Non-vascular plants, such as mosses, liverworts, and hornworts, do not produce seeds. These plants are characterized by their lack of true roots, stems, and leaves, as well as the absence of vascular tissue for efficient water and nutrient transport. Their small stature and reliance on diffusion for internal transport mean they thrive in moist environments.
What Are Seeds?
A seed represents the embryonic stage of a plant’s life cycle, serving as a reproductive unit. Seeds consist of an embryo, stored food reserves, and a protective seed coat. The embryo is a miniature undeveloped plant, while the stored food provides nourishment during germination. The seed coat shields inner structures from environmental stressors and damage. Seeds enable plants to disperse, endure unfavorable conditions through dormancy, and nourish early growth.
How Non-Vascular Plants Reproduce
Non-vascular plants primarily reproduce using spores, which are distinct from seeds. Unlike multicellular seeds, spores are single-celled reproductive units. Their life cycle involves an alternation of generations, cycling between a haploid gametophyte and a diploid sporophyte stage. The visible, green parts of non-vascular plants, like mosses, are the gametophyte, which is the dominant phase.
Male gametophytes produce sperm in antheridia, while female gametophytes produce eggs in archegonia. For fertilization, sperm must swim through water to reach the egg. The resulting diploid zygote develops into a sporophyte, which remains attached to and dependent on the gametophyte for nutrients. The sporophyte then produces haploid spores through meiosis, which are released and dispersed, often by wind. If a spore lands in a moist environment, it germinates into a protonema that eventually grows into a new gametophyte, completing the cycle.
The Fundamental Difference in Plant Reproduction
The core difference between seed and spore reproduction lies in their structural complexity and survival strategies. Spores are unicellular and lack the stored food reserves found in seeds, making them more dependent on immediate favorable conditions for germination. In contrast, seeds contain a multicellular embryo and nutrients, enabling them to remain dormant for extended periods and germinate when conditions are right.
The protective seed coat further enhances a seed’s ability to withstand harsh environments and desiccation, which is a key evolutionary advantage. While non-vascular plants require water for sperm to reach the egg, seed plants have evolved mechanisms like pollen, allowing for reproduction without a constant water supply. This distinction highlights how seeds provide protection, nourishment, and dispersal capabilities, contributing to the success of seed-bearing plants in diverse terrestrial habitats.