darwin and modern science-第66节

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arness　of　their　relation　to　the　higher　Flowering　Plants；　but　the　points　of　agreement　are　so　many　that　it　is　difficult　to　resist　the　conviction　that　a　real　relation　exists；　and　that　the　ancestry　of　the　Angiosperms；　so　long　shrouded　in　complete　obscurity；　is　to　be　sought　among　the　great　plexus　of　Cycad…like　plants　which　dominated　the　flora　of　the　world　in　Mesozoic　times。　　（On　this　subject　see；　in　addition　to　Wieland's　great　work　above　cited；　F。W。　Oliver；　〃Pteridosperms　and　Angiosperms〃；　〃New　Phytologist〃；　Vol。　V。　1906；　D。H。　Scott；　〃The　Flowering　Plants　of　the　Mesozoic　Age　in　the　Light　of　Recent　Discoveries〃；　〃Journal　R。　Microscop。　Soc。〃　1907；　and　especially　E。A。N。　Arber　and　J。　Parkin；　〃On　the　Origin　of　Angiosperms〃；　〃Journal　Linn。　Soc。〃　（Bot。）　Vol。　XXXVIII。　page　29；　1907。）

The　great　complexity　of　the　Bennettitean　flower；　the　earliest　known　fructification　to　which　the　word　〃flower〃　can　be　applied　without　forcing　the　sense；　renders　it　probable；　as　Wieland　and　others　have　pointed　out；　that　the　evolution　of　the　flower　in　Angiosperms　has　consisted　essentially　in　a　process　of　reduction；　and　that　the　simplest　forms　of　flower　are　not　to　be　regarded　as　the　most　primitive。　　The　older　morphologists　generally　took　the　view　that　such　simple　flowers　were　to　be　explained　as　reductions　from　a　more　perfect　type；　and　this　opinion；　though　abandoned　by　many　later　writers；　appears　likely　to　be　true　when　we　consider　the　elaboration　of　floral　structure　attained　among　the　Mesozoic　Cycadophyta；　which　preceded　the　Angiosperms　in　evolution。

If；　as　now　seems　probable；　the　Angiosperms　were　derived　from　ancestors　allied　to　the　Cycads；　it　would　naturally　follow　that　the　Dicotyledons　were　first　evolved；　for　their　structure　has　most　in　common　with　that　of　the　Cycadophyta。　　We　should　then　have　to　regard　the　Monocotyledons　as　a　side…　line；　diverging　probably　at　a　very　early　stage　from　the　main　dicotyledonous　stock；　a　view　which　many　botanists　have　maintained；　of　late；　on　other　grounds。　　（See　especially　Ethel　Sargant；　〃The　Reconstruction　of　a　Race　of　Primitive　Angiosperms〃；　〃Annals　of　Botany〃；　Vol。　XXII。　page　121；　1908。）　　So　far；　however；　as　the　palaeontological　record　shows；　the　Monocotyledons　were　little　if　at　all　later　in　their　appearance　than　the　Dicotyledons；　though　always　subordinate　in　numbers。　　The　typical　and　beautifully　preserved　Palm…　wood　from　Cretaceous　rocks　is　striking　evidence　of　the　early　evolution　of　a　characteristic　monocotyledonous　family。　　It　must　be　admitted　that　the　whole　question　of　the　evolution　of　Monocotyledons　remains　to　be　solved。

Accepting；　provisionally；　the　theory　of　the　cycadophytic　origin　of　Angiosperms；　it　is　interesting　to　see　to　what　further　conclusions　we　are　led。　　The　Bennettiteae；　at　any　rate；　were　still　at　the　gymnospermous　level　as　regards　their　pollination；　for　the　exposed　micropyles　of　the　ovules　were　in　a　position　to　receive　the　pollen　directly；　without　the　intervention　of　a　stigma。　　It　is　thus　indicated　that　the　Angiosperms　sprang　from　a　gymnospermous　source；　and　that　the　two　great　phyla　of　Seed…plants　have　not　been　distinct　from　the　first；　though　no　doubt　the　great　majority　of　known　Gymnosperms；　especially　the　Coniferae；　represent　branch…lines　of　their　own。

