322 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



PHYLUM LEPIDOPHYTA. 

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The Lepidophyta occupy in some respects an isolated position 

 among vascular plants, although there are not wanting students 

 who connect them with the Hepaticae or the Sphenophyllales on the 

 one hand, and with the seed-bearing plants on the other. The phy- 

 lum includes the tiny existing quillworts and herbaceous clubmosses, 

 as well as the often gigantic Lepidodendrales, which occupied such 

 a prominent position in Paleozoic floras. 



Features which serve to unite the somewhat diverse members of 

 this phylum are the small simple leaves, which are spirally arranged, 

 and not vertillate, as in the Arthrophyta. All the known Lepido- 

 phytes are microphyllous, the only suggestion of a departure from 

 a simple type of leaf being the double leaf trace in the Paleozoic 

 Sigillariopsis. A second consistent feature is the presence of branch 

 gaps and the absence of leaf gaps in the vascular cylinder, which is 

 thus cladosiphonic, with an exarch protostele. A third feature is 

 the apparently simple relation between the sporangium and the 

 sporophyll, which can be interpreted as due to progressive sterili- 

 zation according to Bower's well-known theory, or may be explained 

 as a reduction from the same or similar sporangiophoric ancestors 

 that give rise to the Arthrophyte phylum. 



The Lepidophyta are prevailingly strobiloid and are known from 

 the Devonian to the present. Some are homosporous, other hetero- 

 sporous, while still other Paleozoic forms had practically crossed 

 the boundary that separates spore production from seed formation. 

 Further details are best given in the discussion of the various types 

 which follow. The geologic history and mutual relationships of 

 the different members of the phylum are shown in the accompanying 

 diagram and in the light of present knowledge may be expressed in 

 tabular form in the following manner: 



Order Lypidodendrales : Family Bothrodendraceae, Family Lepi- 

 dodendraceae. Family Sigillariaceae. 



Order Lycopodiales : Family Lycopodiaceae, Family Selaginella- 

 ceae. 



Order Isoetales: Family Isoetaceae. 



Order Psilotales : Family Psilotaceae. 



It will make for clearness to consider the existing forms and their 

 fossil representatives before describing the wholly extinct Lepido- 

 dendrales. The Lycopodiales comprise the two families Lycopo- 

 diaceae and Selaginellaceae. The former includes two genera, Ly- 

 copodium and Phylloglossum. Lycopodium has about 100 widely 

 distributed existing species of mostly perennial erect or trailing 

 herbaceous forms of shaded woods, marshes, etc., some tropical 

 species epiphytes. But one land of spores are formed and the 



