The plants of pteridophytes are mostly terrestrial. They prefer shady habitats. Some species of Selaginella and Adiantum are xerophytes. A fern, Acrostichum aureum is a halophyte. Some species e.g., Selaginella oregana, Psilotum flacidum, Lycopodium squarrosum and ferns like Asplenium nidus, Pleopeltis sp. are epiphytes. Marsilea occurs as a terrestrial, amphibious as well as an aquatic plant. There are true aquatics ferns like Salvinia, Azolla and Ceratopteris.

(i) The main independent plant body of pteridophytes is sporophyte. It is differentiated into true roots, true stem and true leaves.
(ii) The primary root is short lived. It is replaced by adventitious roots. The root has a permanent growing apex.
(iii) The stems are usually herbaceous (except in some woody ferns) and branched monopodially or dichotomously.
(iv) The leaves may be small microphyllous (e.g., Lycopodium, Equisetum) or large macrophyllous (e.g., Pteridium, Pteris and other ferns).
(v) All the vegetative parts possess vascular tissues (i.e., xylem and phloem) organized in definite groups or steles. Secondary growth does not occur in most of the living pteridophytes (except in Isoetes).

The pteridophyte generally possess a single apical cell with three cutting faces in the shoot apex. The root tip also has a single apical cell but with four cutting faces.

The plants of pteridophytes are sporophytes. They reproduce asexually by forming spores in sporangia. They are homosporous but a few plants are heterosporous also e.g., Isoetes, Selaginella, Marsilea, Regnellidium, Pilularia, Azolla and Salvinia. In Selaginella the sporangia are borne in relation to sporophylls which constitute a strobilus. In Equisetum they are borne on sporangiophores which constitute a cone. In ferns the sporangia are borne in sori on the sporophylls. The sori are of three types –
(i) Simple sorus : Here all the sporangia mature at the same time.
(ii) Gradate sorus : Here the oldest sporangium lies in the centre and the sporangia on either side show successively younger stages.
(iii) Mixed sorus : It shows mixed arrangement of younger and older sporangia.
In Marsilea, Azolla, Salvinia etc. the sori are present in a box like structure called sporocarp. The sorus may be naked or covered by an inducium. The inducia may be true or false. A true inducium is a specially developed structure whereas a false inducium is formed by curving of the sporophyll margin.

The sporangia are generally stalked structures. Each sporangium is distinguishable into a jacket enclosing a mass of sporogenous tissue. The sporangial jacket may be 2-4 layered. The innermost wall layer is the tapetum. No tapetum is formed in Psilotum and Tmesipteris. It is a nutritive layer which degenerates at maturity of the sporangium. The sporangial jacket in some ferns shows distinctions of annulus and stomium. On the basis of development the sporangia have been classified by Goebel, 1881 into two categories as under :

# (i) Eusporangiate type : Such a sporangium develops from a group of superficial initials. They divide periclinally into outer and inner components. The outer cells form the wall whereas the inner cells give rise to sporogenous tissue.

# (ii) Leptosporangiate type : Such a sporangium arises from a single superficial initial. It divides periclinally into outer and inner components. While the inner cell forms the stalk, the outer gives rise to sporagnium proper. In Marattia alata, the sporangia in a sorus may fuse to form a synangium.

The plants may be homosporous, i.e., produce only one type of spores (e.g., Lycopodium, Pteridium) or heterosporous i.e., produce two different types of spores, smaller microspores and larger – megaspores (e.g., Selaginella, Marsilea etc.). The spore germination is homosporous pteridophytes may be bipolar (e.g., Lycopodium, Equisetum) or tripolar (e.g., Hymenophyllum) or amorphous (e.g., Angiopteris).
The spores germinate to produce haploid gametophyte, called prothallus. The homosporous pteridophytes produce bisexual (monoecious) gametophytes whereas heterosporous one produce unisexual (dioecious) gametophytes.

The archegonia and antheridia are generally of embedded type. The archegonium consists of neck which usually projects from the surface of the prothallus. It contains 1-2 neck canal cells. There is no venter. The egg and the ventral canal cell remain surrounded by the cells of prothallus. The antheridia are generally sessile. They have a 1-cell thick jacket enclosing a mass of androgonial cells. They form the androcytes which are metamorphosed into biflagellate (Lycopodium, Selaginella) or multiflagellate (Equisetum, ferns) antherozoids.

