darwin and modern science-第100章

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n　of　a　stimulus　was　discovered　by　him　in　Sundew　in　1860；　so　that　in　1862　he　could　write　to　Hooker　（〃Life　and　Letters〃；　III。　page　321。）：　　〃I　cannot　avoid　the　conclusion；　that　Drosera　possesses　matter　at　least　in　some　degree　analogous　in　constitution　and　function　to　nervous　matter。〃　　I　propose　in　what　follows　to　give　some　account　of　the　observations　on　the　transmission　of　stimuli　given　in　the　〃Power　of　Movement〃。　　It　is　impossible　within　the　space　at　my　command　to　give　anything　like　a　complete　account　of　the　matter；　and　I　must　necessarily　omit　all　mention　of　much　interesting　work。　　One　well…known　experiment　consisted　in　putting　opaque　caps　on　the　tips　of　seedling　grasses　（e。g。　oat　and　canary…grass）　and　then　exposing　them　to　light　from　one　side。　　The　difference；　in　the　amount　of　curvature　towards　the　light；　between　the　blinded　and　unblinded　specimens；　was　so　great　that　it　was　concluded　that　the　light…sensitiveness　resided　exclusively　in　the　tip。　　The　experiment　undoubtedly　proves　that　the　sensitiveness　is　much　greater　in　the　tip　than　elsewhere；　and　that　there　is　a　transmission　of　stimulus　from　the　tip　to　the　region　of　curvature。　　But　Rothert　（Rothert；　Cohn's　〃Beitrage〃；　VII。　1894。）　has　conclusively　proved　that　the　basal　part　where　the　curvature　occurs　is　also　DIRECTLY　sensitive　to　light。　　He　has　shown；　however；　that　in　other　grasses　（Setaria；　Panicum）　the　cotyledon　is　the　only　part　which　is　sensitive；　while　the　hypocotyl；　where　the　movement　occurs；　is　not　directly　sensitive。

It　was　however　the　question　of　the　localisation　of　the　gravitational　sense　in　the　tip　of　the　seedling　root　or　radicle　that　aroused　most　attention；　and　it　was　on　this　question　that　a　controversy　arose　which　has　continued　to　the　present　day。

The　experiment　on　which　Darwin's　conclusion　was　based　consisted　simply　in　cutting　off　the　tip；　and　then　comparing　the　behaviour　of　roots　so　treated　with　that　of　normal　specimens。　　An　uninjured　root　when　placed　horizontally　regains　the　vertical　by　means　of　a　sharp　downward　curve；　not　so　a　decapitated　root　which　continues　to　grow　more　or　less　horizontally。　　It　was　argued　that　this　depends　on　the　loss　of　an　organ　specialised　for　the　perception　of　gravity；　and　residing　in　the　tip　of　the　root；　and　the　experiment　（together　with　certain　important　variants）　was　claimed　as　evidence　of　the　existence　of　such　an　organ。

It　was　at　once　objected　that　the　amputation　of　the　tip　might　check　curvature　by　interfering　with　longitudinal　growth；　on　the　distribution　of　which　curvature　depends。　　This　objection　was　met　by　showing　that　an　injury；　e。g。　splitting　the　root　longitudinally　（See　F。　Darwin；　〃Linnean　Soc。　Journal　（Bot）。〃　XIX。　1882；　page　218。）；　which　does　not　remove　the　tip；　but　seriously　checks　growth；　does　not　prevent　geotropism。　　This　was　of　some　interest　in　another　and　more　general　way；　in　showing　that　curvature　and　longitudinal　growth　must　be　placed　in　different　categories　as　regards　the　conditions　on　which　they　depend。

Another　objection　of　a　much　more　serious　kind　was　that　the　amputation　of　the　tip　acts　as　a　shock。　　It　was　shown　by　Rothert　（See　his　excellent　summary　of　the　subject　in　〃Flora〃　1894　（Erganzungsband）；　page　199。）　that　the　removal　of　a　small　part　of　the　cotyledon　of　Setaria　prevents　the　plant　curving　towards　the　light；　and　here　there　is　no　question　of　removing　the　sense…organ　since　the　greater　part　of　the　sensitive　cotyledon　is　intact。　　In　view　of　this　result　it　was　impossible　to　rely　on　the　amputations　performed　on　roots　as　above　described。

