Pteridophytes: General features, alternation of generation

Pteridophytes:-

> Pteridophyta:- Gr.- Pteron = feather

                                  phyton = plant 

The name was originally given to those groups of plants which have well developed pinnate or frond like leaves, which are called as ferns.

> Pteridology:- The branch of Botany deals with the study of ferns and fern like plants, is called as Pteridology.

Gr.- Pteron = feather

       logos = study 

> Father of Pteridology:- F. O. Bower

> Father of Indian Pteridology:- S. S. Bir

> Vascular cryptogames:- Pteridophytes are cryptogams which have well developed vascular tissue. Therefore, these plants are also known as vascular cryptogams.

> Reptiles of plant kingdom:- Pteridophytes have horizontal stem, rhizome. Hence Pteridophytes are also known as reptiles of the plant kingdom. In forests, ferns are found in abundance, these ferns are pteridophytes.

> They are represented by about 400 living and fossil genera and some 10,500 species. 

> Palaeobotanical studies reveal that these plants were dominant on the earth during the Devonian period and they were originated about 400 million years ago in the Silurian period of the Palaeozoic era.

General Features of Pteridophytes:-

1. Plant body:-

> Majority of the living Pteridophytes are terrestrial and prefer to grow in cool, moist and shady places e.g., ferns. Some members are aquatic (Eg., Marsileci, Azolla), xerophytic (Eg., Selaginella rupestris, Equisetum) or epiphytic (Eg.,Lycopodium squarrosum).

> Majority of the Pteridophytes are herbaceous but a few are perennial and tree like (Eg., Angiopteris). Smallest Pteridophyte is Azolla (an aquatic fern) and largest is Cyathea (tree fern).

> Plant body is sporophytic and can be differentiated into root, stem and leaves.

> Roots are adventitious in nature with monopodial or dichotomous branching. Internally usually they are diarch.

> Stem is usually branched. Branching is monopodial or dichotomous. Branches do not arise in the axil of the leaves. In many Pteridophytes stem is represented by rhizome. 

> Leaves may be small, thin, scaly (microphyllous Eg., Equisetum), simple and sessile (Eg., Selaginella) or large and pinnately compound (megaphyllous Eg., Dryopteris, Adiantum).

> Vascular tissue is present in stem and root. It consists of xylem and phloem. Xylem consists of tracheids only and phloem has only sieve tubes.

> The steel is protostele (Eg., Rhynia, Lycopodium), siphonostele (Eg., Equisetum), dictyostele Adiantum) or polycyclic (Eg., Angiopteris).

> Cambium is absent; hence, they do not show secondary growth.

2. Reproduction in Pteridophytes:-

> Reproduction takes place by means of spores which are produced inside sporangia.

> The development of the sporangium may be leptosporangiate (sporangium originates from a single cell) or eusporangiate (sporangium develops from a group of cells).

> Sporangia may be borne either on stem or leaves. On the stem they may be terminal (Eg., Rhynia) or lateral (Eg., Lycopodium). On the leaves (sporophylls) they may be ventral, marginal (Pteris, Adiantum) or dorsal (Eg., Polypodiceae). In Equisetum the sporangia are borne on special structures called sporangiophores which constitute a cone. In Marsilea, Azolla, Salvinia sporangia are produced in sporocarps.

> Spores on germination give rise to multicellular gametophytic bodies called prothalli (sing. prothallus).

> In homosporous Pteridophytes prothalli are monoecious (antheridia and archegonia develop on the same prothallus). In heterosporous species prothalli are always dioecious. Microspores on germination give rise to male prothalli and megaspores to the female prothalli.

> Antheridia and archegonia are developed on prothalli.

> Antheridium is surrounded by a single layered sterile jacket.

> Archegonium consists of four vertical rows of neck cells, 1-2 neck canal cells, ventral canal cell and egg.

> Antherozoids are unicellular, biflagellate (e.g., Selaginella) or multiflagellate (e.g., Equisetum and ferns) and motile.

> Antherozoids are attracted towards the neck of the archegonium chemotactically by certain substances like malic acid) present in the mucilaginous substance formed by the degeneration of neck canal cells and venter canal cell.

> Water is essential for fertilization (zooidogamous).

> Fertilization results in the formation of zygote or oospore, which ultimately develops into well-developed sporophyte.

> The fertilized egg divides transversely or vertically. Another cross wall forms a quadrant stage producing stem, leaf, foot and root.

> Plants show heteromorphic alternation of generation. The main plant body is sporophytic and forms a dominant phase in the life cycle.

3. Development of Sporangia in Pteridophytes:- On the basis of development, the sporangia in Pteridophytes are divided into two types:

a. Eusporangiate type

b. Leptosporangiate type

a. Eusporangiate Type:- Sporangium develops from group of superficial cells. These cells divide periclinally into primary wall layers and inner primary sporogenous cells. The outer wall layers form the wall of the sporangium while inner sporogenous cells divide meiotically and form spores.

b. Leptosporangiate Type:- This type of sporangium arises from a single superficial cell. It divides transversely to form an outer and an inner cell. While the inner cell forms the stalk, the entire sporangium develops from the outer cell. The outer cell divides by three successive periclinal divisions and in this way a tetrahedral apical cell is formed.

Alternation of generation in Pteridophytes:-

> Pteridophytes show heteromorphic alternation of generation. 

> The main plant body is sporophytic and forms a dominant phase in the life cycle. 

> Sporophytic plant body develops sporangia in which sporogenous tissue is formed. Sporogenous tissue divides meiotically to form haploid spores.

> Majority of the Pteridophytes are homosporous e.g., Lycopodium, Pteris etc. Spores on germination produce monoecious gametophyte. 

> Some Pteridophytes are heterosporous and produce two types of spores: microspores and megaspores. Microspores on germination produce male gametophyte (prothallus) while megaspores on germination produce female gametophyte (prothallus). So, the prothalli are dioecious. 

Note:- Only 7 living Pteridophytes are heterosporous:

i. Selaginella

ii. Isoetes

iii. Salvinia

iv. Azolla

v. Marsilea

vi. Pilularia

vii. Ragnilidium

> Antheridia and archegonia develop on the same prothallus (monoecious) or on different prothalli (dioecious). 

> The male and female gametes fuse to form zygote which develops into sporophyte. 

> Thus, the life cycle of a Pteridophyte consists of an alternate succession of sporophytic and gametophytic generations.

Affinities of Pteridophytes:- 

Similarities with Gymnosperms:-

i. Plant body is sporophytic, dominant and can be differentiated into root, stem and leaves in both the groups.

ii. Gametophytic phase is of short duration.

iii. Young leaves show circinate vernation.

iv. Vascular tissue is well developed. Xylem lacks vessels (except in order Gnetales of Gymnosperms) and companion cells are absent in phloem).

v. Like Gymnosperms many Pteridophytes are heterosporous (Eg., Marsilea, Selaginella).

vi. Like Pteridophytes many Gymnosperms show ciliate antherozoids (Eg., Cycas, Ginkgo).

vii. Like Gymnosperms, in some Pteridophytes megaspore is retained within the megasporangium (Eg., Selaginella).

viii. Regular alternation of sporophytic and gametophytic phase is present.