Pteridophytes are a diverse group of seedless, vascular plants. They evolved roughly 400 million years ago, distinguishing them from simpler, non-vascular plants like mosses and liverworts. They were the first plants to develop a specialized internal transport system, allowing them to grow taller and colonize diverse terrestrial habitats. Pteridophytes use spores for dispersal, rather than the seeds found in later plant groups.
Defining Characteristics and Structure
The defining feature of pteridophytes is the presence of vascular tissues, making them part of the larger group known as tracheophytes. This specialized system consists of xylem, which transports water and dissolved minerals upward from the roots, and phloem, which moves sugars produced during photosynthesis throughout the plant body. The development of this internal plumbing allowed them to overcome the size constraints that limit non-vascular plants, leading to the evolution of larger, more complex plant bodies.
The main plant body is the sporophyte, a diploid generation recognized as the familiar fern or horsetail plant. This sporophyte is differentiated into true roots, stems, and leaves. Stems often take the form of underground, horizontal structures called rhizomes, from which the roots and leaves emerge. Leaves in ferns are often large and complex, referred to as fronds (megaphylls), while groups like club mosses possess smaller, simpler leaves called microphylls.
The Reproductive Life Cycle
Pteridophytes utilize alternation of generations, alternating between two distinct, free-living forms: the diploid sporophyte and the haploid gametophyte. The dominant plant is the sporophyte, which produces spores housed within structures called sporangia. In ferns, sporangia are often clustered in groups known as sori, typically located on the underside of the fronds.
Inside the sporangia, spore mother cells undergo meiosis, resulting in the production of numerous tiny, haploid spores. Once mature, these spores are released and dispersed, usually by wind. If a spore lands in a suitable, moist environment, it germinates and grows into the small, inconspicuous gametophyte generation, commonly called a prothallus.
The prothallus is a small, heart-shaped, photosynthetic structure independent of the sporophyte. It produces the sexual organs: the antheridia, which generate motile sperm, and the archegonia, which contain a single egg cell. Fertilization requires a layer of water, allowing the flagellated sperm to swim to the egg. The resulting fusion forms a diploid zygote, which develops into a new, mature sporophyte, completing the cycle.
Primary Classification Groups
Pteridophytes are classified into two major lineages: the Lycophytes and the Monilophytes.
Lycophytes
Lycophytes, including club mosses and spike mosses, are characterized by their microphylls—small leaves with a single, unbranched vein. Despite their common names, these plants are not true mosses and often possess cone-like structures called strobili, where their spores are produced.
Monilophytes
The Monilophytes include true ferns, horsetails, and whisk ferns, generally characterized by having megaphylls (larger, more complex fronds). True ferns are the most numerous group, recognized by their large, compound leaves that unroll from a tight coil called a fiddlehead. They thrive in moist, shaded habitats globally.
Horsetails (Equisetum) are distinguished by their unique, jointed, hollow stems that contain abrasive silica. They possess whorls of small, non-photosynthetic leaves and feature reproductive stems bearing strobili at the apex. Whisk ferns (Psilotales) are a highly reduced group lacking true roots and leaves; their green, branched stems conduct photosynthesis and bear small spore-producing structures.