At　this　juncture　a　new　and　brilliant　method　originated　in　Pfeffer's　laboratory。　　（See　Pfeffer；　〃Annals　of　Botany〃；　VIII。　1894；　page　317；　and　Czapek；　Pringsheim's　〃Jahrb。〃　XXVII。　1895；　page　243。）　　Pfeffer　and　Czapek　showed　that　it　is　possible　to　bend　the　root　of　a　lupine　so　that；　for　instance；　the　supposed　sense…organ　at　the　tip　is　vertical　while　the　motile　region　is　horizontal。　　If　the　motile　region　is　directly　sensitive　to　gravity　the　root　ought　to　curve　downwards；　but　this　did　not　occur：　　on　the　contrary　it　continued　to　grow　horizontally。　　This　is　precisely　what　should　happen　if　Darwin's　theory　is　the　right　one：　　for　if　the　tip　is　kept　vertical；　the　sense…organ　is　in　its　normal　position　and　receives　no　stimulus　from　gravitation；　and　therefore　can　obviously　transmit　none　to　the　region　of　curvature。　　Unfortunately　this　method　did　not　convince　the　botanical　world　because　some　of　those　who　repeated　Czapek's　experiment　failed　to　get　his　results。

Czapek　（〃Berichte　d。　Deutsch。　bot。　Ges。〃　XV。　1897；　page　516；　and　numerous　subsequent　papers。　　English　readers　should　consult　Czapek　in　the　〃Annals　of　Botany〃；　XIX。　1905；　page　75。）　has　devised　another　interesting　method　which　throws　light　on　the　problem。　　He　shows　that　roots；　which　have　been　placed　in　a　horizontal　position　and　have　therefore　been　geotropically　stimulated；　can　be　distinguished　by　a　chemical　test　from　vertical；　i。e。　unstimulated　roots。　　The　chemical　change　in　the　root　can　be　detected　before　any　curvature　has　occurred　and　must　therefore　be　a　symptom　of　stimulation；　not　of　movement。　　It　is　particularly　interesting　to　find　that　the　change　in　the　root；　on　which　Czapek's　test　depends；　takes　place　in　the　tip；　i。e。　in　the　region　which　Darwin　held　to　be　the　centre　for　gravitational　sensitiveness。

In　1899　I　devised　a　method　（F。　Darwin；　〃Annals　of　Botany〃；　XIII。　1899；　page　567。）　by　which　I　sought　to　prove　that　the　cotyledon　of　Setaria　is　not　only　the　organ　for　light…perception；　but　also　for　gravitation。　　If　a　seedling　is　supported　horizontally　by　pushing　the　apical　part　（cotyledon）　into　a　horizontal　tube；　the　cotyledon　will；　according　to　my　supposition；　be　stimulated　gravitationally　and　a　stimulus　will　be　transmitted　to　the　basal　part　of　the　stem　（hypocotyl）　causing　it　to　bend。　　But　this　curvature　merely　raises　the　basal　end　of　the　seedling；　the　sensitive　cotyledon　remains　horizontal；　imprisoned　in　its　tube；　it　will　therefore　be　continually　stimulated　and　will　continue　to　transmit　influences　to　the　bending　region；　which　should　therefore　curl　up　into　a　helix　or　corkscrew…like　form；and　this　is　precisely　what　occurred。

I　have　referred　to　this　work　principally　because　the　same　method　was　applied　to　roots　by　Massart　（Massart；　〃Mem。　Couronnes　Acad。　R。　Belg。〃　LXII。　1902。）　and　myself　（F。　Darwin；　〃Linnean　Soc。　Journ。〃　XXXV。　1902；　page　266。）　with　a　similar　though　less　striking　result。　　Although　these　researches　confirmed　Darwin's　work　on　roots；　much　stress　cannot　be　laid　on　them　as　there　are　several　objections　to　them；　and　they　are　not　easily　repeated。

The　method　whichas　far　as　we　can　judge　at　presentseems　likely　to　solve　the　problem　of　the　root…tip　is　most　ingenious　and　is　due　to　Piccard。　　（Pringsheim's　〃Jahrb。〃　XL。　1904；　page　94。）