The　stamens　of　the　Bennettiteae　are　arranged　precisely　as　in　an　angiospermous　flower；　but　in　form　and　structure　they　are　like　the　fertile　fronds　of　a　Fern；　in　fact　the　compound　pollen…sacs；　or　synangia　as　they　are　technically　called；　almost　exactly　agree　with　the　spore…sacs　of　a　particular　family　of　Fernsthe　Marattiaceae；　a　limited　group；　now　mainly　tropical；　which　was　probably　more　prominent　in　the　later　Palaeozoic　times　than　at　present。　　The　scaly　hairs；　or　ramenta；　which　clothe　every　part　of　the　plant；　are　also　like　those　of　Ferns。

It　is　not　likely　that　the　characters　in　which　the　Bennettiteae　resemble　the　Ferns　came　to　them　directly　from　ancestors　belonging　to　that　class；　an　extensive　group　of　Seed…plants；　the　Pteridospermeae；　existed　in　Palaeozoic　times　and　bear　evident　marks　of　affinity　with　the　Fern　phylum。　　The　fern…　like　characters　so　remarkably　persistent　in　the　highly　organised　Cycadophyta　of　the　Mesozoic　were　in　all　likelihood　derived　through　the　Pteridosperms；　plants　which　show　an　unmistakable　approach　to　the　cycadophytic　type。

The　family　Bennettiteae　thus　presents　an　extraordinary　association　of　characters；　exhibiting；　side　by　side；　features　which　belong　to　the　Angiosperms；　the　Gymnosperms　and　the　Ferns。

ii。　　ORIGIN　OF　SEED…PLANTS。

The　general　relation　of　the　gymnospermous　Seed…plants　to　the　Higher　Cryptogamia　was　cleared　up；　independently　of　fossil　evidence；　by　the　brilliant　researches　of　Hofmeister；　dating　from　the　middle　of　the　past　century。　　（W。　Hofmeister；　〃On　the　Germination；　Development　and　Fructification　of　the　Higher　Cryptogamia〃；　Ray　Society；　London；　1862。　　The　original　German　treatise　appeared　in　1851。）　　He　showed　that　〃the　embryo…sac　of　the　Coniferae　may　be　looked　upon　as　a　spore　remaining　enclosed　in　its　sporangium；　the　prothallium　which　it　forms　does　not　come　to　the　light。〃　　（Ibid。　page　438。）　　He　thus　determined　the　homologies　on　the　female　side。　　Recognising；　as　some　previous　observers　had　already　done；　that　the　microspores　of　those　Cryptogams　in　which　two　kinds　of　spore　are　developed；　are　equivalent　to　the　pollen…grains　of　the　higher　plants；　he　further　pointed　out　that　fertilisation　〃in　the　Rhizocarpeae　and　Selaginellae　takes　place　by　free　spermatozoa；　and　in　the　Coniferae　by　a　pollen…tube；　in　the　interior　of　which　spermatozoa　are　probably　formed〃a　remarkable　instance　of　prescience；　for　though　spermatozoids　have　not　been　found　in　the　Conifers　proper；　they　were　demonstrated　in　the　allied　groups　Cycadaceae　and　Ginkgo；　in　1896；　by　the　Japanese　botanists　Ikeno　and　Hirase。　　A　new　link　was　thus　established　between　the　Gymnosperms　and　the　Cryptogams。

