PSILOTOPHYTA

division of spore-bearing vascular plants with the simplest structure known of any extant group. Psilotophytes have no true roots or leaves. a division of spore-bearing vascular plants with only two living genera, commonly known as whisk ferns (order Psilotales). Psilotophytes have an upright stem and an underground stem (rhizome), both with dichotomous branching. They have no true roots. Their leaflike structures are small and scaly or flattened, unlike the true leaves of ferns and seed plants. The shrublike Psilotum, one of the two living genera, is commonly found in tropical regions; Tmesipteris, a hanging epiphyte, is distributed throughout the South Pacific and Oceania. Psilotophytes go through sporophytic (asexual) and gametophytic (sexual) phases during their life cycle. Gametophytes germinate from spores and live underground or on tree trunks. The male organs (antheridia) and the female organs (archegonia) are both located on the surface of the gametophyte, and fertilization occurs when a sperm travels down to an egg inside the archegonium. The embryo sporophyte develops a rhizome and stem, which ultimately grow into a mature sporophyte. Upon reaching maturity, the sporophyte releases its spores, and the cycle is repeated. Additional reading F.O. Bower, The Ferns (Filicales): Treated Comparatively with a View to Their Natural Classification, vol. 1, Analytical Examination of the Criteria of Comparison, vol. 2, The Eusporangiatae and Other Relatively Primitive Ferns, and vol. 3, The Leptosporangiate Ferns (192328), is a classic work of comparative morphology and systematics that emphasizes the need, now being realized, for a broad spectrum of comparative data. A comprehensive summary of paleobotanical knowledge is provided in Thomas N. Taylor, Paleobotany: An Introduction to Fossil Plant Biology (1981). The American Fern Society and the British Pteridological Society assemble the record of current research in the field in their publications American Fern Journal (quarterly), Fiddlehead Forum (bimonthly), The Fern Gazette (annual), and Pteridologist (annual).The abundance and diversity of pteridophytes are the focus of Hermann Christ, Die Geographie der Farne (1910), still an important broad treatment of fern distribution; John T. Mickel, How to Know the Ferns and Fern Allies (1979), the first manual to cover all of North America, with keys, brief descriptions, and illustrations; Rolla M. Tryon and Alice F. Tryon, Ferns and Allied Plants (1982), a good summary of the genera of tropical American pteridophytes with descriptions, maps, discussions, and many illustrations; John T. Mickel and Joseph M. Beitel, Pteridophyte Flora of Oaxaca, Mexico (1988), the best illustrated and most comprehensive pteridophyte manual for Latin America; and R.E. Holttum, A Revised Flora of Malaya: An Illustrated Systematic Account of the Malayan Flora, Including Commonly Cultivated Plants, vol. 2, Ferns of Malaya (1954), a well-illustrated enumeration and description of ferns that presents many of the author's ideas of systematic relationship.Life cycle and habitats are discussed in A.F. Dyer, The Experimental Biology of Ferns (1979), a series of essays on ecology, cytogenetics, reproduction, chemistry, and development; A.F. Dyer and Christopher N. Page (eds.), Biology of Pteridophytes (1985), a collection of symposium papers on a broad range of topics; F. Gordon Foster, Ferns to Know and Grow, 3rd rev. ed. (1984), a well-known book of horticulture with many helpful tips on cultivation; Barbara Joe Hoshizaki, Fern Growers Manual (1975), a good introduction to horticulture with encyclopaedic information on the species in cultivation; and Christopher N. Page, Ferns: Their Habitats in the British and Irish Landscape (1988), with excellent illustrations of habitats and ecology.Studies of form and function include K.R. Sporne, The Morphology of Pteridophytes: The Structure of Ferns and Allied Plants, 4th ed. (1975), a concise summary of ideas on fern structure; B.K. Nayar and S. Kaur, Gametophytes of Homosporous Ferns, The Botanical Review 37:295396 (1971), a thorough summation of the knowledge of the haploid generation of ferns, with an extensive bibliography; John T. Mickel, The Home Gardener's Book of Ferns (1979), a useful compilation of information on fern morphology, diversity, and cultivation; and Lenore W. May, The Economic Uses and Associated Folklore of Ferns and Fern Allies, The Botanical Review 44:491528 (1978), a summary of the diverse uses to which ferns have been put.For the origin and evolution of ferns and fern allies, see I. Manton, Problems of Cytology and Evolution in the Pteridophyta (1950), a milestone in the biology of ferns containing, for the first time, accurate data on chromosomes in relation to evolution and systematics; Richard A. White (ed.), Taxonomic and Morphological Relationships of the Psilotaceae: A Symposium, Brittonia 29:168 (1977), a series of papers on structure, relationships, and fossil history; and J.D. Lovis, Evolutionary Patterns and Processes in Ferns, Advances in Botanical Research 4:229439 (1977), an outstanding summary of the knowledge of fern phylogeny and classification. Also see appropriate sections of Robert F. Scagel et al., An Evolutionary Survey of the Plant Kingdom (1965); Ernest M. Gifford and Adriance S. Foster, Morphology and Evolution of Vascular Plants, 3rd ed. (1989); and David W. Bierhorst, Morphology of Vascular Plants (1971), which provides detailed treatments of vascular plants together with theory and interpretation.Nomenclature for the taxonomy of pteridophytes is provided in Edwin Bingham Copeland, Genera Filicum: The Genera of Ferns (1947), a valuable treatment of the classification and characteristics of ferns, containing many of the author's original correlations. Other works on classification include R.L. Hauke, The Taxonomy of Equisetum: An Overview, New Botanist 1:8995 (1974); J.A. Crabbe, A.C. Jermy, and John T. Mickel, A New Generic Sequence for the Pteridophyte Herbarium, The Fern Gazette 11:141162 (1975), a list of pteridophyte genera in a phylogenetic sequence; and Benjamin llgaard, A Revised Classification of the Lycopodiaceae s. lat., Opera Botanica 92:153178 (1987), a clear, detailed discussion of the taxonomic characters, genera, and species groups of the family, and Index of the Lycopodiaceae (1989), a listing of all the names, references, and type (original) specimens. Warren H. Wagner, Jr. Ernest M. Gifford John T. Mickel