Before fertilization the walls of androgonial cells get dissolved and antherozoids liberated. The neck canal cells and the ventral canal cell disorganise. The neck cells/cover cells separate to give a free passage to incoming antherozoids. Fertilization is affected by water medium (zooidogamous). The antherozoids are attracted towards the egg by a chemotactic stimulus provided by the degeneration of neck canal cell and ventral canal cell, in the form of malic acid.

As a result of fertilization the zygote is formed. It divides into an upper or anterior epibasal cell and a lower or posterior hypobasal cell. In Selaginella, Lycopodium the epibasal cell forms the suspensor and the hypobasal gives rise to embryo proper. By further segmentation a quadrant and then an octant is formed. Usually the epibasal quadrant forms the stem and root and the hypobasal gives rise to foot and root. The young sporophyte of pteridophytes is dependent upon the gametophyte for food which is drawn with the help of its foot. Like bryophytes, the pteridophytes also show heteromorphic alternation of generations.

The stelar theory was proposed by Van Tiegham and Douliot (1886). Some important types of steles found in pteridophytes are :
(i) Protostele : Solid core of xylem surrounded by phloem, pericycle and endodermis. The types of protosteles are :
(a) Haplostele : A protostele having a central smooth core of xylem surrounded by phloem, pericycle and endodermis e.g., Selaginella sp., Lygodium, etc.
(b) Actinostele : A protostele having star-shaped xylem core with radiating ribs e.g., Psilotum, Lycopodium serratum, etc.
(c) Plectostele : A protostele having xylem broken into parallel plates alternating with parallel phloem plates e.g., Lycopodium clavatum.
(d) Mixed protostele : A protostele having several xylem groups scattered and embedded in phloem e.g., Lycopodium cernuum.
(ii) Siphonostele : A stele having central pith. It is formed by medullation (or appearance of pith) in the protostele. The types of siphonosteles are :
(a) Ectophloic siphonostele : The central pith is surrounded by xylem, phloem, pericycle and endodermis. The phloem occurs only outside the xylem e.g., Osmunda.
(b) Amphiphloic siphonostele : The ring of xylem is surrounded on both outer and inner sides by phloem, pericycle and endodermis e.g., Marsilea rhizome, Adiantum pedatum rhizome.
Modification of siphonostele
(i) Cladosiphonic siphonostele : A siphonostele not perforated by leaf gaps e.g., a few species of Selaginella.
(ii) Phyllosiphonic siphonostele : A siphonostele perforated by leaf gaps – e.g., Nephrolepis.
(iii) Solenostele : A siphonostele perforated by leaf gaps which are scattered but not overlapping e.g., Ferns.
(iv) Dictyostele : A siphonostele perforated by several overlapping leaf gaps. Each separate strand is called meristele. e.g., Dryopteris, Pteridium, Pteris, etc.
(v) Polycyclic dictyostele : A dictyostele consisting of two or more concentric rings of meristeles e.g., Pteridium aquilinum.
(vi) Eustele : Much dissected siphonostele having vascular strands separated apart by parenchyma e.g., Equisetum.
Polysetelic condition : Presence of more than one stele e.g., Selaginella kraussiana.

The sporophytes reproduce asexually producing spores in sporangia. When all the spores are alike i.e., almost of the same size, the phenomenon is called homospory. However, in some pteridophytes, two types of spores are formed which differ significantly in their size as also in function. This phenomenon is called as heterospory. It is seen in pteridophytes like Selaginella, Isoetes, Stylites, Marsilea, Regnellidium, Pilularia, Azolla, Salvinia and Platyzoma.
It is believed that during the course of evolution, heterothallism was followed by heterospory. Some homosporous ferns e.g., Equisetum, Ceratopteris produced two types of gametophyte, thus representing incepient heterospory.

The terms apogamy was coined by de Bary (1878). It is defined as formation of sporophyte from a gametophytic cell other than egg without fertilization. It was first observed by Farlow (1874) in Pteris cretica. Thereafter it was observed in several other plants e.g., Lycopodium, Selaginella, Nephrodium, Lastrea, Marsilea etc. The apogamy is of two types, obligate and facultative. If either both of the sex organs are nonfunctional or absent, the apogamy occurring on account of this is said to be obligate. But if a gametophytic cell is induced to form the sporophyte without fertilization, the apogamy is called as facultative or induced.

The formation of gametophyte from a sporophytic cell without meiosis is called as apospory. This phenomenon was first observed by Druery (1884) in Athyrium filix-femina. Thereafter it has been established in several pteridophytes. e.g., Pteridium aquilinum, Asplenium, Trichomanes etc. Induced apospory was seen in Pteris species.

