darwin and modern science-第75章
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
e 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 of the evidence will be found in the writer's article 〃On the present position of Palaeozoic Botany〃； 〃Progressus Rei Botanicae〃； 1907； page 139； and 〃Studies in Fossil Botany〃； Vol。 II。 （2nd edition） London； 1909。） At the same time； the anatomical structure； where it is open to investigation； confirms the suggestion given by the habit； and shows that these early seed…bearing plants had a real affinity with Ferns。 This conclusion received strong corroboration when Kidston； in 1905； discovered the male organs of Lyginodendron； and showed that they were identical with a fructification of the genus Crossotheca； hitherto regarded as belonging to Marattiaceous Ferns。 （Kidston； 〃On the Microsporangia of the Pteridospermeae； etc。〃 〃Phil。 Trans。 Royal Soc。〃 Vol。 198； B。 1906。）
The general conclusion which follows from the various observations alluded to； is that in Palaeozoic times there was a great body of plants （including； as it appears； a large majority of the fossils previously regarded as Ferns） which had attained the rank of Spermophyta； bearing seeds of a Cycadean type on fronds scarcely differing from the vegetative foliage； and in other respects； namely anatomy； habit and the structure of the pollen…bearing organs； retaining many of the characters of Ferns。 From this extensive class of plants； to which the name Pteridospermeae has been given； it can scarcely be doubted that the abundant Cycadophyta； of the succeeding Mesozoic period； were derived。 This conclusion is of far… reaching significance； for we have already found reason to think that the Angiosperms themselves sprang； in later times； from the Cycadophytic stock； it thus appears that the Fern…phylum； taken in a broad sense； ultimately represents the source from which the main line of descent of the Phanerogams took its rise。
It must further be borne in mind that in the Palaeozoic period there existed another group of seed…bearing plants； the Cordaiteae； far more advanced than the Pteridospermeae； and in many respects approaching the Coniferae； which themselves begin to appear in the latest Palaeozoic rocks。 The Cordaiteae； while wholly different in habit from the contemporary fern… like Seed…plants； show unmistakable signs of a common origin with them。 Not only is there a whole series of forms connecting the anatomical structure of the Cordaiteae with that of the Lyginodendreae among Pteridosperms； but a still more important point is that the seeds of the Cordaiteae； which have long been known； are of the same Cycadean type as those of the Pteridosperms； so that it is not always possible； as yet； to discriminate between the seeds of the two groups。 These facts indicate that the same fern…like stock which gave rise to the Cycadophyta and through them； as appears probable； to the Angiosperms； was also the source of the Cordaiteae； which in their turn show manifest affinity with some at least of the Coniferae。 Unless the latter are an artificial group； a view which does not commend itself to the writer； it would appear probable that the Gymnosperms generally； as well as the Angiosperms； were derived from an ancient race of Cryptogams； most nearly related to the Ferns。 （Some botanists； however； believe that the Coniferae； or some of them； are probably more nearly related to the Lycopods。 See Seward and Ford； 〃The Araucarieae； Recent and Extinct〃； 〃Phil。 Trans。 Royal Soc。〃 Vol。 198 B。 1906。）
It may be mentioned here that the small gymnospermous group Gnetales （including the extraordinary West African plant Welwitschia） which were formerly regarded by some authorities as akin to the Equisetales； have recently been referred； on better grounds； to a common origin with the Angiosperms； from the Mesozoic Cycadophyta。
The tendency； therefore； of modern work on the palaeontological record of the Seed…plants has been to exalt the importance of the Fern…phylum； which； on present evidence； appears to be that from which the great majority； possibly the whole； of the Spermophyta have been derived。
One word of caution； however； is necessary。 The Seed…plants are of enormous antiquity； both the Pteridosperms and the more highly organised family Cordaiteae； go back as far in geological history （namely to the Devonian） as the Ferns themselves or any other Vascular Cryptogams。 It must therefore be understood that in speaking of the derivation of the Spermophyta from the Fern…phylum； we refer to that phylum at a very early stage； probably earlier than the most ancient period to which our record of land…plants extends。 The affinity between the oldest Seed…plants and the Ferns； in the widest sense； seems established； but the common stock from which they actually arose is still unknown； though no doubt nearer to the Ferns than to any other group； it must have differed widely from the Ferns as we now know them； or perhaps even from any which the fossil record has yet revealed to us。
iii。 THE ORIGIN OF THE HIGHER CRYPTOGAMIA。
The Sub…kingdom of the higher Spore…plants； the Cryptogamia possessing a vascular system； was more prominent in early geological periods than at present。 It is true that the dominance of the Pteridophyta in Palaeozoic times has been much exaggerated owing to the assumption that everything which looked like a Fern really was a Fern。 But； allowing for the fact； now established； that most of the Palaeozoic fern…like plants were already Spermophyta； there remains a vast mass of Cryptogamic forms of that period； and the familiar statement that they formed the main constituent of the Coal…forests still holds good。 The three classes； Ferns （Filicales）； Horsetails （Equisetales） and Club…mosses （Lycopodiales）； under which we now group the Vascular Cryptogams； all extend back in geological history as far as we have any record of the flora of the land； in the Palaeozoic； however； a fourth class； the Sphenophyllales； was present。
As regards the early history of the Ferns； which are of special interest from their relation to the Seed…plants； it is impossible to speak quite positively； owing to the difficulty of discriminating between true fossil F
