darwin and modern science-第65节

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f　plants：　　i。　The　Origin　of　the　Angiosperms。　　ii。　The　Origin　of　the　Seed…plants。　　iii。　The　Origin　of　the　different　classes　of　the　Higher　Cryptogamia。

i。　　THE　ORIGIN　OF　THE　ANGIOSPERMS。

The　first　of　these　questions　has　long　been　the　great　crux　of　botanical　phylogeny；　and　until　quite　recently　no　light　had　been　thrown　upon　the　difficulty。　　The　Angiosperms　are　the　Flowering　Plants；　par　excellence；　and　form；　beyond　comparison；　the　dominant　sub…kingdom　in　the　flora　of　our　own　age；　including；　apart　from　a　few　Conifers　and　Ferns；　all　the　most　familiar　plants　of　our　fields　and　gardens；　and　practically　all　plants　of　service　to　man。　　All　recent　work　has　tended　to　separate　the　Angiosperms　more　widely　from　the　other　seed…plants　now　living；　the　Gymnosperms。　　Vast　as　is　the　range　of　organisation　presented　by　the　great　modern　sub…kingdom；　embracing　forms　adapted　to　every　environment；　there　is　yet　a　marked　uniformity　in　certain　points　of　structure；　as　in　the　development　of　the　embryo…sac　and　its　contents；　the　pollination　through　the　intervention　of　a　stigma；　the　strange　phenomenon　of　double　fertilisation　（One　sperm　fertilising　the　egg；　while　the　other　unites　with　the　embryo…sac　nucleus；　itself　the　product　of　a　nuclear　fusion；　to　give　rise　to　a　nutritive　tissue；　the　endosperm。）；　the　structure　of　the　stamens；　and　the　arrangement　of　the　parts　of　the　flower。　　All　these　points　are　common　to　Monocotyledons　and　Dicotyledons；　and　separate　the　Angiosperms　collectively　from　all　other　plants。

In　geological　history　the　Angiosperms　first　appear　in　the　Lower　Cretaceous；　and　by　Upper　Cretaceous　times　had　already　swamped　all　other　vegetation　and　seized　the　dominant　position　which　they　still　hold。　　Thus　they　are　isolated　structurally　from　the　rest　of　the　Vegetable　Kingdom；　while　historically　they　suddenly　appear；　almost　in　full　force；　and　apparently　without　intermediaries　with　other　groups。　　To　quote　Darwin's　vigorous　words：　　〃The　rapid　development；　as　far　as　we　can　judge；　of　all　the　higher　plants　within　recent　geological　times　is　an　abominable　mystery。〃　　（〃More　Letters　of　Charles　Darwin〃；　Vol。　II。　page　20；　letter　to　J。D。　Hooker；　1879。）　　A　couple　of　years　later　he　made　a　bold　suggestion　（which　he　only　called　an　〃idle　thought〃）　to　meet　this　difficulty。　　He　says：　〃I　have　been　so　astonished　at　the　apparently　sudden　coming　in　of　the　higher　phanerogams；　that　I　have　sometimes　fancied　that　development　might　have　slowly　gone　on　for　an　immense　period　in　some　isolated　continent　or　large　island；　perhaps　near　the　South　Pole。〃　　（Ibid；　page　26；　letter　to　Hooker；　1881。）　　This　idea　of　an　Angiospermous　invasion　from　some　lost　southern　land　has　sometimes　been　revived　since；　but　has　not；　so　far　as　the　writer　is　aware；　been　supported　by　evidence。　　Light　on　the　problem　has　come　from　a　different　direction。

