PTERIDOPHYTA 259 



feature of the group. Generally the primary tissues were surrounded by 

 a continuous cylinder of secondary wood. 



The strobili were made up of whorled, peltate sporangiophores resem- 

 bling those of Equisetum, except that each bore only four pendent spo- 

 rangia. In Archaeocalamites the cone consisted entirely of sporangio- 

 phores, but in most of the other genera bracts were also present, a whorl 

 of bracts alternating with a whorl of sporangiophores (Fig. 215). These 

 bracts have been interpreted by some botanists as sporophylls. In Cala- 

 mostachys the alternating whorls of bracts and sporangiophores were 

 equidistant, but in Palaeostachya the sporangiophores were situated just 

 above each whorl of bracts, i.e., in their axils. 



The Calamitales were either homosporous or heterosporous, depending 

 on the species. Many megaspores were produced in each megasporan- 

 gium. The difference between the two kinds of spores was not so pro- 

 nounced as in the heterosporous lycopods and ferns. The gametophyte 

 generation is unknown. 



