In this article we will discuss about the four orders under which primofilicidae has been classified. The orders are:- 1. Protopteridales 2. Coenopteridales 3. Cladoxylales 4. Archaeopteridales.
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Order # 1. Protopteridales:
Protopteridium (Fig. 595A & B) from the Middle and Upper Devonian of Eastern Canada, Scotland, Belgium, Bohemia and China almost approaches the Psilophytales in simplicity with fern-like vegetative characters. The stem is advanced in having a. sympodially blanched axis with the branches divided dichotomously. Some of the ultimate divisions are flattened into leaf-like appendages.
There are six to eight species under the genus. In P. hostimense (Fig. 595B) the ultimate branches are pinnate and bear fern-like lamellae near the tips with tissue formed between pinnate segments .These ultimate branches may even be circiniatelycoiled.
Oval sporangia are borne at the tips of some of the smallest subdivisions and on one side of such a sporangium there is a specialised band of cells along which, probably, it dehisced.
The steles of the Protopteridales are of the actinostelic type. The tips of the arms show patches of protoxylem mixed with parenchyma. There are a number of other genera, viz., Arachnoxylon, Reimanrtia, Iridopteris, Aneurophyton, Eospermatopteris (all from Middle and Upper Devonian) all of which show complicated, lobed xylems (actinostele). Iridopteris stem (Fig. 595C) is very unlike the Psilophytean type.
It had a five-lobed xylem, gave out leaf traces and had two parenchymatous patches on each arm. Eospermatopteris developed a tree some six metres high, a metre in diameter at base and bore sporangia which looked like seeds. It is often thought to be the same as Aneurophyton. Rhacophyton, formerly supposed to be a Coenopterid with phyllophores (c.f., below) is now supposed to belong to this group.
It is interesting that Rhacophyton shows the habit of Trimerophyton.
There is no evidence of heterospory in any of the Protopteridales. The stem Character may be linked with the Coenopteridales.
Order # 2. Coenopteridales:
Coenopteridales is a large assemblage of fern-like plants mainly from the late Palaeozoic (Permocarboniferous) sometimes extending to Devonian. It is an artificial assemblage which may be divided into the families Stauropteridaceae, Zygopterida- ceae, Botryopteridaceae and Anachoropteridaceae. Some authorities favour dividing the group into two orders the Phyllophorales and the Inversicatenales.
The former (Stauropteridaceae and Zygopteridaceae) includes those in which the main rachis of the large compound leaves is a phyllophore which is somewhat intermediate between a stem and a rachis. The branch raches are borne on the phyllophore not in one plane but on different planes like branches on a stem axis.
In the Inversicatenales (Botryopteridaceae and Anachoropteridaceae) the main leaf rachis is normal with branches in one plane only.
Phyllophorales:
Family 1. Stauropteridaceae:
Stauropteris (Fig. 596A) is abundantly represented in the Lower and the Upper Carboniferous. The plant is bush-like with highly branches phyllophores which bear alternate branch pairs in different planes and each branches again branches similarly.
At each branching there are two scale-like structures called aphlebiae. The ultimate ramifications of S. oldhami do not bear lamellae but each end in a spherical sporangium with multilayered wall.
The sporangium (Fig. 596B) shows a stomium on top through which the spores escaped. This appears to be a Psilophytean character and this species seems to be homosporous. But S. iurntislandica (Lower Carboniferous) has definitely been found to be heterosporous. There is no annulus but the stomium for spore escapement is present.
Its megasporangium, called Bensonites, is flask-shaped and is about 13 mm long. Its lower part is sterile and there are two megaspores near the tip. The stem of Stauropteris shows a four-lobed xylem with phloem between the lobes.
The lobes sometimes separated from one another forming four separate xylem patches. Stauropteris, with its heterospory, is one of the most advanced among the Primofilices and may be on the main line of the evolution of the Pteridosperms.
Family 2. Zygopteridaceae:
Some 17 form genera with prominent phyllophores and occurring from the Middle Devonian to the Permian are included within the Zygopteridaceae. The type genus is Zygopteris while an important leaf genus is Etapteris (Fig. 596G) whose stem is probably Botrychioxylon. Other important related form genera are Asteropteris, Ankyropteris, etc.
The type genus Zygopteris and some other genera probably had stems which were erect or prostrate or trailing with a more or less complicated central cylinder which may be termed as actinostelic although the central core sometimes contained parenchyma mixed with tracheides. The protoxylem groups were at the tips of the lobes. The stellate primary xylem was encircled by radially arranged secondary wood elements.
The most important characteristic of the group was the elaborately branched leaf and the distinctive vascular organisation in its petiole. The main axis of the frond is the phyllophore which was sometimes 2 cm in diameter and on which the secondary raches were in two to four rows in a vertical plane at right angles to the phyllophore axis.
