Key to the Families' 



(Carried out, in some cases, to genera. The numbers preceding the fam- 

 ily and generic names refer to their sequence in the class to which they 

 belong.) 



PTERIDOPHYTA 



Plants without true flowers, reproducing by spores (without embryos) ; fernlike, moss- 

 like, rushlike, or aquatic plants. 

 A. Plants floating, with small, 2-ranked leaves; sporocarps borne on the under side 



of the stem Salviniaceae, p. 59. 



A. Plants terrestrial or submerged, not floating B. 



B. Stems conspicuously grooved and jointed, their nodes covered by toothed 



sheaths; sporangia borne on the scales of terminal, dry, conelike spikes 



Equisetaceae, p. 59. 



B. Stems not conspicuously grooved, without sheathing joints C. 



C. Leaves closely imbricated, short or long-linear (from a cormlike base); 

 sporangia sessile, axillary. 

 Stem short, cormlike; leaves linear, in a rosette; sporangia borne in a cavity 



on the inner side of the leaf-base Isoetaceae, p. 66. 



Stem elongate, creeping or branching; leaves very short, crowded or imbri- 

 cated. 

 Plants small and mosslike; spores of two sizes. . . . Selaginellaceae, p. 65. 



Plants not resembling mosses ; spores all of one size 



Lycopodiaceae, p. 63. 



C. Leaves (fronds) not closely imbricated D. 



D. Leaves (fronds) 4-foliolate, cloverlike ; aquatic Marsileaceae, p. 102. 



D. Leaves (fronds) not 4-foliolate, broad, flat, fernlike, more or less pin- 

 nately or ternately divided or entire; terrestrial E. 

 E. Sterile and fertile fronds flat, entire; the fertile ones ending in long- 

 stalked, simple spikes Ophioglossaceae, p. 37. 



E. Sterile and fertile fronds not entire F. 



F. Fertile fronds or fertile portions of the fronds conspicuously unlike 



the sterile; sporangia not on the lower surface of green leaves G. 



G. Rootstock almost none; the solitary (rarely 2) fronds appearing to 



rise from a cluster of fleshy roots; lower segments sterile, the 



upper ones fertile and bearing 2-ranked, globular sporangia. 



Botrychium, p. 38. 



G. Rootstock well developed, elongate or stout, the roots fibrous; fronds 

 numerous H. 

 H. Sporangia globose, thin-walled, 2-valved, densely crowded, not 



2-ranked Osmundaceae, p. 40. 



H. Sporangia within firm, 2-ranked, globose and distinct or connected 



in beadlike segments Onoclea, p. 45. 



F. Fertile fronds or segments essentially like the sterile; sporangia borne 

 on the lower surface or on the margins of green segments. 



POLYPODIACEAE, p. 42. 



SPERMATOPHYTA 



Plants with true flowers containing stamens or pistils or both, reproducing by seed 

 (containing an embryo). 

 Ovules not in a closed ovary; trees and shrubs with needlelike or scalelike, mostly 

 evergreen leaves; flowers monoecious or dioecious (Gymnosperms) . 



1 See Introduction, p. 13. 



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