| The Columbia Encyclopedia, Sixth Edition. 2001-07. |
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| Psilotophyta |
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(s l ´´t f´ t) (KEY) , division of vascular plants consisting of only two genera, Psilotum and Tmesipteris, with very few species. These plants are characterized by the lack of roots, and, in one species, leaves are lacking also. The green, photosynthetic stem is well-developed. Like higher plants, e.g., the angiosperms (Magnoliophyta), Psilotophyta has specialized conducting, or vascular, tissue (xylem and phloem). Psilotum, with only two species, is widespread in tropical and subtropical areas, whereas Tmesipteris species is restricted to Australia and neighboring islands. The spore-producing structures are produced in clusters in the axil of a leaflike at the end of a short lateral branch. The gametophyte plant, arising from germination of a spore, is small and colorless, and derives its nutrition through a specialized association with a fungus. Sexual structures on the gametophyte produce eggs and sperm. The motile sperm, with numerous flagella, are able to swim through a film of water to the egg. The fertilized egg, or zygote, first absorbs nourishment from the gametophyte, and later becomes photosynthetic and self-sustaining. The life cycle is very much like that of ferns. |
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| | | The Columbia Encyclopedia, Sixth Edition. Copyright © 2007 Columbia University Press. |
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