Psilophytopsida: History and Phylogeny | Botany

In this article we will discuss about:- 1. History of Psilophytopsida 2. Classification of Psilophytopsida 3. Phylogeny.

History of Psilophytopsida:

One of the most perplexing and enigmatic problems of evolutionary morphol­ogy which is still eluding a solution is that of the origin of land plants. The invasion of the land masses by the aquatic inhabitants which perhaps took place about 400 million years ago is one of the most dramatic and significant events in the history of plants.

The earliest land plants offer us an insight into the origin of stele, root, leaves etc. A thorough onto genetic study of the ‘earliest known land plants’ is unfor­tunately impossible because they are presented today only in the form of fossils. The group psilophytopsida constitutes the earliest known terrestrial plants.

The story of psilophytopsida commenced as early as 1859 with the investiga­tion of plant fragments from the Devonian deposits of Gaspe’ peninsula (Canada) by Sir J.W. Dawson the Canadian geologist. The compressed fossils were extreme­ly fragmentary, but from them Dawson constructed plants which showed close resemblance to the extant genus Psilotum. The plants were named Psilophyton princepis.

The second chapter of the story which formed a firm foundation for further investigations is the discovery by R. Kidston and W.H. Lang (1917). They publish­ed a series of papers in which were described certain primitive vascular plants from old Red Sand Stone near Rhynie in Aberdeenshire-Scotland.

Actually this plant bearing chert was first discovered in 1913 by W. Mackie, but the credit for a thorough investigation goes to Kidston and Lang. The precise age of the bed is not known though it is proved to be not later than the middle old red sand stone.

The chert was formed of a series of old peat beds which were no doubt subjected to periodic inundations (but periodically covered by vegetation), for each bed had a thin layer of sand on the top. Successive layers of peat accumulated to a depth of some 8 feet. Then followed water with soluble silica, most probably from some fumarole and the peat beds were completely sealed with the plants preserved to perfection.

This kind of perfect petrified remains are very rare and the fact that the lower parts of these plants were often found erect as in life, speaks volumes regarding the preservation.

Three distinct genera were described from this chert viz., Rhynia, Horneophylon, and Asteroxylon. In the words of Kidston and Lang: ‘the plants were totally unlike any group of extant and extinct plants and characterised by the sporangia being borne at the tips of certain branches without any relation to leaf or leaf like organs’.

Thus they constituted a new group of pteridophytes with the following general characters:

1. Simple plant body with no differentiation into root, stem and leaf.

2. The naked green stems are spiny having cuticles and stomata. Occasionally leaf like organs may be present.

3. Possessing the simplest type of vasculature.

4. Sporangia are borne at the tips of the ultimate branches.

Classification of Psilophytopsida:

The class consists of a single order psilophytales and five families namely Rhyniaceae, Psilophytaceae, Zosterophyllaceae, Asteroxylaceae and Pseudosporochnaceae.

In this article, only two families, Rhyniaceae and Asteroxylaceae have been discussed.

1. Rhyniaceae:

The family Rhyniaceae consists of genera like Rhynia, Horneophyton Sporognites, Cooksonia, Yarravia, Hicklingia etc.

Of these Rhynia is discussed below:

Members of Rhyniaceae probably represent the most primitive as well as simple organisation of plant body as could be imagined for a vascular plant. The type genus Rhynia of which two well-defined species (R.gwynnevaughnii, R. major) have been identified was discovered in the middle Devonian beds of Scotland.

Rhynia essentially consisted of a rootless subterranean creeping rhizome from which arose cylindrical erect leafless bifurcating shoots with some of them ter­minating in sporangia (Fig. 11) Rhizoids arose from the rhizome.

Of the two species, R. gwynnevaughnii was the smaller and the more advanced one. The height of this plant probably ranged from 17 to 26 cm., with the same general habit. This plant presented some morphological features which were not found in other form. It had an aerial stem studded at irregular intervals with hemispherical bulges or outgrowths similar to but smaller than those of the rhizome.

In some cases it was observed that these swellings near the base of the stem bore rhizoids suggesting a scat of new development. These grew into adven­titious branches having a narrow base at their attachment to the stem and appeared to have easily separated thus forming a ready means of vegetative propagation.

The larger genus R. major, the full height of which is not known probably three times larger than its relative was completely devoid of these hemispherical projections. But for this, the characters were essentially similar in the two species. It thus appears that R. gwynnevaughnii possessed a method of vegetative increase which the larger species dispensed with.

