darwin and modern science-第123节

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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　a　solid　body；　such　as　a　curved　fragment　of　glass；　produces　curvature：　　and　this　may　conceivably　be　the　explanation　of　the　effect　of　gold…beaters　skin　covered　with　shellac。　　But　on　the　whole　it　is　perhaps　safer　to　classify　the　shellac　experiments　with　the　results　of　undoubted　injury　rather　than　with　those　of　contact。

Another　subject　on　which　a　good　deal　of　labour　was　expended　is　the　sleep　of　leaves；　or　as　Darwin　called　it　their　NYCTITROPIC　movement。　　He　showed　for　the　first　time　how　widely　spread　this　phenomenon　is；　and　attempted　to　give　an　explanation　of　the　use　to　the　plant　of　the　power　of　sleeping。　　His　theory　was　that　by　becoming　more　or　less　vertical　at　night　the　leaves　escape　the　chilling　effect　of　radiation。　　Our　method　of　testing　this　view　was　to　fix　some　of　the　leaves　of　a　sleeping　plant　so　that　they　remained　horizontal　at　night　and　therefore　fully　exposed　to　radiation；　while　their　fellows　were　partly　protected　by　assuming　the　nocturnal　position。　　The　experiments　showed　clearly　that　the　horizontal　leaves　were　more　injured　than　the　sleeping；　i。e。　more　or　less　vertical；　ones。　　It　may　be　objected　that　the　danger　from　cold　is　very　slight　in　warm　countries　where　sleeping　plants　abound。　　But　it　is　quite　possible　that　a　lowering　of　the　temperature　which　produces　no　visible　injury　may　nevertheless　be　hurtful　by　checking　the　nutritive　processes　（e。g。　translocation　of　carbohydrates）；　which　go　on　at　night。　　Stahl　（〃Bot。　Zeitung〃；　1897；　page　81。）　however　has　ingeniously　suggested　that　the　exposure　of　the　leaves　to　radiation　is　not　DIRECTLY　hurtful　because　it　lowers　the　temperature　of　the　leaf；　but　INDIRECTLY　because　it　leads　to　the　deposition　of　dew　on　the　leaf…surface。　　He　gives　reasons　for　believing　that　dew…covered　leaves　are　unable　to　transpire　efficiently；　and　that　the　absorption　of　mineral　food…material　is　correspondingly　checked。　　Stahl's　theory　is　in　no　way　destructive　of　Darwin's；　and　it　is　possible　that　nyctitropic　leaves　are　adapted　to　avoid　the　indirect　as　well　as　the　direct　results　of　cooling　by　radiation。

In　what　has　been　said　I　have　attempted　to　give　an　idea　of　some　of　the　discoveries　brought　before　the　world　in　the　〃Power　of　Movement〃　（In　1881　Professor　Wiesner　published　his　〃Das　Bewegungsvermogen　der　Pflanzen〃；　a　book　devoted　to　the　criticism　of　〃The　Power　of　Movement　in　Plants〃。　　A　letter　to　Wiesner；　published　in　〃Life　and　Letters〃；　III。　page　336；　shows　Darwin's　warm　appreciation　of　his　critic's　work；　and　of　the　spirit　in　which　it　is　written。）　and　of　the　subsequent　history　of　the　problems。　　We　must　now　pass　on　to　a　consideration　of　the　central　thesis　of　the　book；the　relation　of　circumnutation　to　the　adaptive　curvatures　of　plants。

Darwin's　view　is　plainly　stated　on　pages　3…4　of　the　〃Power　of　Movement〃。　　Speaking　of　circumnutation　he　says；　〃In　this　universally　present　movement　we　have　the　basis　or　groundwork　for　the　acquirement；　according　to　the　requirements　of　the　plant；　of　the　most　diversified　movements。〃　　He　then　points　out　that　curvatures　such　as　those　towards　the　light　or　towards　the　centre　of　the　earth　can　be　shown　to　be　exaggerations　of　circumnutation　in　the　given　directions。　　He　finally　points　out　that　the　difficulty　of　conceiving　how　the　capacities　of　bending　in　definite　directions　were　acquired　is　diminished　by　his　conception。　　〃We　know　that　there　is　always　movement　in　progress；　and　its　amplitude；　or　direction；　or　both；　have　only　to　be　modified　for　the　good　of　the　plant　in　relation　with　internal　or　external　stimuli。〃

