The diverse plant life on Earth—consisting of nearly 400,000 species—can be divided into three broad categories based on biological characteristics: nonvascular, seedless vascular, and seed plants.
Nonvascular Plants Were the First Plants on Earth
Nonvascular plants that live today include liverworts, mosses, and hornworts—collectively and informally known as bryophytes.
Nonvascular plants are characterized by a lack of extensive vascular tissue, and have no true roots, leaves, or stems. Another trait of this group is the use of spores rather than seeds to reproduce, and a life cycle dominated by the haploid, egg- and sperm-producing gametophyte stage.
Because their sperm typically require water to reach an egg, nonvascular plants are often found in moist habitats and reproduce more successfully close to other members of their species.
The Life Cycle of Nonvascular Plants
In a typical bryophyte, haploid spores produced by the sporophyte will grow via mitosis to form a haploid gametophyte. Once mature, these gametophytes generate haploid gametes of either male (sperm) or female type (eggs), in structures called antheridia or archegonia.
In the presence of water (even as little as a morning dew), the sperm will swim towards the archegonia in order to find and fertilize the eggs. Once fertilization is complete, the now diploid zygote will grow via mitosis from the gametophyte structure, forming a new sporophyte. Once mature, the sporophyte produces haploid spores, and the cycle begins again.
Most Plants on Earth Today Are Seed Plants
While most modern-day plants grow from seeds, nonvascular plants were once the primary colonizers of the terrestrial landscape. Today, these plants continue to thrive in moist environments around the world.
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