The pinnae of the second order were again turned at right angles to the axis on which they were borne. In Etapteris (Fig. 596G) the fronds were very large and the secondary and tertiary pinnules were on the same plane. The phyllophore had an H-shaped bundle (Fig. 597A), the uprights of the H being sometimes carved inwards.
There were protoxylem patches on the tips of each of the four extremities of the H each which, again, gave rise to a leaf trace.
The traces from the extremities of each vertical bar joined to form a strap-like plate (shown left on the figure) which again divided higher up to form two C-shaped strands one of which passed to each branch. The appearance is so characteristic that the Zygoptsrids may be identified only if the phyllophore bundles are known.
Fertile pinnules were mixed with sterile ones. The sporangia of Etapteris lacattei (Fig. 597B) were borne in pedicellate tufts along branch raches which lacked expanded blades. The sporangia were ellipsoid, about 2.5 mm long and borne in tufts of three
to eight. The wall was multi-layered. There was an annulus of thickened cells extending from base to apex on both sides constituting a vertical annulus of unique type. The spores were homosporous and resembled those of ordinary ferns.
Other types of fructifications have also been attributed to the Zygopteridaceae. One is Corynepteris in which the sporangia were sessile, large, ovate and five or six of them were grouped together round a central point to form something like a synangium (Fig. 597D). These synangia were arranged on a stalk (Fig. 597C).
The annulus extended over the point of contact of the sporangia and then over the apices so that dehiscence was probably around the edge of the contact. Suggestions have been made that this shows a connection between Zygopteridaceae and Marattiaceae.
Inversicatenales:
Family 3. Botryopteridaceae:
The Botryopteridaceae, known principally from the Permocarboniferous, has the simpler construction of the Inversicatenales although the more complex Zygopteridaceae is actually more primitive in fossil records. There was no phyllophore and the stem anatomy was much less complex.
Botryopteris (Fig. 598A) is the principal genus with some 20 species spread over European and American Permocarboniferous, specially in coal balls. The stem was slender, only a few mm in diameter, was much branched and bore spirally arranged fronds sometimes showing 2/5 phyllotaxy. Fronds forked at least once and the pinnae were borne on the final branches. In the stem a broad homogeneous cortex surrounded the stele without any endodermis.
The stele was protostelic with a layer of phloem surrounding a cylindrical mass of xylem. The xylem was mesarch with 2 or more protoxylem points some distance in from the surface. In the rachis the branch traces were curved with three arms resembling a curved E (shown in Fig. 598A).
The sporangia were oval or pyriform, short-stalked and borne in clusters (sometimes even a thousand sporangia in a cluster) directly on the branches of the fertile rachis (Fig. 598B). The annulus was plate-like on one side of sporangium somewhat like Osmunda.
The clusters look like sori but probably they were more akin to the pedicellate sporangia of the Zygopterids. There was a large number of spores of the same size (homosporous).
Grammatopteris from the lower Permian of France may be placed within the Botryo- pteridaceae because of the simple stele and the absence of phyllopbores. It had a slender protostelic stem inside a cortex showing leaf traces and a thick armour of leaf bases and adventitious roots.
Although unlike Osrtiundaceae, a relationship of the Zygopteridaceae with the Osmundaceae through Grammatopteris has been suggested by Sahni on a close study of the anatomy of the latter.
Family 4. Anachoropteridaceae:
Anachoropteridaceae, also from the Permocarboniferous, is another Inversicatenale family. Several genera, viz., Anachoropteris, Tubicaulis, Gyropteris, are included here. Tubicaulis shows a stem about 1 cm in diameter. The central xylem was roughly terete and mixed with parenchyma. The cortex of the stem and the petiole contained air spaces.
The petiole and rachis of both Tubicaulis and Anachoropteris had conspicuous C-shaped bundles, the convex side of which was turned towards the stem (reverse of modern ferns) and the two or more protoxylem groups were on the convex side.
Sporangia were borne in tetrads on the apices and along the margins of flattened leaf-like pinnules at the ends of veins (Fig. 598C). Each tetrad was surrounded by a cup forming something like a synangium. The sporangial wall seems to be 1-layered and there was no individual annulus although the whole synangium opened by spreading apart of the sporangia and apical ruptures.
There was a large number of small spores of the same size (homospoious).
The fertile pinnule of Anachoropteris seems to be the nearest approach to living ferns, specially Marattiales.
Order # 3. Cladoxylales:
Cladoxylales is represented by the type genus Cladoxylon with about a dozen species from Middle Devonian to Lower Carboniferous. The position of the group has been greatly disputed. Scott placed it within the Pteridosperms for its polystelic secondary wood.
