Classification of Psilophytopsida | Plants

In this article we will discuss about the classification of psilophytopsida. 

The Psilophytopsida comprise of the fossilised ancient Silurian-Devonian order Psilophytales. The discovery of the different genera from such wide places as Canada, Scotland, Europe and Australia shows that these plants were of world-wide distribution in those days.

The Psilophytopsida and the living Psilotopsida have been included within a single class Psilopsida (or a division Psilophyta) by several authors. But the recent trend is to consider them as separate classes for reasons (Psilotopsida). Previously all the known genera within the class used to be placed within a single order Psilophytales.

The better known genera of this order used to be placed in five families:

1. Rhyniaceae,

2. Zosterophyllaceae,

3. Psilophytaceae,

4. Asteroxylaceae and

5. Trimerophytaceae.

The two metres high, tree-like Middle Devonian genus Pseudosporochnus, formerly supposed to be a member of the Psilophytales is now consi­dered to be a member of the ancient fossil fern taxon Cladoxylales. Moreover, recent investigations have completely changed our idea of the Psilophytales.

Of the principal genera, our original idea of the important genera Psilophyton and Asteroxylon has been shown to be completely wrong.

Psilophyton as constructed by Dawson has been shown to be a mixture of three plants so that some even doubt whether the genus can be retained at all and Asteroxylon proper has been proved to be a Lycopod.

Thus, only four families may now be recognised under Psilophytopsida:

1. Rhyniaceae

2. Psilophytaceae (in a restricted and revised sense)

3. Trimerophytaceae 

4. Zosterophyllaceae.

The so-called Asteroxylon elberfeldense should also belong to the Psilophytales but it needs a new name since the original Asteroxylon (A. mackiei) has now been found to be altogether different and has now been transferred to the Lycopsida.

The confusion about the genus Psilophyton has led some eminent Pteridologists to discard the class name Psilophytopsida. Again because of the fact that this taxon represents different tendencies of evolution leading to all the higher plant groups some of them want to raise this taxon into one or more taxa of a status higher than a class, i.e., division or subdivision.

Thus, Banks (1967) proposed three subdivisions:

(1) Rhyniophytina

(2) Zostryphyllophytina 

(3) Trimerophytina directly under the Division Trachaeophyta.

But, Bierhost (1971) has suggested two classes:

(1) Rhyniopsida (including Trimerophytaceae) 

(2) Zosterophyllopsida directly under the division Trachaeophyta.

Whatever may be the nomenclature of the taxon status of the four families recognised here, they are not necessarily closely related and probably represent an arti­ficial association of three parallel lines of evolution evolved in the earliest history of the land plants as suggested by Delevoryas (1969).

Of the four families

(1) Rhyniaceae 

(2) Psilophytaceae represent a plexus with terminal sporangia while

(3) Trimero­phytaceae is also with terminal sporangia but associated with more bushy, radial branching.

The two may represent two different trends of evolution as suggested by Delevoryas but may also be two tendencies of the same terminal sporangiate plexus as considered by Mehra and, apparently, also by Bierhorst (1971). The fourth family (4) Zosterophyllaceae, no doubt, represents a lateral sporangiate plexus out of which the Lycopsids arose.

Considering all these, this class is divided here into two orders:

(I) Psilophytales (with terminal sporangia— including Trimerophytaceae) and

(II) Zosterophyllales (with lateral sporangia).

Order I. Psilophytales:

The Psilophytales represent the plexus of primitive plants with apical sporangia and probably gave rise to the great majority of modern vascular plants. Three families (Rhyniaceac, Psilophytaceae and Trimerophytaceae) are clearly recognisable here with a forth one (represented by the so-called Asteroxylon elberfeldense) not yet defined.

Family Rhyniaceae:

The Rhyniaceae are the simplest of the Psilophytales, often compared with the sporophyte of Anthoceros. The most important genera are Rhynia and Homeophyton from the Middle Devonian in Scotland.

