darwin and modern science-第76章

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mpossible　to　speak　quite　positively；　owing　to　the　difficulty　of　discriminating　between　true　fossil　Ferns　and　the　Pteridosperms　which　so　closely　simulated　them。　　The　difficulty　especially　affects　the　question　of　the　position　of　Marattiaceous　Ferns　in　the　Palaeozoic　Floras。　　This　family；　now　so　restricted；　was　until　recently　believed　to　have　been　one　of　the　most　important　groups　of　Palaeozoic　plants；　especially　during　later　Carboniferous　and　Permian　times。　Evidence　both　from　anatomy　and　from　sporangial　characters　appeared　to　establish　this　conclusion。　　Of　late；　however；　doubts　have　arisen；　owing　to　the　discovery　that　some　supposed　members　of　the　Marattiaceae　bore　seeds；　and　that　a　form　of　fructification　previously　referred　to　that　family　（Crossotheca）　was　really　the　pollen…bearing　apparatus　of　a　Pteridosperm　（Lyginodendron）。　　The　question　presents　much　difficulty；　though　it　seems　certain　that　our　ideas　of　the　extent　of　the　family　in　Palaeozoic　times　will　have　to　be　restricted；　there　is　still　a　decided　balance　of　evidence　in　favour　of　the　view　that　a　considerable　body　of　Marattiaceous　Ferns　actually　existed。　　The　plants　in　question　were　of　large　size　（often　arborescent）　and　highly　organisedthey　represent；　in　fact；　one　of　the　highest　developments　of　the　Fern…stock；　rather　than　a　primitive　type　of　the　class。

There　was；　however；　in　the　Palaeozoic　period；　a　considerable　group　of　comparatively　simple　Ferns　（for　which　Arber　has　proposed　the　collective　name　Primofilices）；　the　best　known　of　these　are　referred　to　the　family　Botryopterideae；　consisting　of　plants　of　small　or　moderate　dimensions；　with；　on　the　whole；　a　simple　anatomical　structure；　in　certain　cases　actually　simpler　than　that　of　any　recent　Ferns。　　On　the　other　hand　the　sporangia　of　these　plants　were　usually　borne　on　special　fertile　fronds；　a　mark　of　rather　high　differentiation。　　This　group　goes　back　to　the　Devonian　and　includes　some　of　the　earliest　types　of　Fern　with　which　we　are　acquainted。　　It　is　probable　that　the　Primofilices　（though　not　the　particular　family　Botryopterideae）　represent　the　stock　from　which　the　various　families　of　modern　Ferns；　already　developed　in　the　Mesozoic　period；　may　have　sprung。

None　of　the　early　Ferns　show　any　clear　approach　to　other　classes　of　Vascular　Cryptogams；　so　far　as　the　fossil　record　affords　any　evidence；　Ferns　have　always　been　plants　with　relatively　large　and　usually　compound　leaves。　　There　is　no　indication　of　their　derivation　from　a　microphyllous　ancestry；　though；　as　we　shall　see；　there　is　some　slight　evidence　for　the　converse　hypothesis。　　Whatever　the　origin　of　the　Ferns　may　have　been　it　is　hidden　in　the　older　rocks。

It　has；　however；　been　held　that　certain　other　Cryptogamic　phyla　had　a　common　origin　with　the　Ferns。　　The　Equisetales　are　at　present　a　well…　defined　group；　even　in　the　rich　Palaeozoic　floras　the　habit；　anatomy　and　reproductive　characters　usually　render　the　members　of　this　class　unmistakable；　in　spite　of　the　great　development　and　stature　which　they　then　attained。　　It　is　interesting；　however；　to　find　that　in　the　oldest　known　representatives　of　the　Equisetales　the　leaves　were　highly　developed　and　dichotomously　divided；　thus　differing　greatly　from　the　mere　scale…leaves　of　the　recent　Horsetails；　or　even　from　the　simple　linear　leaves　of　the　later　Calamites。　　The　early　members　of　the　class；　in　their　forked　leaves；　and　in　anatomical　characters；　show　an　approximation　to　the　Sphenophyllales；　which　are　chiefly　represented　by　the　large　genus　Sphenophyllum；　ranging　through　the　Palaeozoic　from　the　Middle　Devonian　onwards。　　These　were　plants　with　rather　slender；　ribbed　stems；　bearing　whorls　of　wedge…shaped　or　deeply　forked　leaves；　six　being　the　typical　number　in　each　whorl。　　From　their　weak　habit　it　has　been　conjectured；　with　much　probability；　that　they　may　have　been　climbing　plants；　like　the　scrambling　Bedstraws　of　our　hedgerows。　　The　anatomy　of　the　stem　is　simple　and　root…like；　the　cones　are　remarkable　for　the　fact　that　each　scale　or　sporophyll　is　a　double　structure；　consisting　of　a　lower；　usually　sterile　lobe　and　one　or　more　upper　lobes　bearing　the　sporangia；　in　one　species　both　parts　of　the　sporophyll　were　fertile。　　Sphenophyllum　was　evidently　much　specialised；　the　only　other　known　genus　is　based　on　an　isolated　cone；　Cheirostrobus；　of　Lower　Carboniferous　age；　with　an　extraordinarily　complex　structure。　　In　this　genus　especially；　but　also　in　the　entire　group；　there　is　an　evident　relation　to　the　Equisetales；　hence　it　is　of　great　interest　that　Nathorst　has　described；　from　the　Devonian　of　Bear　Island　in　the　Arctic　regions；　a　new　genus　Pseudobornia；　consisting　of　large　plants；　remarkable　for　their　highly　compound　leaves　which；　when　found　detached；　were　taken　for　the　fronds　of　a　Fern。　　The　whorled　arrangement　of　the　leaves；　and　the　habit　of　the　plant；　suggest　affinities　either　with　the　Equisetales　or　the　Sphenophyllales；　Nathorst　makes　the　genus　the　type　of　a　new　class；　the　Pseudoborniales。　　（A。G。　Nathorst；　〃Zur　Oberdevonischen　Flora　der　Baren…Insel〃；　〃Kongl。　Svenska　Vetenskaps…Akademiens　Handlingar〃　Bd。　36；　No。　3；　Stockholm；　1902。）

