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Pteridophytes
Introduction :
The pteridophytes (Gk. Pteron = feather and phyton = plants ; means plants with feather like fronds or ferns). They are flowerless, seedless, spore producing vascular plant which have successfully invaded the land. Pteridophytes represent an intermediate position between bryophytes and spermatophytes (Gymnosperm and Angiosperm). It is classified by Carolus Linnaeus (1754) under the class cryptogamia. They are also called vascular cryptogams. The group has a long fossil history. It is the most primitive group that flourished in Devonian and Carboniferous periods of Palaeozoic Era.
Pteridophytes are known as reptiles of plant kingdom.
Type of vascular bundle - Concentric, Amphicribal, mesrch condition and closed.
Note : Secondary growth is absent (due to absence of cambium) in pteridophytes but exceptionally secondary growth is present in stem of Isoets.
Habitat :
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.
they are more adapted terrestrial plants as compared to bryophytes. Because –
(i) Vascular tissue is present in pteridophytes.
(ii) They have roots.
Sporophytic plant body
(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).
Apical growth : 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.
LIFE CYCLE OF PHERIDOPHYTES
Plant is sporophyte i.e. diploid.
Most of the pteriophytes are homosporus i.e. only one type of spores are formed during reproduction. eg. Lycopodium, Pteridiu, Equisetum, Dryopteris
Exception : Some pteridophytes are heterosporus i.e. two types of spores microspores and megaspores eg. Selaginella, Isoetes, Marsilea, Salvinia, Azolla, Stylites.
Formation of spores takes place in sporangia. Sporangia is formed at the abaxial surface of leaves.
The leaves on which sporangia are formed are called sporohylls (reproductive leaves) and normal photosynthetic leaves are called tropophyllas (vegetative leaves). Sporngia are present in groups, there groups are called sorus. Many sorus are found on sporophylls. The groups of sorus are called sori.
Key Note :
(A) In pteridophyta, sprophylls are also photosynthetic. This is a unique character of pteridophyta.
(B) In pteridophytes development of sporangia takes place by two method -
(a) Leptosporangiate - Each sporangium develops from single initial cell.
(b) Eusporangiate - Each sporangium develops from a group of initial cells.
Eusporangiate is advanced character because all higher plants are eusporangiate.
Spore mother cells are present in sporangia. Spores are formed in these spore mother cells by meiosis and these spores start the gametophytic generation.
In pteridophyta, the germination of spores is exosporic i.e. germination takes place out side the sporangia (in soil)
In soil, a gametophyte is formed by the germination of each spore, which is known prothallus. The formation of gametophyte takes place in the soil therefore it is free (independent) and autotrophic.
There is no relation between the main sporophytic plant and prothallus. Prothallus (gametophyte plant) is made up of thallus and rhizoids. It is non vascular.
Key Note : IIn plant kingdom gametophyte is always non vascular.
Prothallus (gametophyte) is monoecious but in heterosporus pteridophytes prothallus is dioecious.
In heterosporus pteridophytes
Microspores form - Male gametophyte
megaspores form - Female gametophyte
The formation of sex organs takes place on this gametophyte. Male sex organs are called as antheridium and female sex organs are called as archegonium. The formation of male gametes takes place in Antheridia which are called as antherozoids. Antherooids are spiral and mutliflagellate but exceptionally antherozoied of Selaginell are spindle shaped and Lycopodium is curved (comma) and biflagellate. Egg is formed is archegonia.
Fertilization takes place by zoodiogamy and zygote is formed as a result of fertilization. Zygote develops and forms and embryo. Now this embryo develops and forms a sprophytic plant with root, stem, leaf.
Type of sexual repro. in pteridophyta is oogamous.
Their life cycle is diplo-haplonitc type.
The unique character of life cycle of Petridophyte is - Independent alternation of generation i.e. sporophyte and gametophyte are separate to each other.
Affinities of pteridophytes :
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
Classification :
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
Psilotum
(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
Lycopodium
(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
Equisetum
(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
Pteris
(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.