The　immense　development　of　plants　with　the　habit　of　Cycads；　during　the　Mesozoic　Period　up　to　the　Lower　Cretaceous；　has　long　been　known。　　The　existing　Order　Cycadaceae　is　a　small　family；　with　9　genera　and　perhaps　100　species；　occurring　in　the　tropical　and　sub…tropical　zones　of　both　the　Old　and　New　World；　but　nowhere　forming　a　dominant　feature　in　the　vegetation。　　Some　few　attain　the　stature　of　small　trees；　while　in　the　majority　the　stem　is　short；　though　often　living　to　a　great　age。　　The　large　pinnate　or　rarely　bipinnate　leaves　give　the　Cycads　a　superficial　resemblance　in　habit　to　Palms。　　Recent　Cycads　are　dioecious；　throughout　the　family　the　male　fructification　is　in　the　form　of　a　cone；　each　scale　of　the　cone　representing　a　stamen；　and　bearing　on　its　lower　surface　numerous　pollen…　sacs；　grouped　in　sori　like　the　sporangia　of　Ferns。　　In　all　the　genera；　except　Cycas　itself；　the　female　fructifications　are　likewise　cones；　each　carpel　bearing　two　ovules　on　its　margin。　　In　Cycas；　however；　no　female　cone　is　produced；　but　the　leaf…like　carpels；　bearing　from　two　to　six　ovules　each；　are　borne　directly　on　the　main　stem　of　the　plant　in　rosettes　alternating　with　those　of　the　ordinary　leavesthe　most　primitive　arrangement　known　in　any　living　seed…plant。　　The　whole　Order　is　relatively　primitive；　as　shown　most　strikingly　in　its　cryptogamic　mode　of　fertilisation；　by　means　of　spermatozoids；　which　it　shares　with　the　maidenhair　tree　alone；　among　recent　seed…plants。

In　all　the　older　Mesozoic　rocks；　from　the　Trias　to　the　Lower　Cretaceous；　plants　of　the　Cycad　class　（Cycadophyta；　to　use　Nathorst's　comprehensive　name）　are　extraordinarily　abundant　in　all　parts　of　the　world；　in　fact　they　were　almost　as　prominent　in　the　flora　of　those　ages　as　the　Dicotyledons　are　in　that　of　our　own　day。　　In　habit　and　to　a　great　extent　in　anatomy；　the　Mesozoic　Cycadophyta　for　the　most　part　much　resemble　the　recent　Cycadaceae。　But；　strange　to　say；　it　is　only　in　the　rarest　cases　that　the　fructification　has　proved　to　be　of　the　simple　type　characteristic　of　the　recent　family；　the　vast　majority　of　the　abundant　fertile　specimens　yielded　by　the　Mesozoic　rocks　possess　a　type　of　reproductive　apparatus　far　more　elaborate　than　anything　known　in　Cycadaceae　or　other　Gymnosperms。　　The　predominant　Mesozoic　family；　characterised　by　this　advanced　reproductive　organisation；　is　known　as　the　Bennettiteae；　in　habit　these　plants　resembled　the　more　stunted　Cycads　of　the　recent　flora；　but　differed　from　them　in　the　presence　of　numerous　lateral　fructifications；　like　large　buds；　borne　on　the　stem　among　the　crowded　bases　of　the　leaves。　　The　organisation　of　these　fructifications　was　first　worked　out　on　European　specimens　by　Carruthers；　Solms…Laubach；　Lignier　and　others；　but　these　observers　had　only　more　or　less　ripe　fruits　to　deal　with；　the　complete　structure　of　the　flower　has　only　been　elucidated　within　the　last　few　years　by　the　researches　of　Wieland　on　the　magnificent　American　material；　derived　from　the　Upper　Jurassic　and　Lower　Cretaceous　beds　of　Maryland；　Dakota　and　Wyoming。　　（G。R。　Wieland；　〃American　Fossil　Cycads〃；　Carnegie　Institution；　Washington；　1906。）　　The　word　〃flower〃　is　used　deliberately；　for　reasons　which　will　be　apparent　from　the　following　brief　description；　based　on　Wieland's　observations。