Pichi-Sermolli is of opinion that Protopteridium, Stauropteris, Archaeopteris and Cladoxylon do not belong to the Filicopsida but to a separate group (Progym- nosperms) which is on the line of the evolution of the Pteridosperms. On the other hand, its primary xylem and leaf traces show a link to the Zygopterids while the fertile leaves are Psilophytean. It is better left as another problematic group.
The stem of Cladoxylon (Fig. 599A), up to 5 cm in diameter, was dichotomously branched and spirally bore numerous small (rarely over 2 cm) leaves which were dichotomously dissected in one plane and showed heterophylly (Fig. 599B & C). Fertile leaves (Fig. 599D) were borne on some upper branches. These were dichotomously dissected and bore at the tips of the dissections small sporangia. They were homosporous.
A t.s. of the stem (Fig. 599E) shows a broad homogeneous cortex, the central area of which was polystelic. The steles were many and narrow, often sinuous, and their number increased towards the base. They contained scalariform and pitted tracheides and each stele was surrounded by a secondary xylem.
Pseudosporochnus, another Middle Devonian fossil discovered in a bad state of preservation from New York, Scotland, Norway and Australia, was considered to be a member of the Psilophytales. It had a tree-like habit with a bulbous base, a 2-3 metres high trunk and a bushy crown of branches dichotomously branching once or twice.
It is not known if there was any secondary growth as the anatomy has not been preserved. Some of the ultimate tips of the branches probably served as leaves while other tips swelled into sporangia containing minute spores 4 to 7µ in diameter. Leclerq and Blanks (1962), after studying its anatomy, consider that this specimen does not belong to the Psilophytales at all but to the Cladoxylales.
Order # 4. Archaeopteridales:
Archaecrpteridales, with the type genus Archaeopteris represented by more than 15 species from the Upper Devonian is another problematic group which vacillates between Ferns and Pteridosperms.
Archaeopteris (Fig. 600) was so long known only from sterile and fertile fronds. The fronds were rather large, up to 1 metre in length and bore pinnae on two sides. Some of the fronds were sterile, others bore fertile pinnae either at the middle (Fig. 600A & D) or at the end or at both ends.
The sterile pinnae were wedge-shaped and dichotomously veined. Carluccio, Hueber and Banks (1966) have shown that the fronds had a radial anatomy.
The fertile pinnae bore two rows of club-shaped sporangia (about 2 mm long) in place of pinnules. Arnold (1939) has shown that the sporangia in A. latifolia were of two types (Fig. 600B). A few stouter ones contained only 8 to 16 megaspores about 0.3 mm in diameter.
Others were narrower and contained some 100 microspores 1/10 the size. Heterospory was, therefore, definite. Beck (1960, 1962, and 1966) has subsequently made a spectacular discovery. He has found Archaeopteris fronds attached to a stem formerly identified as Callixylon.
Callixylon had so long been known as a very abundant Devonian stem genus belonging to the Pityaceae of Gordiatales of Gymoosperms which is contemporary with Archaeopteris.
It does not mean that all Callixylon stem specimens have become synonymous with Archaeopteris but simply that this Devonian heterosporous fern genus Archaeopteris had a stem like Callixylon with a distinct pith and abundant secondary wood having bordered pitted tracheides.
It may now be said after Beck that Archaeopteris was a tall tree (Fig. 600C & D). The longest piece of wood discovered so far is 28 ft. long and 5 ft. in diameter at base.
It has, therefore, been argued that the Pteridosperms and other Gymnosperms did not arise from the general group of ferns but from a group of plants combining characters of ferns and Psilophytales, having secondary wood and showing heterospory. A name Progymnosperms has been suggested for this group. Stauropteris, Cladoxylon and Archaeopteris may be members of this group.
Beck has proposed the class name Progymnospermopsida for this taxon but the members come from assorted orders and have not yet been shown to form a coherent taxon. Delevoryas (1969) suggests this group might have arisen out of the Psilophytale group Trimerophyton (Trimerophytina or even Trimerophytopsida) and it may even be possible that the seed habit developed even before the evolution of leaves.
Eusporangidae:
Living ferns (with their fossil allies) in which the sporangium initial is not a single cell but a group of cells resulting in a sporangium with a massive multicellular wall containing a large number of spores used to be placed within a subclass Eusporangidae. The antheridia in these are sunk and also contain a large number of sperms. The first division of the zygote is transverse.
Two orders:
(1) Ophioglossales
(2) Maratti- ales were included within this subclass.
But, it is now generally realised that these two orders are not related and should not be placed within the same subclass.
Some modern authors (c.f. Bierhorst classification) are not even in favour of using the term Filicopsida as done here but are in favour of independent parallel subclasses Primofili- copsida, Ophioglossopsida, Marattiopsida and Filicopsida and even more subclasses. It is, therefore, at least reasonable to break up the subclass into two subclasses, Ophioglossidae and Marattidae.