Without a vascular system, plants are incapable of creating seeds and the flowers, fruits and cones that seeds are stored in. In fact, seeds evolved as plant vascular systems developed to handle them. Yet a select group of plants, known as pteridophytes, have vascular systems but do not reproduce via seed. These plants are descended from ancient plants that preceded the appearance of seed plants.
Pteridophytes are vascular plants with no seeds. These plants have all the standard tropes of the vascular system, from transportation cells called xylem and phloem to organ systems, yet they do not reproduce via seed. These plants are globally distributed, largely terrestrial though some are floating, submerged or emergent aquatic plants. Emergent plants are those with parts below and above the surface of the water. The Australian National Gardens describes pteridophytes as ancient and primitive plants. Non-seeding vascular plants are not nearly as complex as their seeding compatriots.
Types of Pteridophytes
There are two types of pteridophytes: true ferns and fern allies. Ferns are the most developed and successful of non-seeding vascular plants. Boston and maidenhair fern are among the common gardening varieties of these plants. Despite being the most successful pteridophytes, ferns are far less successful at reproducing than seeding vascular plants. Fern allies are related to ferns but do not have the same leaves. While fern leaves are large and developed, thus aiding in photosynthesis, fern allies resemble grass; their leaves are generally barely visible to the naked eye. University of Wisconsin professor Sean Carrington calls fern allies living fossils and explains that most related species have long been extinct.
Pteridophytes reproduce via spores. Spores are released from containers called sporangium and undergo meiosis, or division. As with seeds, there are male and female gametophytes in spores. These separate entities arise from meioses and must commingle for fertilisation. Spores bear a distinct disadvantage to seeds in that they require moist and fertile sites where they can settle immediately and begin to grow; otherwise, the spore will die. This limitation led to the decline of spore plants in competition with seed plants.
Vascular Plant Evolution
The earliest vascular plants, of genera such as Cooksonia and Rhynia, developed approximately 400 million years ago, during the Silurian Period. These plants were incredibly simple organisms without roots, leaves, fruits or flowers. The whisk fern (Psilotum nudum) is a modern fern ally believed to resemble these early vascular plants, all of which reproduced via spores. As vascular plants grew more complex, so did their spores. Scientists believe that seeds evolved directly from a complex spore production process known as heterospory that first appeared in plants known as progymnosperms -- vascular plants that evolved 10 million to 15 million years after the first vascular plants. Over the next few hundred million years, seed plants rose to prominence while complex spore plants didn't evolve in competition. Thus modern pteridophytes aren't too far removed from their ancient kin.
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- Australian National Botanical Gardens; Pteridophytes; Jim Croft; 1999
- Estrella Mountain Community College; Nonvascular Plants and Nonseed Vascular Plants; M.J. Farabee; 2004
- University of Wisconsin; Introduction to the Pteridophytes; Sean Carrington; 2004
- Ohio State University; Ferns and Fern Allies
- The Seed Biology Place; Evolution of Seed Plants; Gerhard Leubner; 200
- University of Florida IFAS Extension; Ferns; C.A. Conover