Bryophytes, a diverse group of plants, lack seeds, a key characteristic distinguishing them from many other plant groups. Understanding this involves exploring their unique biological features and evolutionary history.
What Are Bryophytes?
Bryophytes are non-vascular land plants, lacking specialized internal tissues (xylem and phloem) for water and nutrient transport. This absence of a true vascular system limits their size, keeping them small and close to the ground. They prefer moist environments, though some species endure drier conditions. This group includes about 20,000 species worldwide, such as mosses, liverworts, and hornworts. Instead of true roots, they have hair-like rhizoids that anchor them to surfaces.
Bryophytes and Seed Production
Bryophytes do not produce seeds. These complex structures, characteristic of more evolutionarily advanced seed plants, are absent due to bryophytes’ simpler structure and lack of true vascular tissue. As seedless plants, they rely on alternative methods for reproduction and dispersal.
How Bryophytes Reproduce
Bryophytes reproduce using spores, not seeds, through an alternation of generations between haploid gametophyte and diploid sporophyte stages. The dominant stage is the gametophyte, the green, photosynthetic part that produces gametes (sperm and eggs). Male gametes, which possess flagella, require a film of water to reach female gametes for fertilization.
Following fertilization, a diploid zygote forms and develops into a sporophyte, which remains attached to and dependent on the gametophyte for nutrients. The sporophyte produces haploid spores through meiosis within a sporangium or capsule. These spores are released, dispersed by wind, and can germinate in moist environments to grow into new gametophytes. Bryophytes can also reproduce asexually through fragmentation or the production of gemmae, small propagules that develop into new plants.
Seeds Versus Spores
Seeds offer advantages over spores, contributing to plant colonization of diverse terrestrial environments. Seeds contain a stored food supply that nourishes the developing embryo, allowing it to establish itself before photosynthesis. They are also protected by a tough seed coat, shielding the embryo from physical damage and environmental stressors like desiccation.
Seeds can remain dormant for extended periods, waiting for optimal conditions to germinate, unlike most spores which germinate immediately. This dormancy allows seed plants to survive unfavorable conditions like extreme temperatures or drought. Seeds also facilitate dispersal over long distances by agents like wind, water, or animals, expanding a plant’s geographic range and reducing competition with the parent plant. The evolution of seeds represented an adaptation, enabling plants to become less dependent on water for reproduction and to thrive in more habitats than bryophytes, which remain limited by their reliance on moisture for fertilization and spore dispersal.