124 THE MORPHOLOGY OF PTERIDOPHYTES 



the base and most commonly contained two functional 

 megaspores along with two very small and, presumably, 

 abortive ones, although examples have been found with 

 four, six or eight megaspores. It is believed that the whole 

 structure was shed from the parent plant without prior 

 dehiscence. The microsporangia of S. oldhamia (Fig. i8D) 

 were spherical, were typically eusporangiate in having a 

 thick wall and had a terminal stomium where dehiscence 

 took place, but there was no annulus of thick-walled 

 cells. 



In the past, the Botryopteridaceae were often described as 

 much simpler in their organization than the rest of the 

 Coenopteridales, but recent investigations on both sides of 

 the Atlantic have demonstrated that this is far from the case. 

 Botryopteris antiqua, from the Lower Carboniferous of 

 Scotland, is the earliest known species and was also the 

 simplest in its internal anatomy. It had traihng dorsiventral 

 axes up to 2 mm in diameter, which gave rise to erect, or 

 semi-erect, radial stems bearing petioles, in spiral succession, 

 and roots. The petioles then underwent branching, of up to 

 five successive orders, to produce a multipinnate branch 

 system. There was no flattening of the pinnules to form a 

 lamina anywhere in the frond of this species and the distinc- 

 tion between stem and petiole is purely arbitrary. The three 

 types of stele are illustrated in Fig. i8E, where '2' indicates 

 the one belonging to the trailing dorsiventral axis. It was a 

 soUd rod of tracheids with multiseriate pits or with scalariform 

 or reticulate pits. The single protoxylem group was lateral 

 and almost, but not quite, exarch. The radial stems were 

 about the same diameter, but the stele was circular in cross 

 section with the smallest tracheids (protoxylem?) in the centre 

 (3). The petioles were somewhat smaller, up to 1-4 mm in 

 diameter, and had an oval stele with a lateral protoxylem (4). 

 As the branches of the frond divided and sub-divided, the 

 stele became smaller and smaller until the ultimate pinnules 

 had only a few tracheids or even only one. The sporangia 



