tendrils　was　the　discovery　of　the　delicacy　of　their　sense　of　touch；　and　the　rapidity　of　their　movement。　　Thus　in　a　passion…flower　tendril；　a　bit　of　platinum　wire　weighing　1。2　mg。　produced　curvature　（〃Climbing　Plants〃；　page　171。）；　as　did　a　loop　of　cotton　weighing　2　mg。　　Pfeffer　（〃Untersuchungen　a。d。　Bot。　Inst。　z。　Tubingen〃；　Bd。　I。　1881…85；　page　506。）；　however；　subsequently　found　much　greater　sensitiveness：　　thus　the　tendril　of　Sicyos　angulatus　reacted　to　0。00025　mg。；　but　this　only　occurred　when　the　delicate　rider　of　cottonwool　fibre　was　disturbed　by　the　wind。　　The　same　author　expanded　and　explained　in　a　most　interesting　way　the　meaning　of　Darwin's　observation　that　tendrils　are　not　stimulated　to　movement　by　drops　of　water　resting　on　them。　　Pfeffer　showed　that　DIRTY　water　containing　minute　particles　of　clay　in　suspension　acts　as　a　stimulus。　　He　also　showed　that　gelatine　acts　like　pure　water；　if　a　smooth　glass　rod　is　coated　with　a　10　per　cent　solution　of　gelatine　and　is　then　applied　to　a　tendril；　no　movement　occurs　in　spite　of　the　fact　that　the　gelatine　is　solid　when　cold。　　Pfeffer　（〃Physiology〃；　Eng。　Tr。　III。　page　52。　　Pfeffer　has　pointed　out　the　resemblance　between　the　contact　irritability　of　plants　and　the　human　sense　of　touch。　　Our　skin　is　not　sensitive　to　uniform　pressure　such　as　is　produced　when　the　finger　is　dipped　into　mercury　（Tubingen　〃Untersuchungen〃；　I。　page　504。）　generalises　the　result　in　the　statement　that　the　tendril　has　a　special　form　of　irritability　and　only　reacts　to　〃differences　of　pressure　or　variations　of　pressure　in　contiguous。。。regions。〃　　Darwin　was　especially　interested　in　such　cases　of　specialised　irritability。　　For　instance　in　May；　1864；　he　wrote　to　Asa　Gray　（〃Life　and　Letters〃；　III。　page　314。）　describing　the　tendrils　of　Bignonia　capreolata；　which　〃abhor　a　simple　stick；　do　not　much　relish　rough　bark；　but　delight　in　wool　or　moss。〃　　He　received；　from　Gray；　information　as　to　the　natural　habitat　of　the　species；　and　finally　concluded　that　the　tendrils　〃are　specially　adapted　to　climb　trees　clothed　with　lichens；　mosses；　or　other　such　productions。〃　　（〃Climbing　Plants〃；　page　102。）

Tendrils　were　not　the　only　instance　discovered　by　Darwin　of　delicacy　of　touch　in　plants。　　In　1860　he　had　already　begun　to　observe　Sundew　（Drosera）；　and　was　full　of　astonishment　at　its　behaviour。　　He　wrote　to　Sir　Joseph　Hooker　（〃Life　and　Letters〃；　III。　page　319。）：　　〃I　have　been　working　like　a　madman　at　Drosera。　　Here　is　a　fact　for　you　which　is　certain　as　you　stand　where　you　are；　though　you　won't　believe　it；　that　a　bit　of　hair　1/78000　of　one　grain　in　weight　placed　on　gland；　will　cause　ONE　of　the　gland…bearing　hairs　of　Drosera　to　curve　inwards。〃　　Here　again　Pfeffer　（Pfeffer　in　〃Untersuchungen　a。　d。　Bot。　Inst。　z。　Tubingen〃；　I。　page　491。）　has；　as　in　so　many　cases；　added　important　facts　to　my　father's　observations。　　He　showed　that　if　the　leaf　of　Drosera　is　entirely　freed　from　such　vibrations　as　would　reach　it　if　observed　on　an　ordinary　table；　it　does　not　react　to　small　weights；　so　that　in　fact　it　was　the　vibration　of　the　minute　fragment　of　hair　on　the　gland　that　produced　movement。　　We　may　fancifully　see　an　adaptation　to　the　capture　of　insectsto　the　dancing　of　a　gnat's　foot　on　the　sensitive　surface。

Darwin　was　fond　of　telling　how　when　he　demonstrated　the　sensitiveness　of　Drosera　to　Mr　Huxley　and　（I　think）　to　Sir　John　Burdon　Sanderson；　he　could　perceive　（in　spite　of　their　courtesy）　that　they　thought　the　whole　thing　a　delusion。　　And　the　story　ended　with　his　triumph　when　Mr　Huxley　cried　out；　〃It　IS　moving。〃

Darwin's　work　on　tendrils　has　led　to　some　interesting　investigations　on　the　mechanisms　by　which　plants　perceive　stimuli。　　Thus　Pfeffer　（Tubingen　〃Untersuchungen〃　I。　page　524。）　showed　that　certain　epidermic　cells　occurring　in　tendrils　are　probably　organs　of　touch。　　In　these　cells　the　protoplasm　burrows　as　it　were　into　cavities　in　the　thickness　of　the　external　cell…walls　and　thus　comes　close　to　the　surface；　being　separated　from　an　object　touching　the　tendril　merely　by　a　very　thin　layer　of　cell…　wall　substance。　　Haberlandt　（〃Physiologische　Pflanzenanatomie〃；　Edition　III。　Leipzig；　1904。　　〃Sinnesorgane　im　Pflanzenreich〃；　Leipzig；　1901；　and　other　publications。）　has　greatly　extended　our　knowledge　of　vegetable　structure　in　relation　to　mechanical　stimulation。　　He　defines　a　sense…organ　as　a　contrivance　by　which　the　DEFORMATION　or　forcible　change　of　form　in　the　protoplasmon　which　mechanical　stimulation　dependsis　rendered　rapid　and　considerable　in　amplitude　（〃Sinnesorgane〃；　page　10）。　　He　has　shown　that　in　certain　papillose　and　bristle…like　contrivances；　plants　possess　such　sense…　organs；　and　moreover　that　these　contrivances　show　a　remarkable　similarity　to　corresponding　sense…organs　in　animals。