Internal Structure:

Members of Rhyniaceae are characterised not only by their organs graphical primitiveness but internally also they exhibit many primitive features. Anatomy of the aerial stem was practically similar in both species of Rhynia except that some of the slenderest twigs of R. gwynnevaughnii were destitute of any vascular structure.

A cross section of the stem shows a smooth vascular cylinder occupying the central position in a wide cortex (Fig. 12). The stele consists of a central smooth core of woody elements composed of annular tracheids.

In the smallest possible cylinder the wood shows no differentiation in composition. In larger steles of R.gwynnevaughnii and in almost all the steles of R. major the central tracheids may be distinguished by their smallness from the larger elements of peripheral zone of xylem.

Thus there is some indication of centrarch nature. Surrounding the xylem is a tissue of delicate tubular cells which together with the wood forms the conducting tissue. D.H. Scoott says “there is sufficient reason to regard this tissue as phloem”.

This consists of elongated cells whose end walls are characterised by their oblique nature. Satterthwait and Schopf (1972) have observed the presence of several spores in the phloem region of the shoots of Rhynia major. There is no pericycle or endodermis.

It may be pointed out here that the adventitious branches of R. gwynnevaughnii had no vascular connection with the central stele. The central stele however divided at the region of the dichotomy to provide the branches.

Outside the conducting tissue is a wide cortex differentiated into broad inner cortex and narrow outer cortex. The inner cortex consists of rounded cells often arranged in radial rows with large intercellular spaces. There are indications of this being the assimilatory tissue of the plant.

The narrow outer cortex or the hypoderms is 1-4 layered thick and composed of large clear cells. Outermost layer is the epidermis with thick fusiform cells whose outer walls are cutinized. The continuity of the cells of the surface layer is interrupted by the presence of stomata which are not found on the rhizome.

Stomata are typical of a vascular plant in their structure. But they are not depressed below the level of epidermis. Beneath the stomata hypo-derma is interrupted, thus bringing direct communication with the inner assimilatory cortex. There is no proof for the presence of mycorrhiza.

Reproduction:

All the fossils of Rhynia hitherto discovered are only sporophytes (this is true of other psilophytales also) and there is no evidence of a gametophyte in the fossils though the life cycle undoubtedly should have had a haploid gametophytic stage. The reason for the absence of gametophytes (in the fossil) lies in their delicate nature which renders them unsuitable for fossilization.

In Rhynia, the sporangia are borne distally on certain branches of the stem without any relation to leaf-like organs. The two species of Rhynia differ in the size of sporangia and spores. The sporangia of R. major at 12 mm in length and 4 mm in diameter and those of R. gwynnevaughnii are 3 mm long and 1-1.5 mm broad.

The spores of R. gwynnevaughnii have a diameter of 35-40 µ. and of R. major 65µ. The terminal position of the sporangia which is of an exceptional nature in pteridophytes forms more or less a general character here, the analogy of which could be brought about only with psilotales. Rarely, sporangia are found in pairs in Rhynia. Perhaps these might have been borne so on the plants.

Apart from the size, the sporangium is essentially similar in both species of Rhynia. The cylindrical spore sac tapers towards the distal end and towards the base it has a stout stalk similar to stem in structure (Fig. 13). The strongly constructed sporangium has a wall of cells several layers in thickness showing considerable differentiation.

The superficial layer consists of thick walled, much vertically elongated cells with a strong cuticle. This layer-is similar to that of vegetative epidermis. Beneath the epidermis several layers of intermediate cells are found often badly preserved. Lining the sporangial cavity is a more definite layer of cells originally interpreted as Tapctum.

But the cells appear to be too rigid to be of this nature. Within the sporangium are a number of spores of the same size (homospores) often found still in tetrads. The presence of tetrad indicates undoub­tedly the occurrence of meiosis in the life history Rhynia. There is no indication of any dehiscence mechanism. Spores have cutinised walls typical of pteridophytes.

A new interpretation on the morphology of Rhynia : It has generally been accepted that the fossil specimens obtained from the Rhynic chert are only sporophytes and the gametophytes have not been preserved owing to their delicate nature.

But, in one of his papers D.D. Pant (1962) has questioned these observa­tions and holds that the gametophytes must be present and they have been wrongly interpreted. According to him, of the two species of Rhynia only R. major is the sporophyte and the other (R. gwynnevaghnii) is in reality the gametophyte of R. major.

