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Text File  |  1992-09-02  |  724b  |  18 lines
  1. A simple type of vascular plant. The
  2. pteridophytes comprise four classes: the
  3. Psilosida, including the most primitive
  4. vascular plants, found mainly in the tropics;
  5. the Lycopsida, including the clubmosses and
  6. Selaginella; the Sphenopsida, including the
  7. horsetails; and the Pteropsida, including the
  8. ferns. They are mainly terrestrial,
  9. non-flowering plants characterized by the
  10. presence of a vascular system, the possession
  11. of true stems, roots and leaves, and by a
  12. marked alternation of generations, with the
  13. sporophyte forming the dominant generation in
  14. the life-cycle. They do not produce seeds.
  15. The pteridophytes formed a large and dominant
  16. flora during the Carboniferous period, but
  17. many are now known only from fossils.
  18.