In this article we will discuss about:- 1. Introduction to Pteropsida (Ferns) 2. Plant Body of Pteropsida (Ferns) 3. Gametophyte 4. Classification 5. Phylogeny.
Introduction to Pteropsida (Ferns):
Pteropsida represent the most highly evolved group among the lower vascular plants. In this group are included some of the most beautiful, and most familiar plants called Ferns which are the joy and pride of a gardener.
The delicate, varied and highly attractive foliage of ferns have made them a must in any garden. Of all the pteridophytes, ferns are the most widely distributed. The living ferns are represented by 305 genera and nearly 10,000 species.
Geologically ferns have been known since the carboniferous period. It is believed, however, that the group must have had its ancestors during the Devonian period itself. A unique feature of ferns is that their persistent basic characters are still sufficiently plastic to be receptive to the environmental fluctuations.
Pteropsida are distinct from lycopsida and sphenopsida in several characters. Among the vegetative characters, the megaphyllous leaves with the attendant leaf gaps are most notable. Among the reproductive features, (though some primitive members show some sort of a semblance to the strobilar organisation seen in the previous groups), the aggregation of sporangia on the abaxial or adaxial surface of the leaf into sori is the most significant.
Plant Body of Pteropsida (Ferns):
The sporophyte has an underground rhizomatous stem which may be elongated or tuberous. The branching of the rhizome may or may not be dichotomous. In some cases the rhizome is covered by hairs called ‘ramenta’.
The leaves are compound and once or twice pinnate. They are megaphyllous, having a dichotomous or reticulate type of venation. The size of the leaves varies from few centimeters to several metres (Angiopleris). Usually only the leaves are aerial while the rest of the plant body is subterranean. Some ferns show circinate vernation in the leaves i.e., the young growing parts are coiled inwards and uncoil as they grow.
Like in other pteridophytes, roots are always adventitious.
Wagner and Johnson (1983) have reported a special food storing organ trophopod in a number of ferns such as Asplenium, Platyneuron, Onoclea spp, Dryopteris fragrans etc. According to them the trophopod which is generally over looked in fern description is an organ which is of potential systematic value.
Stelar organisation varies from protostele, solenostele to polycyclic, dictyostels (Pteris). Cortex may be wholly parenchymatous (Ophioglossm) or may be distinguished into outer sclerenchymatous zone and inner parenchymatous zone. Some times muclilage ducts are found in the cortex as in Angiopteris. Xylem has mostly tracheids, but vessels are also reported in Pteris, Marsilea etc. Secondary growth is absent except in Botrychium.
The stele is usually protostelic with variations in xylem groupings. The xylem is exarch and may be mono-di tri or even tetrarch. Root cortex may be homogenous of heterogenous.
The leaves maybe provided with single leaf trace or the trace may be dissected into several meristeles.
The upper and lower epidermal layers enclose the mesophyll which may or may not be differentiated. Distinction of palisade and spongy parenchyma in the mesophyll is seen in Cheilanthes, Pyrrosia etc. Lamina may be hypostomatic or amphistomatic.
This is brought about by a variety of methods such as fragmentation, adventitious buds, embryonic leaf apices, stem tubers, root tubers etc.
Pterospsida are both homosporous and heterosporous (Marsilea). Spore producing organs are varied. They may be fertile spikes (Ophioglossum), tassels (Osmunda), sori (Adiantum, Pteris etc.) or Sporocorps (Marsilea). Spore producing organs are usually borne on the leaves except in some species of Marsilea.
Whatever may be the name given to the spore producing organs they always represent aggregations of sporangia. The sporangia within a sorus are numerous arising from a fertile tissue called Receptacle. The sporangia may or may not be surrounded by a flap or tissue (arising from the receptacle) called Inducium.
Sometimes a sorus is protected by a false inducium which represents the incurving of the leaf margin. The maturity of sporangia within a sorus is varied. Based on this, the son are classified into three types viz., (i) Simple, (ii) gradate arid (iii) mixed. In a simple sorus all sporangia develop simultaneously (eg. Osmunda).
In a gradate sorus sporangia develop in basipetalous succession (e.g. Hymenophyllum) and in a mixed sorus sporangia develop in an irregular sequence (eg. Pteris). It has been widely held that a simple sorus is primitive, a mixed sorus is advanced while a gradate sorus is of the intermediate type.
Sporangia may or may not have a stalk (Ophioglossum). Their development is either of the eusporangiate type or of the leptosporangiate type. The capsule region of the sporangium encloses the spores. Except in primitive members such as Ophioglossum and Angiopteris, the sporangium has a definite dehiscence mechanism brought about by cells of different thickness and differential hygroscopic reaction.
The thick walled cells are called Annulus and the thin walled cells are called the Stomiuim. The annulus may be shield shaped (Osmunda), cap like (Lygodium) or obliquely vertical incompletely overarching the sporangium (Pteris, Adiantum etc.). Spores are wind dessiminated and have a sculptured outer wall (exine) enclosing a thin inner wall (Intine).
Gametophyte of Pteropsida (Ferns):
In homosporous forms the gametophytes are exosporic and in heterosporous forms they are endosporic. Endosporic gametophytes are extremely reduced.
Bower (1923, 1935) has recognised three types of prothalli in homosporous ferns. These are:
(a) Cordate type,
(b) Filamentous type and
(c) Saprophytic type or the mycorhyzic type.
Cordate or heart shaped prothalli are autotrophic and are seen in Adiantum, Osmunda, Pteris etc. The filamentous type is seen in Hymenophyllum. The nutrition here also is autotrophic.
Mycorrhizic prothalli are common in members like Ophioglossum. These prothalli are tuberous or cylindrical and have a radial symmetry as opposed to the bilateral symmetry of the cordate and the filamentous types. Nutrition is saprophytic.
