324 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



like forms were never apparently very different from what they are 

 at present, they represent a very ancient line and were represented 

 as a minor element associated with the diversified arboreal Lepi- 

 dophyte flora of the Paleozoic. 



The genus Phylloglossum consists of a single species of Australia 

 and New Zealand with a reduced tuberous stem and relatively large 

 leaves, closely related to Lycopodium, although by some students 

 considered the most primitive lycopod. 



The second family, the Selaginellaceae, consists of the single 

 genus Selaginella, with about 500 species of mostly mesophytic small 

 plants not differing greatly from the Lycopodiaceae, except that 

 after the mother cell stage the spores differentiate into microspores 

 and megaspores. The foliage is usually dimorphic and both foliage 

 leaves and sporophylls have a ligule. The cones are not highly 

 differentiated from the vegetative leaves. Sometimes but one mega- 

 spore is developed in a sporangium and fertilization may take place 

 before shedding, thus illustrating steps in the acquisition of the seed 

 habit. Fossil species of Selaginella, unless they are fruiting, are 

 not certainly distinguishable from Lycopodites, nevertheless several 

 undoubted species have been recorded from the Carboniferous (Se- 

 laginellites Swissei Zeiller, S. primaevus (Goldenberg) S. elongatus 

 (Gold), S. sdlliatus (Kidst). Less authentic species are recorded 

 from Mesozoic and Cenozoic deposits of widely scattered regions. 



The second order, the Isoetales, consists of the single genus Isoetes, 

 with about 60 existing aquatic or palustrine species, with a reduced 

 tuberous stem and a crowded rosette of grasslike ligulate leaves 

 which are relatively larger than in the majority of Lepidophytes. 

 Microspores and megaspores are produced, both characterized by 

 their large size and by the formation of trabeculae or strands of 

 sterile tissue often completely dividing the sporangial cavity. Sec- 

 ondary thickening occurs and morphological resemblances to the 

 ancient Sigillarias and modern Monocotyledons have frequently 

 been insisted upon, although without logical basis. Fossil forms are 

 exceedingly rare. Saporta recorded a form from the Lower Cre- 

 taceous (Barremian) of Portugal {Isoctites Choffatl). Aside from 

 untrustworthy records Isoetes occurs in the Oligocene of Franco 

 and the Miocene of Switzerland. 



The third order, the Psilotales, is one which has excited a great 

 deal of discussion. It comprises the two genera Psilotum and 

 Tmesipteris; the former with two species of both Tropics and the 

 latter with a single Australasian species, although more are fre- 

 quently recognized. Both genera are considered to be more or less 

 saprophytic. The sporophylls of Psilotum consist of a much 

 shortened bifurcated axis bearing adaxially a bilocular or trilocujar 

 synangium producing but one kind of spores. In Tmesipteris the 