The main points of his argument are as follows:

1. It is true that the gametophytes are less suited for fossilization, but in the chert where Rhynia has been obtained, good fossils of some algae are also obtained. Surely gametophytes of Rhynia could not have been more delicate than the filaments of algae. Hence the gametophytes of Rhynia must be found in the Rhynie chert

2. Kidston and Lang being thorough investigators could not have overlooked the presence of gametophyte but they have wrongly interpreted R. gwynnevaugh­nii as the sporophyte.

3. The adventitious branches present in R. gwynnevaughnii are really young sporophytes developing from the gametophyte. The proof for this is the absence of direct vascular connection. If the so called adventitious branch were to be really a part and parcel of the plant body what prevented it from having a vascular connection while ail other branches have it.

4. Some of the stomata (?) at least represent the archegonia. The two guard cells (?) are in reality the two neck cells. While four rows of neck cells are present in the archegonium, only two are seen in a long section. These two cells may be mistaken for the guard cells.

5. Below the stomata (?) in some sections a darkly staining body is present. According to D.D. Pant this may be a zygote.

6. In the fossil specimens obtained, there was actually no organic connection between the sporangia and the shoots of R. gwynnevaughnii. This was thought to be due to faulty preservation and nobody doubted the relation of the Sporangia to the shoots of R. gwynnevaughnii. But D.D. Pant feels that the absence of organic connection clearly shows that R. gwynnevaughnii is a gametophyte and it could not have had organic connection with the sporan­gium.

‘Merker (1958, 1959) has suggested – “some of the bits of Rhynia identified as rhizomes could have been gametophytes”. Sporne (1966) however disagrees be­cause, according to him no sex organs are found in these “gametophytic bits”. Lyon (1957) has described some germinating spores from the Rhynie chert. Lemoigue (1968) has reported in the shoots of Rhynia gwynnevaughnii some central channels each leading to a large space or cell below which could be an archegonium thus supporting its (R. gwynnevaughnii) gametophytic nature.

2. Asteroxylaceae:

Members of Asteroxylaceae undoubtedly represent advancement over all other families of the order, both morphologically as well anatomically and some of the genera seem to foreshadow the principal extant classes of pteridophytes. The type genus Asteroxylon (Fig. 16) has two species A Mackie and A. elberfeldense.

The former species found in the Rhynie chert was a dichotomizing rhizomatous plant without either roots or rhizoids. But some of the rhizomatic branches dichotomized freely and produced slender leaf like organs which penetrated into the peat like substratum in which the plants grew; other branches grew upwards and became clad with simple non-vasculated leaves.

A transitional region beset with small scales was present at the base of the erect shoot. In the aerial region branching was lateral and exogenous. Cases of dichotomy were also observed and in one instance an endogenous branch was also found. The leaves 5 mm in length covered the shoots somewhat densely and appeared to be spirally arranged recalling the foliage of Lycopodium cernuum.

Kidston and Lang (1913) found in one of the blocks in Rhynie chert certain peculiar appendages being closely associated with Asteroxylon. It was scarcely doubted whether they belonged to the plant. There was however no proof of connection nor unfortunately did the state of preservation of tissue admit of convincing histological connection. As the axes had sporangia at their tips, these were thought to be sporangiophores.

However the fertile shoots were naked. This view was confirmed when another species of Asteroxylon- A. elberfeldens was described from the middle Devonian strata in Germany. This plant possibly grew to a height of one metre.

The lower part of stem was covered with leaves but at a higher level replaced by small projections and further up the branches were smooth bearing sporangia at the tips. Kidston and Lang (1913) suggested “the fertile axes might have replaced the leaves on a distal region of the shoot or have been ultimate sub divisions of a fertile region continu­ing certain vegetative shoots”.

Internal Structure:

Members of Asteroxylaceae probably represent the most complex anatomy which is somewhat strange to the primitive pasilophytes. The type genus Asteroxylon is of great interest because of its structurally advanced type of stele.

No secondary development was observed in the stele. The xylem strand was extremely stellate with four or more very long sometimes forked arms having occasional expansions at the ends and attenuated towards the middle of the star. Though there was no apparent distinction between proto- and metaxylem, the former in clear sections formed groups of small and compressed tracheids within the ends of the xylem rays but close to the surface.

The conducting tissue as a whole was cylindrical, for the phloem not only filled the bays of deep rays of the xylem but also covered the arms of xylem. Small leaf traces were given off from the rays of the stellate xylem. The leaf trace was concentric having protoxylem in the centre.

