darwin and modern science-第73节

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factors。　　The　distribution　of　quantities　is　especially　important　in　the　sexual　reproduction　of　algae；　for　which　a　vigorous　production　of　the　materials　formed　during　carbon…assimilation　appears　to　be　essential。

In　the　Flowering　plants；　on　the　other　hand；　for　reasons　already　mentioned；　the　whole　problem　is　more　complicated。　　Investigations　on　changes　in　the　course　of　development　of　fertilised　eggs　have　hitherto　been　unsuccessful；　the　difficulty　of　influencing　egg…cells　deeply　immersed　in　tissue　constitutes　a　serious　obstacle。　　Other　parts　of　plants　are；　however；　convenient　objects　of　experiment；　e。g。　the　growing　apices　of　buds　which　serve　as　cuttings　for　reproductive　purposes；　or　buds　on　tubers；　runners；　rhizomes；　etc。　　A　growing　apex　consists　of　cells　capable　of　division　in　which；　as　in　egg…cells；　a　complete　series　of　latent　possibilities　of　development　is　embodied。　　Which　of　these　possibilities　becomes　effective　depends　upon　the　action　of　the　outer　world　transmitted　by　organs　concerned　with　nutrition。

Of　the　different　stages　which　a　flowering　plant　passes　through　in　the　course　of　its　development　we　will　deal　only　with　one　in　order　to　show　that；　in　spite　of　its　great　complexity；　the　problem　is；　in　essentials；　equally　open　to　attack　in　the　higher　plants　and　in　the　simplest　organisms。　　The　most　important　stage　in　the　life　of　a　flowering　plant　is　the　transition　from　purely　vegetative　growth　to　sexual　reproductionthat　is；　the　production　of　flowers。　　In　certain　cases　it　can　be　demonstrated　that　there　is　no　internal　cause；　dependent　simply　on　the　specific　structure；　which　compels　a　plant　to　produce　its　flowers　after　a　definite　period　of　vegetative　growth。　　（Klebs；　〃Willkurliche　Entwickelungsanderungen〃；　Jena　1903；　see　also　〃Probleme　der　Entwickelung〃；　I。　II。　　〃Centralbl。〃　1904。）

One　extreme　case；　that　of　exceptionally　early　flowering；　has　been　observed　in　nature　and　more　often　in　cultivation。　　A　number　of　plants　under　certain　conditions　are　able　to　flower　soon　after　germination。　　（Cf。　numerous　records　of　this　kind　by　Diels；　〃Jugendformen　und　Bluten〃；　Berlin；　1906。）　　This　shortening　of　the　period　of　development　is　exhibited　in　the　most　striking　form　in　trees；　as　in　the　oak　（Mobius；　〃Beitrage　zur　Lehre　von　der　Fortpflanzung〃；　Jena；　1897；　page　89。）；　flowering　seedlings　of　which　have　been　observed　from　one　to　three　years　old；　whereas　normally　the　tree　does　not　flower　until　it　is　sixty　or　eighty　years　old。

Another　extreme　case　is　represented　by　prolonged　vegetative　growth　leading　to　the　complete　suppression　of　flower…production。　　This　result　may　be　obtained　with　several　plants；　such　as　Glechoma；　the　sugar　beet；　Digitalis；　and　others；　if　they　are　kept　during　the　winter　in　a　warm；　damp　atmosphere；　and　in　rich　soil；　in　the　following　spring　or　summer　they　fail　to　flower。　　（Klebs；　〃Willkurliche　Aenderungen〃；　etc。　Jena；　1903；　page　130。）　　Theoretically；　however；　experiments　are　of　greater　importance　in　which　the　production　of　flowers　is　inhibited　by　very　favourable　conditions　of　nutrition　（Klebs；　〃Ueber　kunstliche　Metamorphosen〃；　Stuttgart；　1906；　page　115　（〃Abh。　Naturf。　Ges。　Halle〃；　XXV。）　occurring　at　the　normal　flowering　period。　　Even　in　the　case　of　plants　of　Sempervivum　several　years　old；　which；　as　is　shown　by　control　experiments　on　precisely　similar　plants；　are　on　the　point　of　flowering；　flowering　is　rendered　impossible　if　they　are　forced　to　very　vigorous　growth　by　an　abundant　supply　of　water　and　salts　in　the　spring。　　Flowering；　however；　occurs；　if　such　plants　are　cultivated　in　relatively　dry　sandy　soil　and　in　the　presence　of　strong　light。　　Careful　researches　into　the　conditions　of　growth　have　led；　in　the　cases　Sempervivum；　to　the　following　results：　　（1）　　With　a　strong　light　and　vigorous　carbon…assimilation　a　considerably　increased　supply　of　water　and　nutritive　salts　produces　active　vegetative　growth。　　（2）　　With　a　vigorous　carbon…assimilation　in　strong　light；　and　a　decrease　in　the　supply　of　water　and　salts　active　flower…production　is　induced。　　（3）　　If　an　average　supply　of　water　and　salts　is　given　both　processes　are　possible；　the　intensity　of　carbon…assimilation　determines　which　of　the　two　is　manifested。　　A　diminution　in　the　production　of　organic　substances；　particularly　of　carbohydrates；　induces　vegetative　growth。　　This　can　be　effected　by　culture　in　feeble　light　or　in　light　deprived　of　the　yellow…red　rays：　　on　the　other　hand；　flower…production　follows　an　increase　in　light…intensity。　　These　results　are　essentially　in　agreement　with　well…known　observations　on　cultivated　plants；　according　to　which；　the　application　of　much　moisture；　after　a　plentiful　supply　of　manure　composed　of　inorganic　salts；　hinders　the　flower…production　of　many　vegetables；　while　a　decrease　in　the　supply　of　water　and　salts　favours　flowering。

ii。　　INFLUENCE　OF　THE　ENVIRONMENT　ON　THE　FORM　OF　SINGLE　ORGANS。　　（A　considerable　number　of　observations　bearing　on　this　question　are　given　by　Goebel　in　his　〃Experimentelle　Morphologie　der　Pflanzen〃；　Leipzig；　1908。　　It　is　not　possible　to　deal　here　with　the　alteration　in　anatomical　structure；　cf。　Kuster；　〃Pathologische　Pflanzenanatomie〃；　Jena；　1903。）

