Seedless Vascular Plants Slides
Seedless vascular plants include lycophytes, whisk ferns, ferns (Pteridophyta), and horsetails (Equisetophyta). These plants possess vascular tissue (xylem and phloem) and lignin, which enable greater height, structural rigidity, and efficient long-distance transport compared to nonvascular plants. They reproduce via spores and do not produce seeds.
Key Features:
Vascular Tissue: Composed of:
Xylem: Conducts water, made of tracheids and vessel elements, rigid due to lignin.
Phloem: Conducts photosynthates (sugars) using sieve elements.
Lignin: Provides structural support and waterproofing to vascular tissue, facilitating taller growth.
Dominant Sporophyte (2n): The diploid sporophyte generation is more prominent and often larger than the haploid gametophyte (n).
Reproduction via Spores: Spores are produced in sporangia and dispersed by wind.
Water-Dependent Fertilization: Sperm require water to swim to the egg for fertilization.
Spore Types: Some lineages are homosporous (producing one type of spore that develops into a bisexual gametophyte), while others are heterosporous (producing two types of spores: microspores and megaspores, leading to separate male and female gametophytes).
Core Life Cycle Concepts:
Alternation of Generations: The life cycle alternates between a haploid gametophyte (n) and a diploid sporophyte (2n) stage.
Sporophyte Stage (2n):
Mature sporophyte produces sporangia.
Meiosis within sporangia produces haploid spores (n).
Gametophyte Stage (n):
Spores germinate and grow into a haploid gametophyte.
Antheridia: Male gametangia on the gametophyte produce sperm.
Archegonia: Female gametangia on the gametophyte produce eggs.
Fertilization: Sperm swim to the egg, resulting in a diploid zygote (2n).
Development: The zygote grows into a mature sporophyte (2n).
Major Groups of Seedless Vascular Plants:
Lycophyta (Club Mosses):
Characterized by microphylls (small leaves with a single, unbranched vein).
Sporangia are often grouped into cone-like structures called strobili.
Examples: Lycopodium (homosporous), Selaginella deflexa (heterosporous).
Psilotophyta (Whisk Ferns):
Considered the simplest vascular plants, lacking true roots or leaves.
Possess large sporangia.
Pteridophyta (Ferns):
The largest and most diverse group of seedless vascular plants (approximately 13,000 species).
Feature megaphylls (larger leaves with multiple, branching veins and webbing).
Sporangia are typically clustered into sori on the undersides of leaf fronds.
Gametophytes are separate, small, and heart-shaped.
Equisetophyta (Horsetails):
An ancient lineage, known as "scouring rushes" due to silica in their stems.
Leaves are highly reduced, with stems being the primary photosynthetic organs.
Sporangia are borne in strobili, similar to some lycophytes.
Evolutionary Significance:
The evolution of vascular tissue, lignin, and dominant sporophytes enabled these plants to grow taller and expand into new terrestrial environments.
The development of heterospory in some lineages was a crucial step towards the evolution and eventual dominance of seed plants.
Quick-reference glossary
Sorus: cluster of sporangia on a fern leaf.
Sporangia: structures that produce spores.
Spore: a haploid reproductive cell that germinates into a gametophyte (n).
Archegonium: female gametangium producing eggs.
Antheridium: male gametangium producing sperm.
Microphyll: small leaf with a single vein.
Megaphyll: large leaf with multiple veins and webbing.
Strobilus: cone-like structure bearing sporangia in horsetails and some lycophytes.
Heterospory: production of two distinct spore sizes (microspores and megaspores).
Homospory: production of a single spore type that typically gives rise to a bisexual gametophyte.
Xylem: water-conducting tissue with tracheids/vessel elements; lignin provides structural support.
Phloem: food-conducting tissue (sucrose and other photosynthates) via sieve elements.