Genus Rhynia:

The genus Rhynia from the Rhynie chert beds (Middle Devonian) in Scotland was discovered by Mackie in 1913 and fully described by Kidston and Land in 1917. This discovery established the Psilophytales as a separate and distinct taxon. Three species are known of which Rhynia major and R. gwynne-vaughani are the better known.

The plants, apparently, grew in swampy marshes near volcanoes where the atmosphere contained sulphurous vapour and the soil was acid. The reconstruc­tions are from silicified petrifications.

The Sporophyte:

The plant body was a herbaceous sporophyte with dichotomously branching horizontal rhizomes bearing rhizoids on the underside and some of the branches grow­ing up abruptly forming aerial shoots. The aerial shoots of R. major (Fig. 517A) were up to 50 cm long and 6 mm in diameter while those of R. gwynne-vaughani (Fig. 517B) were shorter and more slender.

The aerial branches were cylindrical and sparsely dichotomously branched. These were naked being devoid of any appendage of leaf and usually tapering upwards end­ing in a point or in an erect sporangium.

R. gwynne-vaughani shows hemispheri­cal, oval or lenticular protuberances arising from the lower parts (more mature) of the aerial shoots or from the rhizomatous parts (Fig. 517B). These are constricted at the points of attachment and, in mature ones, have their own vascular bundles not con­nected with those of the main stems.

They are found to be readily detachable and, possibly; was a means of vegetative propagation germinating into new shoots.

The anatomy of the stem is very simple (Fig. 518A). In the centra is a slender, hadrocentric protostele with a small central xylem formed of simple annular tracheides which, in some larger specimens, become smaller towards the centre. This is sur­rounded by four or five layers of elongated cells with oblique ends which represent the phloem although sieve plates have not been observed.

There is no endodermis or peri- cycle. All round this vascular bundle is a massive inner cortex of loose, rounded, parenchymatous cells with lots of air spaces.

This fills most of the stem and was pro­bably the main photosynthetic tissue. The outer cortex is formed by one or two layers of somewhat, larger, comparatively angular, compact (except below stomata) cells at the hypodermal region. The epidermis is a compact layer of cells broken here and there only by the stomata with pairs of guard cells as in other vascular plants.

This is externally covered by a heavy cuticle. Often the smallest branches show no vascular supply.

The sporangia are oval or cylindrical structures with pointed ends (Fig. 518 B) at the apices of the dichotomies. They may be slightly constricted at the bases though continuous with the stem and are always wider. Those of R. major were rather big (about 12 mm long and 4 mm in diameter). The sporangium wall is thick and multi- layered with the outer cells thick-walled and no method of dehiscence is observed.

The thinner, inner cells probably represent the tapetum. The whole interior is filled with spore tetrads or free spores. The spores are spherical, large (up to 65µ in dia­meter in R. major) and covered with a thick cuticle.

The Gametophyte:

As in all the Psilophytopsida, the gametophytes are not known. Lyon (1957) found some germinating spores within the same Rhynie chert beds which show multi­cellular structures developing at the ends of germ tubes. These may represent the gametophytic germination.

Merker (1959 and 1961) has suggested that it is not possible that the gametophytes of such a big group were not fossilised while the large algae had been preserved.

He argued that the underground creeping parts of Rhynia and Honuoph) ‘on are the gametophytes and not rhizomes. But no sex organ has been found on these underground parts and the strong vascular bundle is not normal in a gameto­phyte. His view is, till now, mere speculation.

Puri is of opinion that Rhynia had a homologous gametophyte and, according to him, some of the plants described as sporophytes may be gametophytes, e.g., some smaller R. gwynne-vaughani may be gametophytes of the larger R. major.

These two views suggest a vascular gametophyte which is uncommon, though not completely absent among the Pteridophytos . Moreover all R. gwynne-vaughani show stomata, something unusual in gametophytes.