Haberlandt　and　Nemec　（〃Ber。　d。　Deutschen　bot。　Gesellschaft〃；　XVIII。　1900。　　See　F。　Darwin；　Presidential　Address　to　Section　K；　British　Association；　1904。）　published　independently　and　simultaneously　a　theory　of　the　mechanism　by　which　plants　are　orientated　in　relation　to　gravitation。　　And　here　again　we　find　an　arrangement　identical　in　principle　with　that　by　which　certain　animals　recognise　the　vertical；　namely　the　pressure　of　free　particles　on　the　irritable　wall　of　a　cavity。　　In　the　higher　plants；　Nemec　and　Haberlandt　believe　that　special　loose　and　freely　movable　starch…grains　play　the　part　of　the　otoliths　or　statoliths　of　the　crustacea；　while　the　protoplasm　lining　the　cells　in　which　they　are　contained　corresponds　to　the　sensitive　membrane　lining　the　otocyst　of　the　animal。　　What　is　of　special　interest　in　our　present　connection　is　that　according　to　this　ingenious　theory　（The　original　conception　was　due　to　Noll　（〃Heterogene　Induction〃；　Leipzig；　1892）；　but　his　view　differed　in　essential　points　from　those　here　given。）　the　sense　of　verticality　in　a　plant　is　a　form　of　contact…irritability。　　The　vertical　position　is　distinguished　from　the　horizontal　by　the　fact　that；　in　the　latter　case；　the　loose　starch…grains　rest　on　the　lateral　walls　of　the　cells　instead　of　on　the　terminal　walls　as　occurs　in　the　normal　upright　position。　　It　should　be　added　that　the　statolith　theory　is　still　sub　judice；　personally　I　cannot　doubt　that　it　is　in　the　main　a　satisfactory　explanation　of　the　facts。

With　regard　to　the　RAPIDITY　of　the　reaction　of　tendrils；　Darwin　records　（〃Climbing　Plants〃；　page　155。　　Others　have　observed　movement　after　about　6〃。）　that　a　Passion…Flower　tendril　moved　distinctly　within　25　seconds　of　stimulation。　　It　was　this　fact；　more　than　any　other；　that　made　him　doubt　the　current　explanation；　viz。　that　the　movement　is　due　to　unequal　growth　on　the　two　sides　of　the　tendril。　　The　interesting　work　of　Fitting　（Pringsheim's　〃Jahrb。〃　XXXVIII。　1903；　page　545。）　has　shown；　however；　that　the　primary　cause　is　not　（as　Darwin　supposed）　contraction　on　the　concave；　but　an　astonishingly　rapid　increase　in　growth…rate　on　the　convex　side。

On　the　last　page　of　〃Climbing　Plants〃　Darwin　wrote：　　〃It　has　often　been　vaguely　asserted　that　plants　are　distinguished　from　animals　by　not　having　the　power　of　movement。　　It　should　rather　be　said　that　plants　acquire　and　display　this　power　only　when　it　is　of　some　advantage　to　them。〃

He　gradually　came　to　realise　the　vividness　and　variety　of　vegetable　life；　and　that　a　plant　like　an　animal　has　capacities　of　behaving　in　different　ways　under　different　circumstances；　in　a　manner　that　may　be　compared　to　the　instinctive　movements　of　animals。　　This　point　of　view　is　expressed　in　well…　known　passages　in　the　〃Power　of　Movement〃。　　（〃The　Power　of　Movement　in　Plants〃；　1880；　pages　571…3。）　　〃It　is　impossible　not　to　be　struck　with　the　resemblance　between　the。。。movements　of　plants　and　many　of　the　actions　performed　unconsciously　by　the　lower　animals。〃　　And　again；　〃It　is　hardly　an　exaggeration　to　say　that　the　tip　of　the　radicle。。。having　the　power　of　directing　the　movements　of　the　adjoining　parts；　acts　like　the　brain　of　one　of　the　lower　animals；　the　brain　being　seated　within　the　anterior　end　of　the　body；　receiving　impressions　from　the　sense…organs；　and　directing　the　several　movements。〃

The　conception　of　a　region　of　perception　distinct　from　a　region　of　movement　is　perhaps　the　most　fruitful　outcome　of　his　work　on　the　movements　of　plants。　　But　many　years　before　its　publication；　viz。　in　1861；　he　had　made　out　the　wonderful　fact　that　in　the　Orchid　Catasetum　（〃Life　and　Letters〃；　III。　page　268。）　the　projecting　organs　or　antennae　are　sensitive　to　a　touch；　and　transmit　an　influence　〃for　more　than　one　inch　INSTANTANEOUSLY；〃　which　leads　to　the　explosion　or　violent　ejection　of　the　pollinia。　　And　as　we　have　already　seen　a　similar　transmission　of　a　stimulus　was　discovered　by　him　in　Sundew　in　1860；　so　that　in　1862　he　could　write　to　Hooker　（〃