Gametophytes reproduce vegetatively as well as sexually. The former type of reproduction is very rare. Sexual reproduction is brought about by antheridia and archegonia which have undergone maximum possible simplification.
Embryogeny may be exoscopic (Ophioglossum) or endoscropic with (Helminthostachys) or without (Angiopteris) a suspensor. In leptosporangiate ferns embroyogeny is said to be lateral because the first division is vertical and does not produce epibasal and hypo basal cells.
Classification of Pteropsida (Ferns):
The types of classification proposed for ferns are as varied as ferns themselves. Below is given a few systems of classification.
Hirmer (1927) classified Fillicophyta into four classes viz.:
(c) Protoleptosporangiatae and
Hirmer created protoleptosporangiatae specially to include osmundaceae which exhibits intermediate characters between eusporangiatae and leptosporangiatae.
Pichi-Sermolli (1959) has sub divided Filicopsida (pteropsida) into seven sub-classes, viz., Primofilicidae, Ophiglossidae, Marattidae, Osmundiade, Filicidae, Marsilidae and Salvinidae.
In this article the classification proposed by Reimers (1954) is followed.
The order coenoptaridales comprises only fossil members belonging to the late paleozoic ara. The fossil remains of the plants of this order include stems and frond parts which are very well preserved structurally. The members represent the fossil ferns.
The order Coenopteridales has many alternative names like Palaeopteridales, Primofilicales and Renaultificales. The last mentioned name is in honour of the great French Paleobotanist Renault. The order comprises a heterogenous assemblage of various ferns and has been treated differently by different paleobotanists.
However, there seems to be general agreement in classifying the order into three families namely, Botryopteridacease, Zygopteridaceae and Cladoxylaceae. Burtrand divided the order into two subgroups namely Inversicatenales and Phyllophorales. The group Inversicatenales includes the family Botryopteridaceae while Phyllophorales has two families namely, Zygopteridaceae and Cladoxylaceae.
In this article Botryopteris (Botryopteridaceae) and Zygopteris (Zygopteridaceae) are discussed:
The genus Botryopteris is one of the best known among Coenopteridales. It is the type genus of the family and has 5 species ranging from lower carboniferous to the permian. The name Botryopteris is given to the fossil specimens of stem.
The stems are slender, cylindrical and few millimeters in diameter. They are branched and bear spirally arranged fronds. Anatomically, the main stem has a small mesarch protostele, which is surrounded by a broad cortex. The cortex has a prominent band of sclerotic cells. (Fig. 114).
In the leaf stalk, the xylem strand is solid and has three prototoxylem points. As in B.forensis the strand is deeply indented looking like a trident. In very few species of Botryopteris, sporangia have been found attached to the fronds.
The sporangia are found in clusters which is somewhat rare to ferns. In B. globosa sporangial cluster has thousands of sporangia. The sporangia themselves are small, oval to pyriform in shape.
They measure 2 mm in length and about 1 mm in diameter. Each sporangium has a short stalk and a capsule which is somewhat oval in outline. The wall of the capsule has a broad annulus represented by thick walled cells. The spores are of the same type.
The ferns belonging to the family Zygopteridaceae are more complex and older then Botryopteridaceae. The fossil specimens belong to middle Devonian and possibly have connection with Psilophytales. Zygopteris, the type genus of the family is the best known.
It has several species of which Z. primaria has been studied extensively. The plant body of Zygopteris is tree like with a trunk having a diameter of 20 cm. The stem as such, however is only about 1.5 cm in diameter, the rest of the diameter being made up of an armour of leaf stalks and adventitious roots.
The stem of Zygopteris bears an elaborately branched frond having a number of leaf stalks. These are usually cylindrical and up to 2 cm in diameter. The leaf stalks have a number of pinnae. Occasionally the leaf stalks are given the name Etapteris.
Anatomically the stem of Zygopteris has a xylem cylinder consisting of scalariform tracheids. An unusual feature here is the presence of a layer of secondary wood surrounding the primary xylem. This is perhaps one of the rare instances of secondary vascular tissue in a fern extinct or extant.
The leaf stalk internally shows a vascular strand which is H- shaped with a straight median band with some what fixed lateral arms. Two small protoxylem points lie in the shallow depressions at the end of the median band.
Various names have been given to the fructifications of Zygopteris. Corynopteris is one such fructification genus. The sporangia in the fructification are large and ovate. They are usually sessile and are grouped into spherical son. The wall of the sporangium has a broad band like annulus. The sporangial cavity is filled with homosporous spores.
Phylogeny of Pteropsida (Ferns):
The Coenopteridales represent the most primitive group among the ferns. They are very ancient having originated perhaps, with Psilophytales. One of the prominent features of Coenopterids is the lack of distinction between stem and leaves.
According to Delevoryas (1962) this suggests their affinity with Psilophytales. The relationship of Coenopterids to other ferns is rather obscure. But there is no doubt in the fact that Coenopterids may be regarded as ancestral stock from which the modem ferns sprung up.
This sub-class includes all eusporangiate ferns. The sporangial wall is more than one layered. Spore output is very high.
There are two orders in this sub-class viz., Ophioglossales and Marattiales.
The order includes herbaceous, fleshy sporophytes with a short rhizome. Sporangia are borne on a separate outgrowth called ‘fertile spike’. This arises at the junction of the leaf blade and lamina. Sporangia have a multilayered wall with a high spore output.
There is no special dehiscence mechanism. All the members of the order are homosporous. Gametophytes are tuberous and saprophytic. The order has a single family Ophioglassoceae, with three genera- Ophioglossum, Botrychium and llelminlhoslachys.