124 VASCULAR CRYPTOGAMS 



may be compared to that in the stem of Monocotyledons, as the ordinary 

 arrangement may be to that in the first year in the stem of Dicotyledons. 

 In Stemmatopteris (Cord.) (including Psaronius, Cord., and Zippea, 

 Cord.), from the Bath coal-field, the circumference of the stem is com- 

 posed of a continuous envelope of sclerenchymatous tissue, within which 

 are perpendicular tracts of ' vascular ' tissue not penetrated by meshes. 

 Between these tracts the leaves were given off in perpendicular series, 

 the large single leaf-bundles coming right out from the central paren- 

 chyme, in which they existed as well-formed bundles, filling up more or 

 less completely the central cavity (see fig. 95). There is therefore no 

 closed cylinder with central ' medulla ' as in ordinary ferns. By some 

 authors it has been proposed to establish the fern-stems which display 

 this character as a separate group under the name Psaronieae, but there 

 is every reason to identify the stem of Stemmatopteris insignis (Cord.) 

 with the fronds of Pecopteris arborescens (Schl), which bear fructi- 

 fication indistinguishable from that of Cyathea ; and, this character 

 being the more important, the genus must be placed under Cyathe- 

 aceiE. 



Fossil Equisetace^. 



Remains of the genus Equisetites, evidently very nearly allied to 

 Equisetum. if not identical with it, are found in greater or less abun- 

 dance in various strata from the Carboniferous to the Tertiary, attaining 

 their maximum development in the Trias. The stems of these fossil 

 horsetails are from one and a half to six inches in diameter, and may have 

 attained a height of from twenty-five to thirty-five feet. They are cylin- 

 drical, and are marked with alternate ridges and furrows. At the nodes 

 are tubular leaf-sheaths split at the margin into numerous short teeth, 

 each of which terminates in an elongated bristle ; in some species the 

 number of these teeth appears to have amounted to as many as one 

 hundred. The nodal diaphragms are clearly seen in E. arenaceus 

 (Brongn.), the remains of which occur in extraordinary abundance in the 

 Upper Trias ; and, in some species at least, the furrows and ridges of 

 each internode are alternate respectively with those of the internodes 

 next above and next below. Remains of rhizomes have been found closely 

 resembling in structure those of Equisetum. Nearly allied to Equise- 

 tites are the genera Schizoneura (Schmp.) and Phyllotheca (Brongn.); 

 the latter differing from the type in its spreading sheath-teeth, and in 

 the ridges and furrows of adjacent internodes not being alternate. The 

 fructification of Equisetites has only been found in a very imperfect con- 

 dition. That of Phyllotheca bears a close resemblance to the cone- 



