CYCADOFILICALES 21 



belong to Cycadofilicales, but they include forms known to be such 

 (Lyginodendron, Heterangium, Medullosa) ; others almost certainly 

 such because of their evident relationships {Colpoxylon, Sulcliffia, 

 Megaloxylon, Calamopitys, Cycadoxylon); and others possibly such 

 because they are Cycadofilices. Each of the genera cited represents 

 a stem family of the same name, except that Heterangium is included 

 in the Lyginodendrineae (Lyginodendreae), and Colpoxylon and 

 Sutdiffia in the Medullosineae (Medulloseae). Cycadofilicales are 

 known to extend beyond the range of these stem genera, for seed- 

 bearing fronds have been discovered the structure of whose stems is 

 unknown. The stems already described, however, show a wide 

 anatomical range for the group, being protostelic {Heterangium and 

 Sutdiffia), siphonostelic {Lyginodendron, Megaloxylon, Cycadoxylon, 

 etc.), and polystelic {Medullosa, Cladoxylon, Colpoxylon, etc.); while 

 the xylem of the stem ranges from exarch {Sutdiffia) , through mesarch 

 (the general condition), to endarch {Cycadoxylon) . The leaf traces 

 also range from the usual simple forking to a profuse branching and 

 anastomosing. So far as vascular anatomy is concerned, therefore, 

 the Cycadofilicales exhibit as wide a variation and the same kind of 

 variation as do the existing ferns, and they add to these characteristics 

 those of existing gymnosperms. 



THE LEAF OF CYCADOFILICALES 



To distinguish the leaves of Cycadofilicales among the innumerable 

 paleozoic "fronds" that have been preserved is at present impossible. 

 It has been suggested (60) that the prevailing dichotomy of paleozoic 

 fronds may be indicative of Cycadofilicales rather than of ferns, but 

 such a character cannot be used with any definiteness. In fact, 

 Scott has said recently (82) that from the frond alone it cannot be 

 told whether a paleozoic plant is a fern or one of the Cycadofilicales. 

 The best that can be done is to select those leaves whose connection 

 with stems or with seeds testifies to their nature. In this way most of 

 the large frond genera have contributed representatives to the Cycado- 

 filicales, but it may be that some of their other members arc ferns. 

 At least, so artificial an assemblage as a large frond genus would 

 suggest such a possibility. The certain or probable cases of leaves 

 belonging to the Cycadofilicales are as follows: 



