Characteristic Features of Pteridophytes
The Pteridophytes are an assemblage of seedless vascular plants that have successfully invaded the land and reproduce by means of spores.
The Pteridophytes are also called ‘Vascular cryptogams’. The word ‘cryptogams’ is a synthesis of two Greek terms Kruptos-hidden and gamos-wedded i.e. the single term cryptogams represents all plants that reproduce by means of spores and, do not produce seeds. The word ‘vascular’ refers having a conducting system. It is, therefore, the pteridophytes are also referred to as ‘vascular cryptogams’. However, the name ‘pteridophyta’ (Greek, Pteron = feather) was originally give to this group of plants because of their pinnate or feather like leaves.
The pteridophytes have their long fossil history and their first appearance on earth are traced during the Silurian period of late Palaeozoic age (about 400 million years back). They flourished well during Devonian, Mississippian and Pennsylvanian periods of the late Paleozoic, the late Paleozoic is, therefore, regarded as the “Age of Pteridophyta”. The auborescent lycopods, the giant horsetails and tree ferns dominated the earth of those ancient ages. The lycopods like Lycopodium, Selaginella, Isoetes etc. and sphenopsides like Equisetum of present day are the more relicts of the mighty lycopsids and sphenosids which ones were represented by 120 feet Lepidodendron, Sigillaria and Calamies.
Characteristic Features of Pteridophytes
Plant Body: Adult Sporophyte
Majority of the living pteridophyts are terrestrial and flourish in moist and shady places. Some mebers (e.g., Azolla, Marsilea, Salvinia) are aquatic, usually occurring in ponds. A few pteridophytes are xerophytic (e.g., Selaginella nupestris, Equisetum arvense).
(b) External Features-
- The plant body pteridophytes is sporophytic and differentiated into root, stem and leaves. The mature sporophyte is nutritionally independent of the gametophyte. They show much variation in their form, size and habit ranging from small annuals (e.g., Azolla, Salvinia) to large tree like perennials (e.g., Angiopteris, Osmundo)
- The stem is generally branched. The branching is monopodial or dichotomous and the branches do not arise from the axils of leaves (axillary) as in spematophytes.
- The leaves are sealy (e.g., Equisetum), small and sessile (e.g., Lycopodium, Selaginella), or large petiolate and compound (e.g., ferns). On the basis of the structure of leaves, pteridophytes are classified as; (i) microphyllous pteridophytes-those with simple uninerved leaves not forming a leat gap in the stem stele (e.g., Equisetum. Lycopodium, Selaginella), and (ii) megaphyllous pteridophytes-Those with large pinnatified leaves having a complex series of veins and forming prominent leaf gaps in the stem stele (e.g., Pteris, Lastrea).
- The primary roots are ephemeral and are soon replaced by adventitious roots. The roots have a permanent growing apex.
- The leaves and stem show filiform trichomes. Stomata occur on both the upper and lower surface of the leaf.
(c) Internal Features-
- The root and stem possess well developed vascular system, made up of xylem and phloem. Cambium is, however, absent and hence the secondary growth is lacking.
- The stele is a protostele (e.g., Lycopodium, Selaginella), shiphomostele (e.g., Equisetum, Ophioglossum pendulum, Botrychium), dictrostele (e.g., Pteris, Nephrolepis, Polystichum) or polycydic (e.g., Angiopteris, Moratia).
- The xylem consists of tracheids and the phloem has sieve tubes only.
- The photosynthetic tissue is restricted mainly to the leaves. In megaphyllous leaves, the internal tissue is differentiated into spongy and palisade cells.
Life Cycle (Reproduction) of Pteridophytes
(a) Reproduction Structures-
- Pteridophytes reproduce by spores which are produced in sporangia. The sporangia are usually borne on the ventral surface of the leaf, but in some members (e.g., Ophioglossum) the fertile spike bearing sporangia is produced from the axil of the leaf. In Psilotum and some fossil members (e.g. Rhynia, Horneophyton) the sporangia are cauline.
- The plants are either homosporous (e.g. Equisetum, Lycopodium, Pteris) or heterosporous (e.g. Selaginella, Marsilea, Salvinia).