It　may　at　once　be　allowed　that　the　view　here　given　has　not　been　accepted　by　physiologists。　　The　bare　fact　that　circumnutation　is　a　general　property　of　plants　（other　than　climbing　species）　is　not　generally　rejected。　　But　the　botanical　world　is　no　nearer　to　believing　in　the　theory　of　reaction　built　on　it。

If　we　compare　the　movements　of　plants　with　those　of　the　lower　animals　we　find　a　certain　resemblance　between　the　two。　　According　to　Jennings　（H。S。　Jennings；　〃The　Behavior　of　the　Lower　Animals〃。　　Columbia　U。　Press；　N。Y。　1906。）　a　Paramoecium　constantly　tends　to　swerve　towards　the　aboral　side　of　its　body　owing　to　certain　peculiarities　in　the　set　and　power　of　its　cilia。　But　the　tendency　to　swim　in　a　circle；　thus　produced；　is　neutralised　by　the　rotation　of　the　creature　about　its　longitudinal　axis。　　Thus　the　direction　of　the　swerves　IN　RELATION　TO　THE　PATH　of　the　organism　is　always　changing；　with　the　result　that　the　creature　moves　in　what　approximates　to　a　straight　line；　being　however　actually　a　spiral　about　the　general　line　of　progress。　　This　method　of　motion　is　strikingly　like　the　circumnutation　of　a　plant；　the　apex　of　which　also　describes　a　spiral　about　the　general　line　of　growth。　　A　rooted　plant　obviously　cannot　rotate　on　its　axis；　but　the　regular　series　of　curvatures　of　which　its　growth　consists　correspond　to　the　aberrations　of　Paramoecium　distributed　regularly　about　its　course　by　means　of　rotation。　　（In　my　address　to　the　Biological　Section　of　the　British　Association　at　Cardiff　（1891）　I　have　attempted　to　show　the　connection　between　circumnutation　and　RECTIPETALITY；　i。e。　the　innate　capacity　of　growing　in　a　straight　line。）　　Just　as　a　plant　changes　its　direction　of　growth　by　an　exaggeration　of　one　of　the　curvature…elements　of　which　circumnutation　consists；　so　does　a　Paramoecium　change　its　course　by　the　accentuation　of　one　of　the　deviations　of　which　its　path　is　built。　　Jennings　has　shown　that　the　infusoria；　etc。；　react　to　stimuli　by　what　is　known　as　the　〃method　of　trial。〃　　If　an　organism　swims　into　a　region　where　the　temperature　is　too　high　or　where　an　injurious　substance　is　present；　it　changes　its　course。　　It　then　moves　forward　again；　and　if　it　is　fortunate　enough　to　escape　the　influence；　it　continues　to　swim　in　the　given　direction。　　If　however　its　change　of　direction　leads　it　further　into　the　heated　or　poisonous　region　it　repeats　the　movement　until　it　emerges　from　its　difficulties。　　Jennings　finds　in　the　movements　of　the　lower　organisms　an　analogue　with　what　is　known　as　pain　in　conscious　organisms。　　There　is　certainly　this　much　resemblance　that　a　number　of　quite　different　sub…injurious　agencies　produce　in　the　lower　organisms　a　form　of　reaction　by　the　help　of　which　they；　in　a　partly　fortuitous　way；　escape　from　the　threatening　element　in　their　environment。　　The　higher　animals　are　stimulated　in　a　parallel　manner　to　vague　and　originally　purposeless　movements；　one　of　which　removes　the　discomfort　under　which　they　suffer；　and　the　organism　finally　learns　to　perform　the　appropriate　movement　without　going　through　the　tentative　series　of　actions。

I　am　tempted　to　recognise　in　circumnutation　a　similar　groundwork　of　tentative　movements　out　of　which　the　adaptive　ones　were　originally　selected　by　a　process　rudely　representative　of　learning　by　experience。

It　is；　however；　simpler　to　confine　ourselves　to　the　assumption　that　those　plants　have　survived　which　have　acquired　through　unknown　causes　the　power　of　reacting　in　appropriate　ways　to　the　external　stimuli　of　light；　gravity；　etc。　　It　is　quite　possible　to　conceive　this　occurring　in　plants　which　have　no　power　of　circumnutatingand；　as　already　pointed　ou