Other Genera of Rhyniaceae:

Homeophylon (originally named Homea but name changed due to nomenclatural defect) was similar to Rhynia and was discovered from the same place but was smaller and the rhizomes were short and tuberous presenting a jointed appearance (Fig. 519A). The rhizome is devoid of any vascular supply.

The vascular supply enters the rhizome tuber from the stem but fades out after expanding like a bell. There are some mycorrhazal fungi inside the rhizome parenchyma.

The sporangium (Fig. 519B) is slightly wider than the stem apex and in all respects show that it is simply a modification of its apex. It has a sterile columella which is a projection of the stem phloem below with the spore sac overarching on it as in Sphagnum or Andreaea.

This columella sometimes shows a tendency to bifurcate like the stem. The tapetal cells form an extension of the columella lining the inside of the spore sac. The presence of this columella is of phylogenetic importance.

Sporogonites specimens, discovered from Upper Silurian of Australia and Lower Devonian of Norway, are rather imperfectly preserved. The sporangium shows a sterile basal half, there being a dome-shaped spore sac only in the upper half. No vascular strand is noted in the stem so the it a relationship with the Bryophyta is strongly suggested.

This lack of vascular supply was explained to be due to imperfect preservation. But, Andrews (1959) found the stalked sporangia borne parallelly on a crust-like film which he supposed to be the thallus of a Hepatic.

Besides Rhynia, Horneophyton and the doubtful genus Sporogonites, a number of other genera assignable to this family have been discovered. Cooksonia (Fig. 520A) from Upper Silurian and Lower Devonian of Wales show slender, naked, dichoto­mously branched stem fragments with apical sporangia similar to Rhynia.

Hedeia (Fig. 520B) and Tarravia (Fig. 520G) were discovered by Land and Cookson from the Upper Silurian of Australia as fructifications only.

The former has a corymbose bunch of apical sporangia while in the latter there is an apical sporangial structure which is a synangium of radial symmetry formed by the lateral fusion of 5 to 6 lineal- oval sporangia. These structures were, apparently, more complex than the later Hornea or Horneophyton.

Hicklingia (Fig. 520D) from Lower to Middle Devonian of Scotland had a bushy appearance formed by the repeated dichotomy with apical oval but bell-shaped sporangia on the dichotomies.

Nothia aphylla (Fig. 520E) is the name given by Lyon (1964) to the Lower Devonian unattached, dichotomously bran­ched, leafless fructification which was wrongly associated by Kidston and Lang with Asteroxylon mackiei.

Taeniocrada (Fig. 520F) from Upper Silurian and Lower Devo­nian with a flattened, dichotomously branched, thallus-like stem, was probably of a submerged, aquatic habit, and showed apical sporangia on branched fertile clusters.

Family Psilophytaceae:

The concept of the Class Psilophytopsida begin with the discovery by Sir William Dawson in 1858 from the Lower Devonian Gaspe sandstone bed in Canada. This very primitive pteridophyte was quite un-thought of in those days and was named as Psilophyton princeps by Dawson in 1859 and was fully described by him later (1871).

Later on this was also found in Scotland and still later the generic name was being applied very loosely to numerous fragmentary and imperfect specimens from all over the world, sometimes on very incomplete evidence. Now even Dawson’s reconstruc­tion has been proved to be incorrect.

Dawson’s original reconstruction is described below:

Psilophyton Princeps Dawson:

The sporophytic plant (Fig. 521 A) grew in dense clumps and, possibly, in wet places along streams or in marshes. They were probably not more than a metre high with the stem not exceeding 1 cm in, diameter. The general habit is the same as that of Rhynia with a creeping rhizome bearing rhizoids and erect, aerial, dichotomously branched shoots. The aerial shoots are much branched and the tips of stems circinately coiled.

