Bryophytes, which include mosses, liverworts, and hornworts, do not produce seeds. These simple plants represent one of the earliest lineages of terrestrial flora. Instead of seeds, bryophytes reproduce and disperse using single-celled spores. This distinction reflects major differences in their physical structure, life cycle, and evolutionary adaptations compared to seed-bearing plants.
Non-Vascular Structure and Water Dependence
Bryophytes are non-vascular plants, lacking the specialized internal transport tissues known as xylem and phloem. Xylem moves water and minerals, while phloem distributes sugars produced by photosynthesis. This absence restricts their ability to efficiently move substances over long distances. Consequently, bryophytes rely on slower processes like osmosis and diffusion to absorb water and nutrients directly through their surfaces.
This structure limits bryophytes to a small stature, typically forming low-lying mats or cushions. They lack true roots, anchoring themselves instead with simple, thread-like rhizoids. Rhizoids serve primarily for attachment rather than extensive water uptake. The need for direct surface moisture means these plants thrive mainly in damp, shaded habitats, such as forest floors and stream banks.
A constantly moist environment is necessary for the bryophyte reproductive cycle. Unlike seed plants, bryophytes require water for the successful transfer of male gametes to the female reproductive structures. The sperm cells possess flagella, meaning they must swim through a film of water, dew, or rain to reach the egg cell for fertilization. This dependency ties them ecologically to wet locations.
Reproduction Through Spores and Alternation of Generations
Bryophytes reproduce through the alternation of generations, switching between two distinct forms: a haploid gametophyte and a diploid sporophyte. The familiar green, cushion-like plant body of a moss is the gametophyte generation. This is the dominant and long-lived phase, producing male and female gametes through mitosis.
Following fertilization, the resulting diploid zygote remains attached to and dependent on the gametophyte. This zygote develops into the sporophyte generation, which appears as a slender stalk topped with a capsule. The capsule is the sporangium, where specialized cells undergo meiosis to generate numerous haploid spores.
A spore is a single, microscopic, haploid cell released for dispersal, unlike a seed, which is multicellular. When a spore lands in a favorable, moist location, it germinates and grows into a new gametophyte. Spores do not contain a pre-formed embryo or stored food reserves. They must immediately begin photosynthesis to survive, making them highly vulnerable to dry conditions.
The Evolutionary Leap of Seed-Bearing Plants
The evolutionary transition to seed-bearing plants, known as spermatophytes, represents a step away from the limitations of bryophyte reproduction. A seed is a complex, multicellular package consisting of a diploid embryo, a food supply (endosperm), and a protective outer seed coat. This structure protects the embryo from damage and desiccation, while the stored food sustains the young plant during germination.
The seed’s protective features allow it to enter a state of dormancy, delaying growth until environmental conditions are optimal. The ability to survive dry periods and harsh weather is a major advantage over the delicate bryophyte spore. Furthermore, seed plants developed pollen, which transports the male gamete without the need for an external film of water, freeing them from the aquatic requirement for fertilization.
The robust nature of the seed enabled plants like gymnosperms and angiosperms to colonize vast, diverse, and often arid terrestrial environments. The seed is a dispersal unit that protects the next generation and provides nutritional resources for its initial establishment. This gives seed plants a superior ability to spread and thrive compared to the simple, resource-poor bryophyte spore.