74 THE MORPHOLOGY OF PTERIDOPHYTES 



Strands. This is most interesting, for it represents the 

 culmination of a trend which was also taking place, at 

 the same time, among several groups of early gymnosperms, 

 from the sohd protostele, through medullated protosteles 

 (first with mixed pith and then with pure pith) to a pith 

 surrounded by separate strands of primary wood. 



From a distance, Bothrodendron must have looked very 

 similar to Lepidophloios, for it had a stout trunk with a 

 crown of branches covered with small lanceolate leaves and 

 its cones {Bothrodendrostrobus) were borne in a cauhflorous 

 manner. It differed, however, in the external appearance of 

 the trunk, for it had circular leaf scars that were almost 

 flush with the surface. 



The underground organs of all the genera of Lepido- 

 dendrales so far described were so similar that they are all 

 placed in the form genus Stigmaria, and many are placed in 

 a single artificial species, S. ficoides. The base of the trunk 

 bifurcated once and then immediately again, to produce 

 four horizontal axes, each of which continued to branch 

 dichotomously many times in a horizontal plane. These 

 Stigmarian axes were most remarkable structures in many 

 respects. Thus, even at their growing points, perhaps lo m 

 from the parent trunk, they were frequently as thick as 

 4 cm. They bore lateral appendages, commonly called 'root- 

 lets', in a spiral arrangement. These were up to i cm in 

 diameter and were completely without root hairs. Internally 

 they show a remarkable resemblance to the rootlets of the 

 modern Isoetes in having had a tiny stele separated from the 

 outer cortex by a large space, except for a narrow flange of 

 tissue (Fig. 12J). In origin, they were endogenous, although 

 only just so. The axes on which they were borne were 

 pecuhar in being completely without metaxylem. In the 

 centre was either pith or a pith-cavity, round which were 

 protoxylem regions directly in contact with a zone of 

 secondary wood. This consisted of scalariform tracheids 

 interspersed with small wood-rays, but there were also very 