The leaf trace however travelled only upto the base of the leaf. The branch traces gradually assumed the stellate shape. There was no pith in A. Mackie. In A. elberfeldense, however, there was a pith in the centre of the xylem: Except for this the two species were identical anatomically.

The stele was clearly contrasted with the cortex though seldom was there any indication of pericycle or endodermis. When fully differentiated, the cortex was having three distinct zones. The innermost region of cortex was compact.

The middle cortex had trabecular structure consisting of vertically radiating plates of trabeculae separating the lacunae. Peripheral zone of cortex consisted of tangentially extended cells. Cortical cells contained aseptate fungal hyphae and resting spores of the type genus Paleomyces.

Stomata were observed both in stem and leaves. But they were unlike those of Rhynia in being depressed below the epidermal surface.

Reproduction:

In Asteroxylon Mackie, the fertile shoots were not intact with the plant body but found as separate twigs. There was in fact double breach of continuity first, between the vegetative shoots and the supposed fertile axes, and secondly, between the latter and the sporangia. In both the cases however the association was highly suggestive. The sporangia were borne on slender naked, leafless branches of the Hostimella type.

These axes were thought as the deciduous fertile branches of Asteroxylon owing to the similarity in anatomy and stomatal structure. In A. elbarfeldense there was organic connection between the vegetative shoots and the reproductive axis.

The sporangia were ovoid or ellipsoid in shape with a definite dehiscence at the wider free end which shows a simple annulus. All the plants of Asteroxylon were homosporous. No stage of spore germination has been noticed nor the gametophytic stage yet been found belonging to any one of them.

According to Lyon (1964) these axes with pear shaped sporangia have been wrongly attributed to Asteroxylon. She has described some fertile specimens in which there were reniform sporangia among the leaves. Based on this, Sporne (1956) observed “these suggest that the affinities of Asteroxylon are with the Lycopsida instead of the Psilophytopsida”.

Phylogeny of Psilophytopsida:

There can be no doubt that psilophytopsida represent the simplest vascular plants. Even among psilophytopsida, Rhynia perhaps is the simplest. It consists of a naked leafless shoot, bearing sporangia at the apices of the branches. From the morphological stand point Asteroxylon certainly represents considerable advance­ment over the simple Rhynia.

The presence of photosynthetic laterals in Asteroxylon is no doubt a definite advancement in that it represents a significant step in the evolution of the leaves. How far actually the leaves of Asteroxylon could be regarded as true leaves is open to question, because the leaves lack a vascula­ture. But the presence of stomata clearly indicates that the photosynthesis laterals are proceeding along the right path to be evolved into leaves.

Anatomically also, Asteroxylon shows evolved characters in possessing a structurally complex stele. The possession of pith (A. elberfelclense) and the leaf traces are additional evidences for the advanced nature of Asteroxylon. The spore bearing organs of psilophytopsida are perhaps unique in vascular plants, the like of which could be seen only in Psilotum.

The group as a whole shows such simple feature that it certainly defeats the tide the first land plants. Eames (1964) observes; “the plant body of psilophytales is a mere axis or thallus carrying on all vegetative and reproductive functions with little or no elaboration of form and provided with those features essential to existence on land and in the air”.

Is Rhynia the first land Plant?

As per the present knowledge Rhynia is the earliest known land plant, but certainly it cannot be regarded the first land plant. The first plant part should still possess imprints of an aquatic habitat and should show features which make the plant a little bit unsure of itself on a terrestrial habitat.

But the structural attributes of Rhynia, its photosynthetic shoot, thick cuticle, well developed stele, presence of stomata in the stem, all clearly point out that it is quite at home in the terrestrial habitat. Surely, this cannot be expected of the first land plant. Among the living groups of pteridophytes only psilotales (specially Psilotum) show close resemblance to psilophytales.

The relationship is expressed mainly in the following:

(a) Plant body being a rootless axis with emergences than leaves;

(b) The sporangium being an axis tip with massive undifferentiated wall and

(c) The presence of a simple protostele.

Eames (1964) agrees with this and states, “psilotaceae and psilophytaceae seem to belong together in the major group in a natural classification”. According to him psilotales and psilophytales should be placed in a common group psilopsida.

Psilotales no doubt represent the surviving remnants of the ancient psilophytalean stock retaining the major morphological features of the ancestral types.

A study of the group provides us an insight into various morphological problems like the origin and differentiation of the sporophyte, origin of vascular plants, origin and evolution of stele etc.