Occurrence and evolution of Pteridophytes
Pteridophyta is a group of ancient plants. They formed the dominant part of the earths during Palaeozoic and Mesozoic periods. Majority of Pteridophytes became extinct. They are preserved in the form of fossils or coal deposits. Only a few living Pteridophyta are left.
The vegetative plant body is a sporophyte in Pteridophytes. Sporophyte is differentiated into stem, leaves and roots. This spoi ophyte is an independent plant. The sporophyte of Pteridophyta possesses well-developed vascular tissue consisting of both xylem and phloem. The vascular tissue is confined to the central region of the stem forming the stele. It is clearly demarcated from the cortex by a well-developed endodermis.
Pteridophytes possess all terrestrial characters. The epidermis of the aerial parts is covered by cuticles. The roots are adventitious. It possesses a root cap and root hair. Branching of the stem is dichotomous or lateral. Secondary growth is absent in the living forms.
The gametophytic and sporophytic stages regularly alternate with each other in Pteridophytes.
The sporophyte forms the vegetative plant. It is asexual stage of the life cycle. The sporophyte forms the dominant stage. There are two types of sporophytes:
i. Microphyllous: The sporophytes with small and scale like leaves are called microphyllous.
ii. Megaphyllous: The sporophyte with large and prominent leaves is megaphyllous.
Spores: Spores are produced in sporangia. Sporangia are borne on the leaves or in their axils. When the sporangia are borne on the leaves then the condition is known as phyllosporous. When sporangia borne in the axils of the leaves then the condition is known as stachyosporous. The leaves which bear the sporangia are called sporophylls. These sporophylls may be like ordinary vcgetative leaves. Sometimes they modify to form definite cones or strobili (sing strobilus).
The plants producing spores of the same size are called homosporous plants. The plants producing spores of two different sizes are called heterosporous. The larger spores are called megaspores (macrospores). They are produced in megasporangia. The smaller spores are called microspores. They are produced in m icrosporangia.
Sporangium: Each sporangium has a well developed wall. It encloses sporogenous tissue or a mass of archesporial cells. These cells separate from each other on maturity and form spore mother cells. One or two peripheral layers persist for the nourishment of the developing spores. These nourishing cells form tapetum. The spore mother cells are diploid. The spore mother cell undergoes meiosis and produces four haploid spores. These spores germinate to form gametophyte or prothallus.
The gametophyte is small, insignificant and short lived. It forms a simple thallus called prothellus. It bears rhizoids for the absorption of water. Gametophytes are independent in the majority of forms. Their cells are green due to the presence of chloroplasts. The gametophytes may be monoecious or dioecious in the homosporous forms. But in the heterosporous forms they are always dioecious. The microspore produces a tiny male prothallus. The megaspore contains a large amount of reserve food material. It gives rise to the female protballus. The sex organs are antheridia and arehegonia. Sex organs are embedded in the tissue of the prothallus near the surface. The antherozoids are coiled flagellated. Thus Pteridophytes depends on water for fertilization. It is primitive characters.
Germination of oospore: The female gamete or oosphere is never released from the archegonium. Diploid oospore is produced as a result of fertilization. This diploid oospore gives rise to the sporophyte. The young sporophyte (embryo) is dependent upon the gametophyte during early stages of development. But soon it becomes independent. The embryo possesses a foot for the absorption of nourishment from the gametophyte
Alternation of Generation
Pteridophyta show a regular alternation of sporophytic and gametophytic generations. The sporophytic stage is dominant. They have separate and independent gametophytic stage. Haploid spore is the beginning point of the gametophytic stage. Diploid oospore is the beginning point of the sporophyte.
Classification of Pteridophyta:
Pteridophyta are divided into the following four divisions.
Division I: Psilopsida
Their sporophytes are differentiated into underground portion rhizome and the aerial branches. They are without any roots. Branching of the stem is generally dichotomous. Leaves may be present or absent. Sporangia are borne singly at the tips of long or reduced branches. Gametophytes are subterranean (under ground). It has following orders:
- Psilophytales: They all are present in fossil forms.
- Psilotales: They are all living. Example: Psilotum.
Division II: Lycopsida (Club Mosses)
Their sporophytes are differentiated into stem, leaves and roots. Leaves are small (microphyllous). They may have a ligule. Steal is variable from protostelic to siphonostelie. Sporangia are borne singly on the leaves or in their axils. Sporophylls form definite strobili or cones. Both homosporous and heterosporous forms are present. It has following orders:
- Lycopodiales: Example: Lycopodium.