Formation of sporophyte from egg without fertilization is called as parthenogenesis. Farmer and Digby (1907) observed that in homosporous, leptosporangiate ferns, apospory was always followed by parthenogenesis. This phenomenon has been observed in several species of Selaginella and Marsilea.

The pteridophytes occupy an intermediate place between bryophytes and spermatophytes. They represent affinities with both the groups.

# (i) Similarities with bryophytes
(a) Both have terrestrial mode of life.
(b) Water is indispensable for the process of fertilization.
(c) Male gametes are flagellated.
(d) The structure and ontogeny of sex organs i.e. antheridium and archegonium is based on similar pattern.
(e) Both the groups have definite alternation of sporophytic and gametophytic generations.
(f) Sexual reproduction is of oogamous type. Zygote is retained within the venter of archegonium to form embryo.
(g) Sex organs are surrounded by sterile jacket.
(h) The young sporophyte is partially or wholly dependent on the gametophyte for nourishment.

# (ii) Similarities with spermatophytes
(a) In both the cases, sporophytic plant body is large, independently existing and dominant phase of life cycle.
(b) The plant body is differentiated into true stem, leaves and roots.
(c) Vascular tissue is present.
(d) Spores are produced inside the sporangia.
(e) Presence of distinct alternation of generations.
(f) Process of photosynthesis is mainly confined to leaves. Stomata are present on the leaves.

The pteridophyta have a long fossil history. Their first traces were identified in the silurian period of paleozoic age (about 400 million years age). Pteridophytes flourished well during devonian, mississipian and pensylvanian periods of late paleozoic age. This period can be well recognised as "age of pteridophyta".

Eichler (1883) classified the plant kingdom into Cryptogamia and Phanerogamia. The Cryptogamia was further divided into Thallophyta, Bryophytes and Pteridophyta. Engler (1909) included Bryophyta and Pteridophyta under Embryophyta. Sinnott (1935) introduced the term Tracheophyta. A years later Eames (1936) classified Tracheophyta into four groups namely, Psilopsida Lycopsida, Sphenopsida and Pteropsida, and Pteropsida into Filicinae, Gymnospermae and Angiospermae. Smith (1938) classified pteridophytes into four classes namely Psilophytineae, Lycopodineae, Equisetineae and Filicineae. Oswald and Tippo (1942) classified pteridophytes in to four sub-phyla.
(i) Psilopsida
(ii) Lycopsida
(iii) Sphenopsida
(iv) Pteropsida

# Salient features of sub-phyla
- (i) Sub-phylum : Psilopsida
(a) These are the oldest known vascular plants, most of them (except Psilotum and Tmesipteris) are fossils.
(b) Plant body is relatively less differentiated.
(c) Roots are absent, instead dichotomously branched rhizome is present.
(d) Aerial axis is either naked or have small spirally arranged leaves.
(e) Sporangia are cauline (i.e., directly borne on the axis or stem); they are lateral or terminal in position. e.g., Psilotum, Tmesipteris.

- (ii) Sub-Phylum : Lycopsida
(a) Plant body is differentiated into root, stem and leaves.
(b) Leaves small (i.e., microphyllous) with a single unbranched vein.
(c) Sporangia develop in the axil of the sporophylls.
(d) Sporophylls generally form compact strobili. e.g., Lycopodium, Selaginella, etc.

- (iii) Sub-Phylum : Sphenopsida
(a) Stem differentiated into nodes and internodes.
(b) Leaves microphyllous, present in whorls at each node.
(c) Sporangia are borne on the sporangiophores which form compact cones at the apex of the fertile branches. e.g., Equisetum.

- (iv) Sub-Phylum : Pteropsida
(a) Plant body well differentiated into root, stem and leaves.
(b) Leaves megaphyllous, pinnately compound.
(c) Sporangia develop on the ventral surface of the sporophylls, usually aggregated into sori. e.g., Dryopteris, Pteris, Pteridium, Polypodium, etc.

(i) Ornamental value : Many ferns are grown as ornamental plants in gardens for their large, Show and graceful foliage. e.g., species of Lycopodium, Nephrolepis, Selaginella, Lygodium, Anemia, Cyathea etc.
(ii) Medicines : An anthelmintic drug is obtained from the rhizomes and petioles of the fern Dryopteris. Lycopodium clavatum is used in skin diseases. Equisetum arvense has diuretic properties.
(iii) Food : The sporocarps of Marsilea are rich source of starch and used by tribals for their nutritive value.
(iv) Soil conservation : Plants like Selaginella are useful in soil conservation.