- The sporangia bearing leaves are called sporophylls. The sporophylls with megasporangia are called megasporophylls and those with microsporangia are called microsporophylls.
- The development of sporangia may be : (1) Eusporangiate as found in Psilotum, Lycopodium, Selaginella and Equisetum or (ii) Leptosporangiate as found in Salvinia, Marsilea, Pteris etc.
- Sporophylls are either uniformly distributed (e.g. Pteris, Adiantum and other ferns) or are aggregated into compact cones (known as strobili) at the apex of the stem (e.g. Selaginella, Lycopodium, Equisetum).
- In aquatic forms like Azolla, Salvinia and Marsilea the sporangia are produced within specialized structures, called sporocarps.
- In filicales the sporangia are produced in clusters, called sori (singular-sorus). The sori are of the following three types : Simple sorus: In this type of sorus, all the sporangia appear, grow and mature together i.e. g., Botrychium); Gradate sorus: In this type, the central part of the sorus has mature sporangia and the peripheral part immature sporangina (e.g. Marsilea); mixed sorus. Here mature and immature sporangia aggregated irregularly (e.g., Pteris).
- The spore wall possess various types of ornamentations.
- The spore, on germination, result in gametophytic generation. The germination of spore may be bipolar (e.g. Lycopodium, Equisetum), tripolar (e.g. Hymenophyllum) or amorphous or irregular (e.g. Angrropteris).
(b) The Gametophytes-
- In homosporous (e.g. Lycopodium, Equisetum), the development of gametophyte is exosporic (e.g. the gametophyte develops outside the spore wall), whereas in heterosporous species (e.g. Selaginella, Marsilea) it is endosporic (i:g. the gametophyte develops within the spore wall). There is much variation in the shape of the gametophyte archigonia are borne on.
- Most of the homosporous species are monoecious, i.e. antheridia the same prothallus. Same homosporous forms (e.g. Pteris, Pteridium) are also dioecious.
- The homosporous species are always dioecious. The microspore germinates to produce the male gametophyte and the megaspore to the female gametophyte.
- The sex organs are embedded in the prothallus (e.g. Lycopodium, Selaginella, Equisetum). Or and projected (e.g. Psilotum, Pteridium).
- The antheridium is surrounded by a sterile jacket which is always single layered in species with projected antheridia.
- The antherozoids are unicellular, uninucleate and spirally coiled with two apical flagella. Contrany to it they are multiflagellate in Psilotales and Equisetales.
- The antherozoide are liberated by bursting of the antheridium or separation of the terminal cell, known as opercular cell or cover cell.
- The archegonium is differentiated into neck and venter, the neck is projected and the venter is embedded in the prothallus.
- The neck consists of four vertical rows of cells and usually two neck canal cells. But some species have more than two (e.g. Lycopodium) or only one (e.g. Pteridium) neck canal cells. The neck canal cells degenerate before fertilization resulting in a mucilaginous substance which attracts antherozoides towards archegonium.
- Water is essential for fertilization. The antherozoide and egg fuse to form a diploid zygote or oospore which functions as the mother cell of the sporophytic generation.
(c) The Young Sporophyte-
- The zygote or cospore divides to from the embryo. The first division of the cospore determines the polarity (i.e. basal and apical part) of the young sporophyyte. The basal pole from foot and roots, whereas the apical pole gives rise to shoot apex.
- The embryo of pteridophytes are considered to be of two types on the basis of polarity (i) Exoscopic embryo-In which the apical pole is directed outward, i.e., towaeds the neck of the archegonium (e.g., Psilotum, Equisetum, Ophiloglossum); and (ii) Endoscopic embryo- In which the apical pole is directed towards the base of the archegonium, (e.g., Lycopodium, Selaginella, Isoetes).
- In both, exoscopic and endoscopic types of embryo development the first division of the oospore is transverse. But in leptosporangiate ferns, the first division is vertical and the embryo shows lateral polarity.
(d) Alternation of Generations-
The pteridophytes show a typical heteromorphic alternation of generations. Morphologically distinct sporophytic and gametophytic generations alternate with each other in regular succession. The main plant body is sporophytic and forms a dominant phase in the life cycle. Two types of life cycle are found in pteridophytes with respect to homosporous and heterosporous species.
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