1865.] DAWSON — COAL-FORMATION. 131 



i Of the genus Leioderma or Asolanus I know but one species, inde- 

 pendently of those specimens of old trunks of the ordinary Sigillaria 

 in which the ribs have disappeared. My species, ^, Sydnensis, is 

 founded on specimens collected by Mr. Brown at Sydney, Cape 

 Breton, which are especially remarkable for the curious modification 

 which they present of the Stigmarian root. The specimens described 

 by Mr. Brown under the name of JS. alternans* , and which have 

 been copied by Geinitz and Goldenberg, belong, I believe, to this 

 species. (Fig. 28, Plate YII.) 



On the genus Syringodendron of Sternberg I have no observations 

 to make. I have seen only fragments of stems ; and these seem to be 

 very rare. 



I include under Sigillarice the remarkable fossils known as Stig- 

 maria, being fully convinced that all the varieties of these plants 

 known to me are merely roots of Sigillaria ; 1 have verified this fact 

 in a great many instances, in addition to those so well described by 

 Mr. Binney and Mr. Brown. The different varieties or species of 

 Stigmaria are no doubt characteristic of different species of Sigillaria, 

 though in very few cases has it proved possible to ascertain the va- 

 rieties proper to the particular species of stem. The old view, that 

 the /Stigmarice were independent aquatic plants, still apparently 

 maintained by Goldenberg and some other palgeobotanists, evidently 

 proceeds from imperfect information. Independently of their ascer- 

 tained connexion with Sigillaria, the organs attached to the branches 

 are not leaves, but rootlets. This was made evident long ago by the 

 microscopic sections published by Goeppert, and I have ascertained 

 that the structure is quite similar to that of the thick fleshy rootlets 

 of Cycas. The lumps or tubercles on these roots have been mistaken 

 for fructification ; and the rounded tops of stumps, truncated by the 

 falhng in of the bark or the compression of the empty shell left by 

 the decay of the wood, have been mistaken for the natural termina- 

 tion of the stemf. The only question remaining in regard to these 

 organs is that of their precise morphological place. Their large pith 

 and regular areoles render them unlike true roots; and hence 

 Lesquereux has proposed to regard them as rhizomes. But they cer- 

 tainly radiate from a central stem, and are not known to produce 

 any true buds or secondary stems. In short, while their function is 

 that of roots, they may be regarded, in a morphological point of view, 

 as a peculiar sort of underground branches. They all ramify very 

 regularly in a dichotomous manner, and, as Mr. Brown has shown, in 

 some species at least, give off conical tap-roots from their underside. 



In all the Stigmarice exhibiting structure which I have examined, 

 the axis exhibits only scalariform vessels. Corda, however, figures a 

 species with wood- cells, or vessels with numerous pores, quite like those 

 found in the stems of Sigillaria proper ; and, as Hooker has pointed 

 out, the arrangement of the tissues in Stigmaria is similar to that in 



* Quart. Journ. Geol. Soc. vol. v. p. 354 et seq. 



t For examples of the manner in which a natural termination may be simu- 

 lated by the collapse of bark or by constriction owing to lateral pressure, see 

 my papers, Quart. Journ. Geol. Soc. vol. x. p. 35, and vol. vii. p. 194. 