Andrew　Knight's　celebrated　experiment　showed　that　roots　react　to　centrifugal　force　precisely　as　they　do　to　gravity。　　So　that　if　a　bean　root　is　fixed　to　a　wheel　revolving　rapidly　on　a　horizontal　axis；　it　tends　to　curve　away　from　the　centre　in　the　line　of　a　radius　of　the　wheel。　　In　ordinary　demonstrations　of　Knight's　experiment　the　seed　is　generally　fixed　so　that　the　root　is　at　right　angles　to　a　radius；　and　as　far　as　convenient　from　the　centre　of　rotation。　　Piccard's　experiment　is　arranged　differently。　　（A　seed　is　depicted　below　a　horizontal　dotted　line　AA；　projecting　a　root　upwards。）　　The　root　is　oblique　to　the　axis　of　rotation；　and　the　extreme　tip　projects　beyond　that　axis。　　Line　AA　represents　the　axis　of　rotation；　T　is　the　tip　of　the　root　just　above　the　line　AA；　and　B　is　the　region　just　below　line　AA　in　which　curvature　takes　place。　　If　the　motile　region　B　is　directly　sensitive　to　gravitation　（and　is　the　only　part　which　is　sensitive）　the　root　will　curve　（down　and　away　from　the　vertical）　away　from　the　axis　of　rotation；　just　as　in　Knight's　experiment。　　But　if　the　tip　T　is　alone　sensitive　to　gravitation　the　result　will　be　exactly　reversed；　the　stimulus　originating　in　T　and　conveyed　to　B　will　produce　curvature　（up　towards　the　vertical）。　　We　may　think　of　the　line　AA　as　a　plane　dividing　two　worlds。　　In　the　lower　one　gravity　is　of　the　earthly　type　and　is　shown　by　bodies　falling　and　roots　curving　downwards：　　in　the　upper　world　bodies　fall　upwards　and　roots　curve　in　the　same　direction。　　The　seedling　is　in　the　lower　world；　but　its　tip　containing　the　supposed　sense…organ　is　in　the　strange　world　where　roots　curve　upwards。　　By　observing　whether　the　root　bends　up　or　down　we　can　decide　whether　the　impulse　to　bend　originates　in　the　tip　or　in　the　motile　region。

Piccard's　results　showed　that　both　curvatures　occurred　and　he　concluded　that　the　sensitive　region　is　not　confined　to　the　tip。　　（Czapek　（Pringsheim's　〃Jahrb。〃　XXXV。　1900；　page　362）　had　previously　given　reasons　for　believing　that；　in　the　root；　there　is　no　sharp　line　of　separation　between　the　regions　of　perception　and　movement。）

Haberlandt　（Pringsheim's　〃Jahrb。〃　XLV。　1908；　page　575。）　has　recently　repeated　the　experiment　with　the　advantage　of　better　apparatus　and　more　experience　in　dealing　with　plants；　and　has　found　as　Piccard　did　that　both　the　tip　and　the　curving　region　are　sensitive　to　gravity；　but　with　the　important　addition　that　the　sensitiveness　of　the　tip　is　much　greater　than　that　of　the　motile　region。　　The　case　is　in　fact　similar　to　that　of　the　oat　and　canary…grass。　　In　both　instances　my　father　and　I　were　wrong　in　assuming　that　the　sensitiveness　is　confined　to　the　tip；　yet　there　is　a　concentration　of　irritability　in　that　region　and　transmission　of　stimulus　is　as　true　for　geotropism　as　it　is　for　heliotropism。　　Thus　after　nearly　thirty　years　the　controversy　of　the　root…tip　has　apparently　ended　somewhat　after　the　fashion　of　the　quarrels　at　the　〃Rainbow〃　in　〃Silas　Marner〃〃you're　both　right　and　you're　both　wrong。〃　　But　the　〃brain…function〃　of　the　root…tip　at　which　eminent　people　laughed　in　early　days　turns　out　to　be　an　important　part　of　the　truth。　　（By　using　Piccard's　method　I　have　succeeded　in　showing　that　the　gravitational　sensitiveness　of　the　cotyledon　of　Sorghum　is　certainly　much　greater　than　the　sensitiveness　of　the　hypocotylif　indeed　any　such　sensitiveness　exists。　　See　Wiesner's　〃Festschrift〃；　Vienna；　1908。）

Another　observation　of　Darwin's　has　given　rise　to　much　controversy。　　（〃Power　of　Movement〃；　page　133。）　　If　a　minute　piece　of　card　is　fixed　obliquely　to　the　tip　of　a　root　some　influence　is　transmitted　to　the　region　of　curvature　and　the　root　bends　away　from　the　side　to　which　the　card　was　attached。　　It　was　thought　at　the　time　that　this　proved　the　root…tip　to　be　sensitive　to　contact；　but　this　is　not　necessarily　the　case。　　It　seems　possible　that　the　curvature　is　a　reaction　to　the　injury　caused　by　the　alcoholic　solution　of　shellac　with　which　the　cards　were　cemented　to　the　tip。　　This　agrees　with　the　fact　given　in　the　〃Power　of　Movement〃　that　injuring　the　root…tip　on　one　side；　by　cutting　or　burning　it；　induced　a　similar　curvature。　　On　the　other　hand　it　was　shown　that　curvature　could　be　produced　in　roots　by　cementing　cards；　not　to　the　naked　surface　of　the　root…　tip；　but　to　pieces　of　gold…beaters　skin　applied　to　the　root；　gold…beaters　skin　being　by　itself　almost　without　effect。　　But　it　must　be　allowed　that；　as　regards　touch；　it　is　not　clear　how　the　addition　of　shellac　and　card　can　increase　the　degree　of　contact。　　There　is　however　some　evidence　that　very　close　contact　from