If　we　look　closely　into　the　development　of　a　flowering　plant；　we　notice　that　in　a　given　species　differently　formed　organs　occur　in　definite　positions。　　In　a　potato　plant　colourless　runners　are　formed　from　the　base　of　the　main　stem　which　grow　underground　and　produce　tubers　at　their　tips：　　from　a　higher　level　foliage　shoots　arise　nearer　the　apex。　　External　appearances　suggest　that　both　the　place　of　origin　and　the　form　of　these　organs　were　predetermined　in　the　egg…cell　or　in　the　tuber。　　But　it　was　shown　experimentally　by　the　well…known　investigator　Knight　（Knight；　〃Selection　from　the　Physiological　and　Horticultural　Papers〃；　London；　1841。）　that　tubers　may　be　developed　on　the　aerial　stem　in　place　of　foliage　shoots。　　These　observations　were　considerably　extended　by　Vochting。　　（Vochting；　〃Ueber　die　Bildung　der　Knollen〃；　Cassel；　1887；　see　also　〃Bot。　Zeit。〃　1902；　87。）　　In　one　kind　of　potato；　germinating　tubers　were　induced　to　form　foliage　shoots　under　the　influence　of　a　higher　temperature；　at　a　lower　temperature　they　formed　tuber…bearing　shoots。　　Many　other　examples　of　the　conversion　of　foliage…shoots　into　runners　and　rhizomes；　or　vice　versa；　have　been　described　by　Goebel　and　others。　　As　in　the　asexual　reproduction　of　algae　quantitative　alteration　in　the　amount　of　moisture；　light；　temperature；　etc。　determines　whether　this　or　that　form　of　shoot　is　produced。　　If　the　primordia　of　these　organs　are　exposed　to　altered　conditions　of　nutrition　at　a　sufficiently　early　stage　a　complete　substitution　of　one　organ　for　another　is　effected。　　If　the　rudiment　has　reached　a　certain　stage　in　development　before　it　is　exposed　to　these　influences；　extraordinary　intermediate　forms　are　obtained；　bearing　the　characters　of　both　organs。

The　study　of　regeneration　following　injury　is　of　greater　importance　as　regards　the　problem　of　the　development　and　place　of　origin　of　organs。　　（Reference　may　be　made　to　the　full　summary　of　results　given　by　Goebel　in　his　〃Experimentelle　Morphologie〃；　Leipzig　and　Berlin；　1908；　Section　IV。）　　Only　in　relatively　very　rare　cases　is　there　a　complete　re…formation　of　the　injured　organ　itself；　as　e。g。　in　the　growing…apex。　　Much　more　commonly　injury　leads　to　the　development　of　complementary　formations；　it　may　be　the　rejuvenescence　of　a　hitherto　dormant　rudiment；　or　it　may　be　the　formation　of　such　ab　initio。　　In　all　organs；　stems；　roots；　leaves；　as　well　as　inflorescences；　this　kind　of　regeneration；　which　occurs　in　a　great　variety　of　ways　according　to　the　species；　may　be　observed　on　detached　pieces　of　the　plant。　　Cases　are　also　known；　such；　for　example；　as　the　leaves　of　many　plants　which　readily　form　roots　but　not　shoots；　where　a　complete　regeneration　does　not　occur。

The　widely　spread　power　of　reacting　to　wounding　affords　a　very　valuable　means　of　inducing　a　fresh　development　of　buds　and　roots　on　places　where　they　do　not　occur　in　normal　circumstances。　　Injury　creates　special　conditions；　but　little　is　known　as　yet　in　regard　to　alterations　directly　produced　in　this　way。　　Where　the　injury　consists　in　the　separation　of　an　organ　from　its　normal　connections；　the　factors　concerned　are　more　comprehensible。　　A　detached　leaf；　e。g。；　is　at　once　cut　off　from　a　supply　of　water　and　salts；　and　is　deprived　of　the　means　of　getting　rid　of　organic　substances　which　it　produces；　the　result　is　a　considerable　alteration　in　the　degree　of　concentration。　　No　experimental　investigation　on　these　lines　has　yet　been　made。　　Our　ignorance　has　often　led　to　the　view　that　we　are　dealing　with　a　force　whose　specific　quality　is　the　restitution　of　the　parts　lost　by　operation；　the　proof；　therefore；　that　in　certain　cases　a　similar　production　of　new　roots　or　buds　may　be　induced　without　previous　injury　and　simply　by　a　change　in　external　conditions　assumes　an　importance。　　（Klebs；　〃Willkurliche　Entwickelung〃；　page　100；　also；　〃Probleme　der　Entwickelung〃；　〃Biol。　Centralbl。〃　1904；　page　610。）

A　specially　striking　phenomenon　of　regeneration；　exhibited　also　by　uninjured　plants；　is　afforded　by　polarity；　which　was　discovered　by　Vochting。　　（See　the　classic　work　of　Vochting；　〃Ueber　Organbildung　im　Pflanzenreich〃；　I。　Bonn；　1888；　also　〃Bot。　Zeit。　1906；　p