SPHENOPSIDA lOI 



Calamitales 



This group reached the peak of its development in the Upper 

 Carboniferous, when a large number of arborescent species 

 was co-dominant with the Lepidodendrales in coal-measure 

 swamp forests ; yet by the end of the Permian the group had 

 become extinct. The first representatives to appear, in the 

 Upper Devonian, were the Asterocalamitaceae, a group 

 which differed from the later Calamitaceae in a number of 

 interesting details. Asterocalamites { = Archaeocalamites) 

 (Fig. i6A) had woody stems up to i6 cm in diameter, 

 strongly grooved on the outside, with the grooves con- 

 tinuing through successive nodes (i.e. not alternating). The 

 leaves, up to lo cm long, were in whorls at the nodes and 

 forked many times dichotomously, in a manner strongly 

 reminiscent of Calamophyton leaves. At intervals along the 

 more slender branches, there were fertile regions, in which 

 there were superimposed whorls of peltate sporangio- 

 phores, each bearing four reflexed sporangia (Fig. i6B). 

 Sometimes the fertile regions were interrupted by a whorl 

 of leaves, but these were apparently normal leaves and could 

 not be regarded as bracts. The absence of any regular 

 association between bracts and sporangiophores makes an 

 interesting comparison with the cones of the later Calami- 

 taceae. 



Protocalamites was one of the earhest representatives of 

 the Calamitaceae, being present in the Lower Carboniferous 

 rocks of Pettycur, Scotland. Its stems were ridged, with the 

 ridges alternating in successive internodes, like those of 

 most members of the family, but they differed in one im- 

 portant respect. Examination of a transverse section of a 

 petrified stem (Fig. i6C) reveals a marked development of 

 centripetal wood, as well as centrifugal (i.e. the primary 

 wood was mesarch). As in Catamites and in Equisetum, the 

 protoxylem tended to break down to form a carinal canal. 

 Secondary wood was laid down to the outside of the meta- 

 xylem, but the primary wood-rays were so wide that it gives 



