PTEROPSIDA 117 



illustrates the 'islands of parenchyma', as seen in transverse 

 section, which are a common feature of the Zygopteridaceae 

 too. Another feature, shared with the Coenopteridales, is the 

 presence of 'aphlebiae' at the base of the lateral branch (or 

 petiole?). These were similar in position to the stipules of 

 many flowering plants and received separate vascular 

 bundles (i). 



Fig. 17E illustrates another type of stem structure, found 

 in Cladoxylon taeniatum and several other species, in which 

 each of the xylem strands has an outer region of radially 

 arranged tracheids which are thought to have been formed 

 from a cambium. The arrows in the figure indicate that three 

 of the stem steles were involved in the origin of branch 

 traces. Successive branches, petioles and pinnae of descend- 

 ing order had progressively simpler vascular structures, 

 without secondary wood, and are described under separate 

 form-generic names. Thus, Fig. 17F shows the Hierogramma 

 type of stelar arrangement, in which there were six xylem 

 regions, each with islands of parenchyma. Lateral branches 

 from this had four xylem areas and are known as Syncardia 

 (Fig. 17G). Clepsydropsis (Fig. 17H) was probably the next 

 type of branch, or petiole, although there has been some dis- 

 agreement among palaeobotanists about this. Its stele, as 

 seen in transverse section, had the shape of an hour-glass 

 (hence the generic name) and from it lateral pinna traces 

 were given off" alternately, along with a pair of aphlebia traces 

 (i). It should be noted that similar clepsydroid steles are 

 known from a number of plants belonging to the Coenop- 

 teridales. 



Pseudosporochnus is represented in Middle Devonian 

 rocks of Germany, Scotland, Scandinavia and North 

 America, but our knowledge of its morphology is based 

 chiefly on the German species P. Krejcii (Fig. 17 1). It had an 

 erect stem with a swollen base and a bushy crown of branches 

 which forked dichotomously and terminated in small 

 sporangia. According to Kraiisel and Weyland^^ there were 



