darwin and modern science-第101章

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ard　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　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