The base of the stem bears short, rigid, curved, spinous structures (Fig. 521B) 0.3 to 2 mm long. These were described as leaves by Dawson but seem to be mere super­ficial emergences flattened in a vertical plane and devoid of any vascular supply or stomata. The presence of a dark substance inside some of them suggests that they were of a glandular nature.

The anatomy of the stem is not very clear but, on the whole, resembles Rhynia. The central xylem contains spiral as well as annular (Fig. 521G) and, perhaps, some scalariform tracheides. The epidermis with stomata and the inner cortex, possibly, resembled Rhynia and the latter probably served as the photosynthetic tissue. The outer cortex is collenchymatous and the stele is mesarch.

The sporangia (Fig. 521A) are usually borne in pairs (because of the bifurcation of the stem tip) suspended from stem tips. They are broader than the stem, obovoid with acute pointed ends, somewhat curved, up to 6.5 mm long and 1^.5 mm in diameter. The sporangial wall is multi-layered. The spores are 60 to 100 µ in diameter and, in spite of the irregularity in size, do not suggest heterospory.

The gametophyte is completely unknown.

The above is a description of the later reconstruction made by Dawson in 1871 out of fragments which were never found together. Recent intensive investigations by Hueber (1964) and Hueber and Banks (1967) in the same Gaspe bed from where Dawson obtained his material and also of the very material used by Dawson as avail­able has shown that Dawson’s reconstruction actually involved three different plants.

The rhizome structure shown by Dawson did not belong to this plant but probably be­longed to Taeniocrada.

Apart from this, Hueber and Banks found in the Gaspe bed three different plants all assignable to the genus Psilophyton:

(1) A plant corresponding to the plant described by Dawson but without any basal rhizome. The t.s. of the stem (Fig. 522B) is similar to Rhynia but with scalariform (sometimes pitted) tracheides. This (Fig. 522A) is now Psilophyton princeps as amended by Hueber and Banks (1967).

(2) Another similar plant with terminal sporangia with smooth stem (not showing any enation or spine) first named Dawsonites (Fig. 522G) but is now considered not as a separate genus but as a species or variant of Psilophyton as amended by Hueber and Banks.

(3) A third plant which was noted by Dawson and called var, ornatum has got lateral and not apical sporangia very unlike Psilophyton although the stem is spinous. Provisionally named Psilophyton ornatum, it has now been given a new generic name Sawdonia by Hueber (1971) so that the name now becomes Sawdonia orata (Dawson) Hueber (Fig. 522B) and belongs to the Zosterophyllales.

Asteroxylon elberfeldense:

The family Asteroxylaceae has so long been considered as a family within the order Psilophytales. It has now been clearly demonstrated that A. mackiei on which the family is based did not have terminal sporangia and is a Lycopod so that it has been transferred to the Lycopsida.

But, there is a problematic plant named Asteroxylon elberfeldense by Krausel and Weyland (Fig. 523) from German Middle Devonian rocks. This has got the vegetative stem closely covered with enations and underground branches as in Asteroxylon mackiei (for description see Lycopsida) but in organic association with a sporangial shoot showing apical sporangia as in the Psilophytales.

This plant is, therefore, a psilophyte and not an Asteroxylon. It needs a new generic name and also a new family status. The stem of elberfeldense is even more developed in being siphonostelic with a central pith and showing the pre­sence of scalariform and round-pitted tracheides.

Family Trimerophytaceae:

The genus Trimerophyton from the Devonian deserves placing in a separate family as it probab­ly represents a separate line of evolution leading to the Progymnosperm Archaeopteris and then the seed ferns. It is included within the Psilophytopsida as it belongs to the terminal sporangial plexus.

But, as ex­plained before, some Pteridologists consider it as a separate line altogether, parallel to Psilophy­topsida and Zosterophyllopsida.

Trimerophyton robustius (Fig. 524) has a stout erect axis, about 1 cm in diameter bearing lateral branches, each one of which in its turn branch trichotomously and then dichotomously ultimately ending in sporangia. There is no leafy appendage but still it presents a bushy appearance. The radial appearance of the plants gives them a distinct position.