The　fructification　is　attached　to　the　stem　by　a　thick　stalk；　which；　in　its　upper　part；　bears　a　large　number　of　spirally　arranged　bracts；　forming　collectively　a　kind　of　perianth　and　completely　enclosing　the　essential　organs　of　reproduction。　　The　latter　consist　of　a　whorl　of　stamens；　of　extremely　elaborate　structure；　surrounding　a　central　cone　or　receptacle　bearing　numerous　ovules。　　The　stamens　resemble　the　fertile　fronds　of　a　fern；　they　are　of　a　compound；　pinnate　form；　and　bear　very　large　numbers　of　pollen…sacs；　each　of　which　is　itself　a　compound　structure　consisting　of　a　number　of　compartments　in　which　the　pollen　was　formed。　　In　their　lower　part　the　stamens　are　fused　together　by　their　stalks；　like　the　〃monadelphous〃　stamens　of　a　mallow。　　The　numerous　ovules　borne　on　the　central　receptacle　are　stalked；　and　are　intermixed　with　sterile　scales；　the　latter　are　expanded　at　their　outer　ends；　which　are　united　to　form　a　kind　of　pericarp　or　ovary…wall；　only　interrupted　by　the　protruding　micropyles　of　the　ovules。　　There　is　thus　an　approach　to　the　closed　pistil　of　an　Angiosperm；　but　it　is　evident　that　the　ovules　received　the　pollen　directly。　　The　whole　fructification　is　of　large　size；　in　the　case　of　Cycadeoidea　dacotensis；　one　of　the　species　investigated　by　Wieland；　the　total　length；　in　the　bud　condition；　is　about　12　cm。；　half　of　which　belongs　to　the　peduncle。

The　general　arrangement　of　the　organs　is　manifestly　the　same　as　in　a　typical　Angiospermous　flower；　with　a　central　pistil；　a　surrounding　whorl　of　stamens　and　an　enveloping　perianth；　there　is；　as　we　have　seen；　some　approach　to　the　closed　ovary　of　an　Angiosperm；　another　point；　first　discovered　nearly　20　years　ago　by　Solms…Laubach　in　his　investigation　of　a　British　species；　is　that　the　seed　was　practically　〃exalbuminous；〃　its　cavity　being　filled　by　the　large；　dicotyledonous　embryo；　whereas　in　all　known　Gymnosperms　a　large　part　of　the　sac　is　occupied　by　a　nutritive　tissue；　the　prothallus　or　endosperm；　here　also　we　have　a　condition　only　met　with　elsewhere　among　the　higher　Flowering　Plants。

Taking　all　the　characters　into　account；　the　indications　of　affinity　between　the　Mesozoic　Cycadophyta　and　the　Angiosperms　appear　extremely　significant；　as　was　recognised　by　Wieland　when　he　first　discovered　the　hermaphrodite　nature　of　the　Bennettitean　flower。　　The　Angiosperm　with　which　he　specially　compared　the　fossil　type　was　the　Tulip　tree　（Liriodendron）　and　certainly　there　is　a　remarkable　analogy　with　the　Magnoliaceous　flowers；　and　with　those　of　related　orders　such　as　Ranunculaceae　and　the　Water…lilies。　　It　cannot；　of　course；　be　maintained　that　the　Bennettiteae；　or　any　other　Mesozoic　Cycadophyta　at　present　known；　were　on　the　direct　line　of　descent　of　the　Angiosperms；　for　there　are　some　important　points　of　difference；　as；　for　example；　in　the　great　complexity　of　the　stamens；　and　in　the　fact　that　the　ovary…wall　or　pericarp　was　not　formed　by　the　carpels　themselves；　but　by　the　accompanying　sterile　scale…leaves。　　Botanists；　since　the　discovery　of　the　bisexual　flowers　of　the　Bennettiteae；　have　expressed　different　views　as　to　the　nearness　of　their　relation　to　the　higher　Flowering　Plants；　but　the　points　of　agreement　are　so　many　tha