258 



PLANT MORPHOLOGY 



cence. Sporophylls are absent. The prothallium is aerial and cushion- 

 like, with erect, green, ribbon-like lobes. The sperms are numerous, 

 large, and multiciUate. The archegonia have one or two neck canal cells. 

 The embryo lacks a suspensor. The Equisetales are a distinct order, 

 superficially unUke any other group of living pteridophytes. 



4. Calamitales 



This is a Paleozoic order closely related to the Equisetales and often 

 combined with it. It ranged from the Devonian to the Triassic. The 



A B 



Fig. 215. Longitudinal sections of cones of Calamitales. A, Palaeostachya, showing 

 peltate sporangiophores in axils of bracts, diagrammatic; B, Archaeocalamites, showing 

 axis bearing sporangiophores only. (A, after Scott; B, after Renault.) 



principal genus is Catamites (Fig. 214). The Calamitales were tree-like 

 forms, some reaching a height of 20 or 30 m. and a diameter of 1 m. The 

 hollow stems bore whorled leaves and branches, the leaves being either 

 free or united at the base. Although mostly small and narrow, the leaves 

 were larger than in modern horsetails, while in Archaeocalamites they were 

 large, dichotomously divided into narrow segments, and somewhat fern- 

 like in appearance. 



The vascular anatomy was of an advanced type. The stem w^as an 

 ectophloic siphonostele, the primary xylem occurring in isolated, collateral 

 vascular bundles arranged in a circle around a hollow pith. The bundles 

 were prevailingly endarch but were mesarch in Protocalamites. The 

 young stem of the Calamitales was essentially similar to an adult stem of 

 Equisetum but became different as a result of secondary thickening, a 



