174 



PHYLUM TRACHEOPHYTA: 



sporangia, or entirely absent; leaves, when present, 

 always small and simply structured microphylls. 



Reproduction: in living species the mature sporan- 

 gium when dry opens along vertical rows of thin- 

 walled cells, and the spores are shed; in most fossil 

 forms, there usually were no cells specialized to open 

 the mature sporangium; the sporangium probably 

 merely split due to internal water pressure or decay; 

 in living species the shed spores germinate into simple 

 or forked cylindrical gametophytes which typically 

 become covered with multicellular "hairs"; gameto- 

 phytes unknown in fossil species. 



CLASS PSILOTAE 



Diagnosis: the only recognized class; character- 

 istics as for the subphylum; perhaps the entire sub- 

 phylum is an assemblage of unrelated lycopsids, 

 sphenopsids, and/or pteropsids that share the single 

 attribute of simplification owing to their ancestry in 

 freshwater to semiaquatic, commonly marshland, 

 habitats. 



Subphylum LYCOPSIDA ( = LEPIDOPSIDA; 

 MICROPHYLLOPSIDA) (Club Mosses, 

 Spike Mosses, and Quillworts) 



Diagnosis: known from Cambrian to Recent, rep- 

 resented by the oldest apparent vascular plant known, 

 Aldanophylon of the Cambrian; modern forms peren- 

 nial herbs; fossil forms included trees; sporophyte 

 with simple organs, microphyllous leaves, simple or 

 branching stem, and roots; leaves alternate or op- 

 posite along the stem, rarely, whorled at the nodes; 

 sporophyte organs usually containing veins through- 

 out; sporangia occur singly on the under or upper 

 surface of leaves and produce one or two kinds of 

 spores; sporophyte and gametophyte both function- 

 ally independent at maturity; gametophytes of living 

 species small but visible, but mostly subterranean and 

 essentially a membranous sheet with multicellular 

 sex organs (upon the same plant in one-spore species, 

 upon separate male and female gametophytes in two- 

 spore species); or gametophyte much reduced and 

 confined within a spore. 



Occurrence: living forms in three orders; Lyco- 

 podiales include about 15 United States and 200 total 

 members of the genus Lycopodium and one Australian 

 species o( Phyltoglossum, most species being inhabitants 

 of warm, moist, tropical areas, but some found in 



both cool and dry areas; Selaginellales include about 

 25 United States and 500 total species of Selaginella, 

 the only living genus, mostly in the tropics but almost 

 world wide; Isoetales include about 20 United States 

 and 80 total species in the single genus, Isoetes, mostly 

 aquatic forms in slow-moving to standing water but 

 also semiaquatic in temporary waters. 



GAMETOPHYTE 



Structure: microscopic to about 1-inch in diame- 

 ter; either aerial or subterranean; aerial types are 

 chlorophyllous and gain nutrients mostly by photo- 

 synthesis; subterranean types normally lack chloro- 

 phyll, obtain nutrients from a fungus (Phycomycete) 

 association, and although distinctive, resemble psi- 

 lopsid gametophytes. 



Reproduction: fertilization, embryo formation and 

 development is similar to that of psilopsids and other 

 pteridophytes (see Figures 11.9 and 11.11), but the 

 embryo has a primary root; like psilopsids and sphe- 

 nopsids the gametophyte eventually dies and decays 

 after reproduction. 



SPOROPHYTE 



Structure: many fossil trees with leaves a few feet 

 long; living species are mostly small, short-stemmed, 

 perennial herbs, but some are vine-like; structure 

 consists of stems, leaves, and roots; sporangia borne 

 singly upon the upper surface of a leaf and either 

 near or within the leaf a.xil. 



Sporangia: most are on undifferentiated leaves; 

 in some, leaves are somewhat modified and grouped 

 into a cone that produces spores of one or two types. 



Stems: simple or branched, green and photosyn- 

 thetic; branching often equal forking, but straight 

 main stems do occur; organized similar to, but more 

 complex than, the psilopsids. 



Leaves: present and mostly alternate or opposite, 

 rarely whorled; narrow, typically small and with a 

 single, rarely two, unbranched midveins (micro- 

 phylls) otherwise of generalized structure; all except a 

 fossil order and the Lycopodiales have a ligule (a 

 small scale-like appendage on the upper surface near 

 the leaf axil) and are called ligulate (see Figures 11.7 

 and 11.8), fossil order and the Lycopodiales are 

 eligulate; ligule might represent an undeveloped 

 branch; the term ligule refers to any tongue-like struc- 

 ture, so the word does not necessarily refer to the 

 same structure in all plant groups. 