Bierhorst (1971) would like to include the bushy plants of Psilophytaceae within this family. In that case Asteroxylon elberfeldense also should come here.

Order II. Zosterophyllales:

The ZosteroPhyllales represent the lateral sporangiate plexus of this primitive group which prevailed during the Lower Devonian. Some Pteri­dologists consider this as a taxon large than an order, viz., Class Zosterophyllopsida evolving parallel to the Psilophytopsida. There is good reason to believe that these plants gave rise to the Lycopsida.

The genera may be placed within a single family Zosterophyllaceae.

Family Zosterophyllaceae:

Of the genera included within this family Zosterophyllum (Fig. 525-I) is the best known. Zosterophyllum fossils have been obtained from the Upper Silurian to Lower Devonian Old Red Sandstone in Scotland. The plants were probably partially sub­merged.

The most important peculiarity is that the sporangia are borne on very short lateral stalks spirally arranged on the leafless aerial branches—giving the appearance of spikes. The sporangia are rather wide and each provided with an apical split for spore dispersal. The spores are of variable size but the size difference is not connected with heterospory. The stem anatomy is as in Rhyniaceae.

Of the other genera within this Lower Devonian family, mention may be made of:

(1) Bucheria (Fig. 525A) of which only the shoots bearing lateral sporangia are known

(2) Gossilinga (Fig. 525G) with lateral sporangia on a dichotomously branched, deli­cate plant

(3) Pectinophyton (Fig. 525D) bearing peculiar lateral, flat sporangia on incurved stalks. The most interesting however is

(4) The so-called Psilophyton ornatum (Fig. 525B) with lateral and not apical sporangia.

This has got lateral and not apical sporangia although it was formerly mixed up with Psilophyton Dawson. The taxon now needs a new generic name which has now been supplied by Hueber as Sawdonia so that the name now becomes Sawdonia ornata (Dawson) Hueber.

Phylogeny of the Psilophytopsida:

The discovery of Psilophyton in Canada by Dawson was at first disbelieved as a vascular plant was not expected at such an early age. Later discoveries from Rhynie chert beds (Rhynia, Horneophyton, Asteroxylon) brought this group into great prominence and its simplicity gave rise to the belief that these are the ancestors of all the Pteridophytes.

Later, however, the position re Psilophyton and Asteroxylon had to be modified.

It has been suggested by several authors (Zimmermann, Eames, Smith, Takhtajan, etc.) that the Psilophytopsida is the ancestral stock of vascular plants from which all the other groups have evolved. The living Psilotopsida, also showing a rootless, dicho­tomously branched sporophyte, has often been considered as a direct descendent of the Psilophytopsida.

Several authors have combined Psilophytopsida and Psilotopsida into a common group Psilopsida.

The Lycopsida may be connected with the lateral sporangiate plexus of the Psilophytopsida (i.e., the Zosterophyllales) through the simplest of the former—the fossil Asteroxylaceae and Protolepidodendraceae.

The Sphenopsida may be connected to the terminal spor- angiate plexus of the Psilophytopsida through the Hyeniales and the Filicopsida through the fossil Protopteridaceae. In the latter direction, probably the bushy Tri- merophytace played a special part and, ultimately, this line lead to the Spermaphytes.

It should also be mentioned that, of late, strong arguments have been put against the theory of the origin of the other taxa of Vascular Plants from the Psilophytopsida as they are known today.

It is now established that a well-developed vascular flora was already present in the Cambrian so that the origin of this group of plants must have taken place in the Pre-Cambrian ages–Baragwanathia, a Lycopsid, with well- developed leaves, flourished during the Silurian—much earlier than the leafless Rhyniaceae.

