11th Class Biology Plant Kingdom Pteridophyta

(Gk. Pteron = father/fern; phyton = plants)

The term pteridophyta was first introduced by Haeckel in (1866). The pteridophytes 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). They are also called vascular cryptogams. (The term cryptogams was coined by Linnaeus (1737) which means plants without seeds). The group has a long fossil history. Pteridophytes flourished well during devonian, mississipian and pensylvanian periods of late paleozoic age. This period can be well recognised as "age of pteridophyta".

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 aquatic ferns like Salvinia (Salvinia is root less pteridophyte), Azolla and Ceratopteris.

Sporophytic plant body

(1) The main independent plant body of pteridophytes is sporophyte. It is differentiated into true roots, true stem and true leaves.

(2) The primary root is short lived. It is replaced by adventitious roots. The root has a permanent growing apex.

(3) The stems are usually herbaceous (except in some woody ferns) and branched monopodially or dichotomously.

(4) The leaves may be small microphyllous (e.g., Lycopodium, Equisetum) or large macrophyllous (e.g., Pteridium, Pteris and other ferns).

(5) All the vegetative parts possess vascular tissues (i.e., xylem and phloem) organized in definite groups or steles.

Spore producing organs

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 :

(1) Simple sorus : Here all the sporangia mature at the same time.

(2) Gradate sorus : Here the oldest sporangium lies in the centre and the sporangia on either side show successively younger stages.

(3) 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.

Sporangium

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 :

(1) 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. e.g., Selaginella, Equisetum, Lycopodium.

(2) 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. e.g., Salvinia, Azolla.

Spore

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.

Sex organs and Fertilization

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 venter 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.

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 venter canal cell, in the form of malic acid.

Embryogeny

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.

Stelar system

The stelar theory was proposed by Van Tiegham and Douliot (1886). Some important types of steles found in pteridophytes are :

(1) Protostele : Solid core of xylem surrounded by phloem, pericycle and endodermis. The types of protosteles are :

Haplostele : A protostele having a central smooth core of xylem surrounded by phloem, pericycle and endodermis e.g., Selaginella sp., Lygodium, etc.

Actinostele : A protostele having star-shaped xylem core with radiating ribs e.g., Psilotum, Lycopodium serratum, etc.

Plectostele : A protostele having xylem broken into parallel plates alternating with parallel phloem plates e.g., Lycopodium clavatum.

Mixed protostele : A protostele having several xylem groups scattered and embedded in phloem e.g., Lycopodium cernuum.

(2) Siphonostele : A stele having central pith. It is formed by medullation (or appearance of pith) in the protostele. The types of siphonosteles are :

Ectophloic siphonostele : The central pith is surrounded by xylem, phloem, pericycle and endodermis. The phloem occurs only outside the xylem e.g., Osmunda.

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

(1) Cladosiphonic siphonostele : A siphonostele not perforated by leaf gaps e.g., a few species of Selaginella.

(2) Phyllosiphonic siphonostele : A siphonostele perforated by leaf gaps e.g., Nephrolepis.

(3) Solenostele : A siphonostele perforated by leaf gaps which are scattered but not overlapping e.g., Ferns.

(4) Dictyostele : A siphonostele perforated by several overlapping leaf gaps. Each separate strand is called meristele. e.g., Dryopteris, Pteridium, Pteris, etc.

(5) Polycyclic dictyostele : A dictyostele consisting of two or more concentric rings of meristeles e.g., Pteridium aquilinum.

(6) Eustele : Much dissected siphonostele having vascular strands separated apart by parenchyma e.g., Equisetum.

Heterospory

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.