# PTERIDOPHYTA (SEEDLESS VASCULAR PLANTS) Pteridophytes are the first true land plants possessing vascular tissues (xylem and phloem) but lacking seeds. They represent a significant evolutionary step from bryophytes. This group includes four main sub-divisions: Psilopsida, Lycopsida, Sphenopsida, and Pteropsida (of which the class Fillicinae, or ferns, are a part). ## General Characteristics of Pteridophytes * **Vascular Tissue:** They are *tracheophytes*, meaning they possess vascular tissues (xylem and phloem) for the transport of water and nutrients. * **Reproduction:** They are seedless and reproduce via spores. * **Life Cycle:** Exhibit a distinct *heteromorphic alternation of generations*, where the diploid sporophyte and haploid gametophyte are morphologically different. The **sporophyte generation is dominant**. * **Habitat:** Typically found in moist, shaded environments as water is required for fertilization. ## Sub-Divisions of Pteridophyta ### 1. Psilopsida (Whisk Ferns) * **Structure:** Simplest vascular plants. They lack true roots and leaves. * ***Rhizome:*** An underground stem for anchorage, possessing *rhizoids*. * ***Aerial Stems:*** Green, photosynthetic stems with small scale-like appendages. * **Reproduction:** *Sporangia* (spore-producing structures) are located at the tips or on the sides of aerial branches. * **Gametophyte:** A colorless, subterranean structure that lives in a symbiotic relationship with a fungus (*mycorrhiza*) for nutrition. * **Examples:** *Psilotum* (living), *Rhynia* (extinct). ### 2. Lycopsida (Club Mosses, Spike Mosses, Quillworts) * **Structure:** Plant body has a branching rhizome with aerial stems. * ***Microphylls:*** Stems and branches are covered in small, scale-like leaves, each with a single, unbranched vein of vascular tissue. * **Reproduction:** Sporangia are borne on terminal clusters of leaves called ***strobili*** (cones), which have a club-shaped appearance. * **Gametophyte:** Mostly an underground structure. * **Examples:** *Lycopodium* (Club moss), *Selaginella* (Spike moss). ### 3. Sphenopsida (Horsetails) * **Structure:** Also known as *arthrophytes* due to their jointed stems. * ***Rhizome:*** Gives rise to two types of aerial stems: fertile and sterile. * ***Aerial Stems:*** Slender, green, hollow, and jointed. * ***Leaves:*** Whorls of scale-like leaves are present only at the nodes (joints). Photosynthesis occurs mainly in the stem. * **Reproduction:** Spores are produced in *cones* (strobili) located at the tips of fertile shoots. * **Gametophyte:** A small, thalloid structure that grows on moist soil. * **Example:** *Equisetum* is the only surviving genus. ### 4. Pteropsida - Class Fillicinae (Ferns) * **General:** The most abundant and diverse group of seedless vascular plants, thriving in warm, moist tropics. * **Size:** Varies from tiny aquatic forms to large tree ferns. * **Dominance:** The **sporophyte generation** is large, complex, and independent. * **Leaves (Fronds):** Large, complex leaves (megaphylls) that serve both photosynthesis and reproduction. Young fronds exhibit ***circinate vernation*** (coiled like a fiddlehead). ## Life Cycle of a Fern (e.g., Adiantum) ### Sporophyte Generation (2n) * This is the main, recognizable fern plant. * **Structure:** * ***Rhizome:*** An underground, horizontal stem covered in brown scales called *ramenta*. * ***Roots:*** Adventitious roots arise from the rhizome. * ***Fronds (Leaves):*** Large, pinnately compound leaves. The main stalk is the *rachis*. Leaflets are *pinnae* (1st order) and *pinnules* (2nd order). * **Reproduction:** * ***Sori:*** Clusters of sporangia found on the underside of leaflets, often protected by a bent leaflet margin (*false-indusium*). * ***Sporangium:*** A capsule on a stalk containing spore mother cells. Its wall has a specialized ring of cells (*annulus*) and a weak point (*stomium*). * ***Spore Formation:*** Spore mother cells undergo **meiosis** to produce haploid (n) spores. * ***Spore Dispersal:*** The annulus contracts as it dries, tearing the sporangium at the stomium and flinging the spores out. ### Gametophyte Generation (n) * A germinating spore grows into the gametophyte, also called a **prothallus**. * **Structure:** * A small, heart-shaped, green, photosynthetic structure. * It is independent and anchors itself with *rhizoids*. * **Reproductive Organs (on the same prothallus):** * ***Antheridia:*** Globular structures that produce motile, multi-ciliated sperm (*antherozoids*). * ***Archegonia:*** Flask-shaped structures containing a single egg (*oosphere*). ### Fertilization 1. Water is essential for fertilization. 2. Antherozoids are released and swim towards the archegonia, attracted by chemical signals (*chemotaxis*). 3. One antherozoid fertilizes the egg, forming a diploid (2n) **zygote**. 4. The zygote develops into an embryo, which initially depends on the gametophyte but quickly grows into an independent **sporophyte**, completing the cycle. ## Summary * Pteridophytes are the first vascular plants, but they are seedless and rely on spores for reproduction. * The life cycle is an alternation of generations, with the **diploid sporophyte** being the dominant, long-lived phase. * The **haploid gametophyte** (prothallus in ferns) is a small but independent, photosynthetic plant. * Water is a limiting factor for reproduction, as it is required for the motile sperm to reach the egg. * The four main groups show a progression in structural complexity. | Group | Common Name | Key Distinguishing Features | Example | |--------------|--------------|-------------------------------------------------------------|-------------| | **Psilopsida** | Whisk Ferns | No true roots or leaves; dichotomously branching stems. | *Psilotum* | | **Lycopsida** | Club Mosses | Have microphylls (leaves with a single vein); spores in strobili. | *Lycopodium*| | **Sphenopsida** | Horsetails | Jointed, hollow, photosynthetic stems; scale-like leaves in whorls. | *Equisetum* | | **Fillicinae** | Ferns | Large leaves (fronds) with complex venation; circinate vernation. | *Adiantum* | **Biological Significance:** Pteridophytes represent a crucial evolutionary link between non-vascular bryophytes and seed-bearing plants. Their development of vascular tissue was a key adaptation for life on land, allowing them to grow taller and colonize more diverse habitats.