Moreover, the Psilophytopsida itself seems to be a complex group of yet uncertain phylogeny. The Psilophytopsida grew in a very abnormal atmosphere fall of sulphurous vapour and in a marshy acidic soil. One may even suspect that the leaflessness of the Rhyniaceae may be rather due to reduction. Even the course of evolution within the Psilophytopsida is not yet clear.

It is reasonable to- presume that this Silurian-Devonian group had a long earlier history before the vascular plants actually evolved and the predominating Lower to Middle Devonian forms probably represent a very specialised group which became adapted to the peculiar environment. Nevertheless, the environment alone cannot explain the general simplicity in all its characters.

Later discoveries from still earlier (Lower Devonian and Silurian) beds have brought to light very complicated forms, which existed earlier than the simpler Rhyniaceae, so that their interrelations are still not well understood.

It is now the opinion of many authorities (viz., Andrews, Leclercq, Delevoryas, Axelrod) that during the Pre-Devonian period their was a complex plexus of Psilophyton-like plants, not necessarily related to one another, which we are now placing in an artificial group the Psilophytopsida.

These were probably of polyphyletic origin. Further discovery and research only may clarify the position.

Delevoryas (1969) thinks that there are at least three independent and powerful evolutionary lines (classes Rhyniophytina, Zosterophyllophytina and Trimerophytina) within the Psilophytopsida. Trimerophytina probably evolved the Pterophytes, osterophyllum represented a leteral plexus giving rise to the Lycopsida (also c.f. Asteroxylon mackiei) and the Rhyniaceae gave rise to the other Pteridophyta. Mehra (1968) has also adopted the idea that a apical sporangiate plexus and a lateral sporangiate plexus gave rise to all groups of plants.

The Psilophytopsida no doubt represents the simplest group of vascular plants. But whether this simplicity is due to reduction or whether this group represents the actual ancestors of the present-day vascular plants is open to dispute. They certainly form a very artificial group.

However, they throw some light on the origin of the vascular plants from the algae or some Anthoceros-like pre-Bryophyte. The simplicity of this group lies in the very simple rootless, dichotomously branched sporophytic plants lacking true leaves, the very simple protostelar vascular strand and in the sporan­gia which are evidently just the specialised branch tips.

The vegetative sporophytes of the three families Rhyniaceae, Psilophytaceae and Asteroxylaceae (which is now included within the Lycopsida) represent the trend of evolution of leaves through enations although the exact relationships between the three families are not yet definite.

The Rhyniaceae is the simplest with no leaf or such appendage, with the simplest stelar constitution (annular tracheides only) and simple sporangia which show the columella (a prolongation of the stem vascular system) in Homoephyton demonstrating the stem nature of the sporangium. The extreme simplicity of the Rhyniaceae suggests an elaboration of the Anthoceros type sporophyte.

Sporogonites apparently shows no vascular supply and this is now assumed to be a Bryophyte. In Psilophytaceae the simplicity is retained but the xylem shows spiral tra­cheides and there are some enations at the base of the stems which are certainly not true leaves. The sporangium is as pimple. In Asteroxylaceae the structures become more complex.

The vascular system is actinostelic and even siphonostelic with a pith; spiral, scalariform and even pitted tracheides are found to have evolved. The enations also become more leaf-like showing stomata and leaf traces although reaching only up to the bases of the leaves.

Trimerophyton of Trimerophytaceae is another very interesting plant which pro­bably connects Psilophytales with Archaeopteris and ultimately the Gymnosperms, according to Delevoryas.

It may be concluded that the Psilophytopsida represents a primitive and pro­blematic group of plants, the understanding of which throw much light on the origin of all the vascular plant taxa.

It at least shows two lines of evolution:

(1) A termi­nal sporangiate plexus of plants (including the bushy Trimerophyton) leading to the Filicopsida and the Spermaphyta 

(2) A smaller lateral sporangiate plexus leading to the Lycopsida and, according to some, may have some connection with the living Psilotopsida.