PTEKIDOPHYTA. 

 Sub-Class 2. Heterosporous Equisetinae. 



225 



The two orders which come under this head are united by the characteristics, 

 that the verticillate leaves are not united into sheaths (Fig. 226), and that be- 

 tween each whorl of fertile leaves there is also a whorl of barren ones. The 



FIG. 226. A. fragment of Annularia. 



fertile whorls in ATJNULAKI^; are situated about midway between the barren ones 

 (Fig. 227), but in ASTEKOPHYLLITE.E they occur immediately above a barren 

 whorl (Fig. 228) and contain only half as many members as the latter. The 

 lower whorls bear macrosporang'a with one macrospore, the upper, micro- 

 sporangia with many microspores. 



FIG. 227. Fragment of Annularia longi- 

 /olia, with sporangia ; the leaves have 

 partly fallen off : a barren whorls ; s fer- 

 tile whorls. 



FIG. 228. Fragment of cone of Astero- 

 phyttites (Volkmannia elongata) -. a and s 

 as in Fig. 227. 



The ANNUIABI^E were distichous (Fig. 226), and presumably floating plants. 

 The ASTEKOPHYLLITE^E had verticellate branches. These also died out after the 

 Carboniferous period, at the close of the Palaeozoic. 



