Pteridophyta (from Greek: pteron=feather, phyton=plant) are the most primitive vascular plants, representing an evolutionary step between non-vascular bryophytes and seed-bearing gymnosperms. Key characteristics distinguishing pteridophytes from other plant groups: Vascular tissue (xylem and phloem) — absent in bryophytes, present in all seed plants. No seeds — reproduce by spores (cryptogams). Sporophyte is the dominant, photosynthetic generation — gametophyte is small, independent, and called a prothallus (unlike bryophytes where gametophyte dominates, and unlike seed plants where gametophyte is completely dependent on sporophyte). True roots, stems, and leaves (megaphylls or microphylls depending on the group). They flourished during the Carboniferous period (~359-299 million years ago) as dominant forest trees (Lepidodendron, Sigillaria — giant lycopsids; Calamites — giant sphenopsids), eventually fossilising to form much of the world's coal deposits.

Sphenopsida, also called Equisetopsida or the horsetail class, represents one of the most ancient and morphologically distinctive lineages of vascular plants, with a fossil record extending back approximately 400 million years. During the Carboniferous period, sphenopsids were represented by giant arborescent (tree-like) forms called Calamites, reaching heights of up to 30 metres and dominating vast coal swamp forests. Today, the class has a single surviving genus, Equisetum, containing approximately 15-25 species that are globally distributed except in Australasia. Equisetum plants are characterised by their jointed, hollow, ribbed aerial stems; whorled, scale-like, non-photosynthetic microphylls (fused into sheaths at nodes); siliceous (silica-impregnated) stem walls that make the plants rough and historically useful for scouring; green photosynthetic stems that compensate for the non-functional leaves; underground rhizomes enabling vegetative reproduction; and reproduction by spores produced in sporangia on umbrella-shaped sporangiophores arranged in terminal strobili (cone-like structures) on fertile shoots.

Understanding the distinctions among the four pteridophyte classes helps in classification and identification. Psilopsida (Psilotum): the most primitive class, sometimes called "living fossils" of pteridophytes. True roots absent (rhizoids); stems dichotomously branched with small, scale-like appendages (enations, not true leaves); sporangia fused in synangia on short lateral branches; prothallus is subterranean, mycorrhiza-dependent. Lycopsida (Lycopodium, Selaginella, Isoetes): true roots, stems, and microphylls; sporangia in strobili (cones); Selaginella is notable for being heterosporous (producing two types of spores: microspores and megaspores), representing an intermediate evolutionary stage toward seed plant reproduction. Sphenopsida (Equisetum): jointed stems, whorled microphylls, siliceous stems, strobili; single surviving genus. Pteropsida (ferns): the largest and most familiar class; true megaphylls (large, complex leaves called fronds) showing circinate vernation; sporangia clustered in sori on lower leaf surface; diverse and abundant, with thousands of living species.

Pteridophytes, despite being overshadowed in modern ecosystems by flowering plants, have significant ecological and economic roles. Coal formation: ancient giant pteridophytes (Lepidodendron, Sigillaria, Calamites) formed the vast coal deposits of the Carboniferous period, which power much of modern industrial civilisation. Soil formation and erosion control: ferns and other pteridophytes colonise disturbed habitats, contributing to succession and preventing erosion; Equisetum is particularly aggressive in colonising disturbed soils. Nitrogen fixation: Azolla, a floating water fern, has a mutualistic association with the nitrogen-fixing cyanobacterium Anabaena azollae in its leaves; Azolla is widely used as a green manure/biofertiliser in rice paddies in Asia, fixing significant amounts of atmospheric nitrogen. Ornamental horticulture: many fern species (Nephrolepis, Adiantum, Asplenium, Platycerium) are popular ornamental and houseplants. Traditional medicine: various ferns are used in traditional medicine systems worldwide. Indicator plants: the presence of certain fern species indicates specific soil conditions, helping botanists assess soil chemistry and moisture.