It　remained　uncertain；　however；　from　which　line　of　Cryptogams　the　gymnospermous　Seed…plants　had　sprung。　　The　great　point　of　morphological　comparison　was　the　presence　of　two　kinds　of　spore；　and　this　was　known　to　occur　in　the　recent　Lycopods　and　Water…ferns　（Rhizocarpeae）　and　was　also　found　in　fossil　representatives　of　the　third　phylum；　that　of　the　Horsetails。　　As　a　matter　of　fact　all　the　three　great　Cryptogamic　classes　have　found　champions　to　maintain　their　claim　to　the　ancestry　of　the　Seed…　plants；　and　in　every　case　fossil　evidence　was　called　in。　　For　a　long　time　the　Lycopods　were　the　favourites；　while　the　Ferns　found　the　least　support。　　The　writer　remembers；　however；　in　the　year　1881；　hearing　the　late　Prof。　Sachs　maintain；　in　a　lecture　to　his　class；　that　the　descent　of　the　Cycads　could　be　traced；　not　merely　from　Ferns；　but　from　a　definite　family　of　Ferns；　the　Marattiaceae；　a　view　which；　though　in　a　somewhat　crude　form；　anticipated　more　modern　ideas。

Williamson　appears　to　have　been　the　first　to　recognise　the　presence；　in　the　Carboniferous　flora；　of　plants　combining　the　characters　of　Ferns　and　Cycads。　　（See　especially　his　〃Organisation　of　the　Fossil　Plants　of　the　Coal…Measures〃；　Part　XIII。　〃Phil。　Trans。　Royal　Soc。〃　1887　B。　page　299。）　　This　conclusion　was　first　reached　in　the　case　of　the　genera　Heterangium　and　Lyginodendron；　plants；　which　with　a　wholly　fern…like　habit；　were　found　to　unite　an　anatomical　structure　holding　the　balance　between　that　of　Ferns　and　Cycads；　Heterangium　inclining　more　to　the　former　and　Lyginodendron　to　the　latter。　　Later　researches　placed　Williamson's　original　suggestion　on　a　firmer　basis；　and　clearly　proved　the　intermediate　nature　of　these　genera；　and　of　a　number　of　others；　so　far　as　their　vegetative　organs　were　concerned。　　This　stage　in　our　knowledge　was　marked　by　the　institution　of　the　class　Cycadofilices　by　Potonie　in　1897。

Nothing；　however；　was　known　of　the　organs　of　reproduction　of　the　Cycadofilices；　until　F。W。　Oliver；　in　1903；　identified　a　fossil　seed；　Lagenostoma　Lomaxi；　as　belonging　to　Lyginodendron；　the　identification　depending；　in　the　first　instance；　on　the　recognition　of　an　identical　form　of　gland；　of　very　characteristic　structure；　on　the　vegetative　organs　of　Lyginodendron　and　on　the　cupule　enveloping　the　seed。　　This　evidence　was　supported　by　the　discovery　of　a　close　anatomical　agreement　in　other　respects；　as　well　as　by　constant　association　between　the　seed　and　the　plant。　　（F。W。　Oliver　and　D。H。　Scott；　〃On　the　Structure　of　the　Palaeozoic　Seed；　Lagenostoma　Lomaxi；　etc。〃　〃Phil。　Trans。　Royal　Soc。〃　Vol。　197　B。　1904。）　　The　structure　of　the　seed　of　Lyginodendron；　proved　to　be　of　the　same　general　type　as　that　of　the　Cycads；　as　shown　especially　by　the　presence　of　a　pollen…chamber　or　special　cavity　for　the　reception　of　the　pollen…grains；　an　organ　only　known　in　the　Cycads　and　Ginkgo　among　recent　plants。

Within　a　few　months　after　the　discovery　of　the　seed　of　Lyginodendron；　Kidston　found　the　large；　nut…like　seed　of　a　Neuropteris；　another　fern…like　Carboniferous　plant；　in　actual　connection　with　the　pinnules　of　the　frond；　and　since　then　seeds　have　been　observed　on　the　frond　in　species　of　Aneimites　and　Pecopteris；　and　a　vast　body　of　evidence；　direct　or　indirect；　has　accumulated；　showing　that　a　large　proportion　of　the　Palaeozoic　plants　formerly　classed　as　Ferns　were　in　reality　reproduced　by　seeds　of　the　same　type　as　those　of　recent　Cycadaceae。　　（A　summary