72 THE MORPHOLOGY OF PTERIDOPHYTES 



it follows that the axes bearing them must also have been 

 large, even when young.^eb Details of Sitypical Lepidodendron 

 leaf base are illustrated in Fig. 12C. Within the area of the 

 leaf scar (2) are to be seen three smaller scars, representing 

 the leaf-trace (3) and the two parichnos strands (4). Above 

 this hes the Hgule pit (i) and, in some species, below it are 

 two depressions that were once thought to be associated 

 with the parichnos system, but are now known to be caused 

 by shrinkage of thin-walled cells within the leaf cushion. 



Lepidophloios is distinguished by its leaf bases being 

 extended horizontally, instead of vertically. Otherwise, the 

 anatomy of the trunks is indistinguishable from that of 

 Lepidodendron. Indeed, it has been suggested that the differ- 

 ences do not warrant a separation into two genera. However, 

 there were differences ; in the way the cones were borne. In 

 Lepidodendron, they were nearly always terminal, whereas 

 in Lepidophloios, they were borne some distance behind the 

 branch tip in a cauliflorous manner. 



The cones of both genera are known as Lepidostrobus and 

 they consisted of a central axis around which sporophylls 

 were arranged in a compact spiral, their apices overlapping 

 so as to protect the sporangia. Further protection was 

 afforded by a dorsal projection, or 'heel', as illustrated in 

 the ideaHzed longitudinal section. Fig. 12D. The cones 

 varied in length from 5 cm to over 40 cm and must have 

 looked Hke those of modern conifers. Some cones contained 

 only megasporangia, others only microsporangia, while 

 others were hermaphrodite. In the latter, the megasporo- 

 phylls were at the base and the microsporophylls towards 

 the apex, as illustrated in Fig. 12D. This is the reverse of 

 the arrangement in gymnosperms and angiosperms, where 

 the microsporangial organs lie below the megasporangial 

 whenever they happen to be associated in a hermaphrodite 

 *flower'. The sporangia of Lepidostrobus were elongated and 

 attached throughout their length to the *stalk' of the sporo- 

 phyll, which was relatively narrow, compared with the 



