i7o STUDIES IN FOSSIL BOTANY 



branches of the same size also have a solid woody axis, 

 and it is only in the larger stems that a pith is present. 

 As regards the leaf-traces and cortical structures, the 

 Halonia shows no peculiarities ; a thick layer of 

 periderm forms its outer boundary. From the central 

 cylinder small cylindrical steles are given off, which 

 pass obliquely outwards to supply the tubercles 

 (see Fig. 69, b', from Lepidophloios fuliginosus'). The 

 tubercular steles are surrounded by their own leaf- 

 traces, which no doubt supplied the leaves, borne on 

 the lateral appendages of which the tubercles are the 

 persistent bases. 



In Lepidodendron Hickii similar tubercles have 

 been observed on some of the smaller branches. In 

 this case the branch itself has the ordinary structure of 

 the species, with a medullate stele, while the small 

 strands running out to the tubercles are without any 

 pith. A similar condition has been found in Lepido- 

 dendron obovatum and in Lepidophloios fuliginosus (Fig. 

 69). All these cases afford striking examples of the 

 so-called unequal dichotomy described in the last 

 chapter (p. 163). 



What then was the nature of these Halonial 

 branches, which evidently did not constitute a 

 separate genus, but occurred as terminal ramifications 

 on certain Lepidodendroid stems ? Their position 

 and structure alike prove that they were neither 

 roots nor root -bearing rhizomes. Evidently they 

 bore some kind of lateral appendages of the nature 

 of branches, but different from the ordinary vegetative 

 twigs. 



The distinction between Halonia and Ulodendron is 