- Selaginefia: Selaginella
- lepidodendrales: Fossils
- Isoetales: Both fossil and living
Division III: Sphenopsida (Horse Tails)
Sporophyte differentiated into root, stem and leaves. Stem has underground rhizome and aerial branches. Stem is differentiated into nodes and internodes. lnternodes have characteristic ridges and furrows. Leaves are small (microphyllous). They are arranged in whorls on the nodes. Lateral branches are also present in whorls. They are siphonostelie. Sporangia are borne on sporangiophores. Sporangiophores are grouped on axis to form definite cones. It has following orders.
- flyeniales: All fossils
- Sphenophyllales: All fossils
- Equisetales: Equisetum.
Division IV: Pteropsida (Ferns)
Sporophyte is differentiated into stem, leaves and roots. Leaves are large (microphyllous) and spirally arranged. They are siphonostelic. Sporangia groups together to form sorus (sing sorus – soral evolution). It is covered by an indusium. Young leaves are circinate (coiled). This class is subdivided into the following sub-classes
Sub-class (a): Primofilices: It is extinct sub-class. It is present only in fossil forms.
Sub-class (1): Eusporangiatae: Sporangia develop from a group of cells. Sporangia borne on the leaf blade or on a fertile spike. Antheridia are embedded in the gametophyte. It has following orders:
- Ophioglossales: Bottychium
- Marratiales: Marratia.
Sub-class (c) Leptosporngiatae: Sporangial wall is one cell in thickness. They are homosporous. Antheridia produce only a few antherozoids. Necks of archegonium protrude above the surface of the thallus. Sporangia have an annulus. This sub-class has only one order, Filicales. Example: Dryopteris
Sub-class (d) Hydropteridae: These are heterosporous ferns. Sporangial wall is thin and without annulus. The sporangia are enclosed in special thick walled structures called the sporocarps. It has single orders Salviniales: Example, Manila:
Types of Steles
Different types of steles are found in Pteridophytes. These are:
The xylem is in the form of a solid mass in Protostele. It forms the central core of the stele. It is completely surrounded by the phloem. Pericycle and endodermis surrounds the phloem. There are two forms of Protostele:
(a) Haplostele: The protostele in which xylem core is smooth and rounded is called Haplostele.
(b) Actinostlele: The protostele in which xylem core is star like is called Actinostlele.
In piectostele xylem forms several plates. These plates lie parallel to each other among the phloem elements. The protoxylem lies at the tips of the xylem plates or on one side
In siphonostele the vascular tissue is in the form of a hollow cylinder. Parenchymatous cells in the centre forms the pith. In some cases, the vascular supply of a branch in siphonostele departs from the main vascular cylinder. It forms a gap in the main cylinder. Leaf gaps are present in the macrophyllous plants. But there are no gaps in microphyllous. There are following types of siphonostele:
(a) Ectopholic Siphonostele: In this case xylem is bounded by the phloem. Pericycle and the endodermis are present only on the outer side.
(b) Amphiphloic Siphonostele: In this case, xylem is bounded by phloem. Pericycle and endodermis are present towards the older and the inner side.
(c) Solenostele: This is a type of siphonostele in which the gaps of different leaf traces do not overlap. Only one gap is present in the main cylinder.
(d) Dictysotele: This is a type or siphonostele in which different gaps overlap with each other. In this case, several gaps are present at a particular level.
(e) Polycyclic: The siphonostele in which two cylinders of vascular tissue are present in the stele is called polycyclic. In such a stele both the cylinders may be solenostelic or dictyostetic.
4. Monostelic and polystelic
Monostelic plants have only one stele. Polystelic plants have two or more steles.
Types of Xylem
1. Protoxylem: In this case, the tracheids are annular with spiral thickening. These tracheids elongates even after maturity.
2. Metaxylem: In this case, the thickenings on tracheids are scalariform in the form simple pits. They do not elongate after maturity.
Exarch: In this case, protoxylem is lying on the periphery of metaxylem.
Mesarch: In this case, protoxylem is lying in the middle of metaxylein.
Endarch: In this case, the protoxylem is lying towards the inner side of the metaxylem.