The　available　data；　though　still　very　fragmentary；　certainly　suggest　that　both　Equisetales　and　Sphenophyllales　may　have　sprung　from　a　common　stock　having　certain　fern…like　characters。　　On　the　other　hand　the　Sphenophylls；　and　especially　the　peculiar　genus　Cheirostrobus；　have　in　their　anatomy　a　good　deal　in　common　with　the　Lycopods；　and　of　late　years　they　have　been　regarded　as　the　derivatives　of　a　stock　common　to　that　class　and　the　Equisetales。　　At　any　rate　the　characters　of　the　Sphenophyllales　and　of　the　new　group　Pseudoborniales　suggest　the　existence；　at　a　very　early　period；　of　a　synthetic　race　of　plants；　combining　the　characters　of　various　phyla　of　the　Vascular　Cryptogams。　　It　may　further　be　mentioned　that　the　Psilotaceae；　an　isolated　epiphytic　family　hitherto　referred　to　the　Lycopods；　have　been　regarded　by　several　recent　authors　as　the　last　survivors　of　the　Sphenophyllales；　which　they　resemble　both　in　their　anatomy　and　in　the　position　of　their　sporangia。

The　Lycopods；　so　far　as　their　early　history　is　known；　are　remarkable　rather　for　their　high　development　in　Palaeozoic　times　than　for　any　indications　of　a　more　primitive　ancestry。　　In　the　recent　Flora；　two　of　the　four　living　genera　（Excluding　Psilotaceae。）　（Selaginella　and　Isoetes）　have　spores　of　two　kinds；　while　the　other　two　（Lycopodium　and　Phylloglossum）　are　homosporous。　　Curiously　enough；　no　certain　instance　of　a　homosporous　Palaeozoic　Lycopod　has　yet　been　discovered；　though　well…preserved　fructifications　are　numerous。　　Wherever　the　facts　have　been　definitely　ascertained；　we　find　two　kinds　of　spore；　differentiated　quite　as　sharply　as　in　any　living　members　of　the　group。　　Some　of　the　Palaeozoic　Lycopods；　in　fact；　went　further；　and　produced　bodies　of　the　nature　of　seeds；　some　of　which　were　actually　regarded；　for　many　years；　as　the　seeds　of　Gymnosperms。　　This　specially　advanced　form　of　fructification　goes　back　at　least　as　far　as　the　Lower　Carboniferous；　while　the　oldest　known　genus　of　Lycopods；　Bothrodendron；　which　is　found　in　the　Devonian；　though　not　seed…bearing；　was　typically　heterosporous；　if　we　may　judge　from　the　Coal…measure　species。　　No　doubt　homosporous　Lycopods　existed；　but　the　great　prevalence　of　the　higher　mode　of　reproduction　in　days　which　to　us　appear　ancient；　shows　how　long　a　course　of　evolution　must　have　already　been　passed　through　before　the　oldest　known　members　of　the　group　came　into　being。　　The　other　characters　of　the　Palaeozoic　Lycopods　tell　the　same　tale；　most　of　them　attained　the　stature　of　trees；　with　a　corresponding　elaboration　of　anatomical　structure；　and　even　the　herbaceous　forms　show　no　special　simplicity。　　It　appears　from　recent　work　that　herbaceous　Lycopods；　indistinguishable　from　our　recent　Selaginellas；　already　existed　in　the　time　of　the　Coal…measures；　while　one　herbaceous　form　（Miadesmia）　is　known　to　have　borne　seeds。

The　utmost　that　can　be　said　for　primitiveness　of　character　in　Palaeozoic　Lycopods　is　that　the　anatomy　of　the　stem；　in　its　primary　ground…plan；　as　distinguished　from　its　secondary　growth；　was　simpler　than　that　of　most　Lycopodiums　and　Selaginellas　at　the　present　day。　　There　are　also　some　peculiarities　in　the　underg