PALEOBOTANY AND EVOLUTION. iii 



by quoting the authors' words : " The view of the affinities 

 of Lyginodendron and Hetei'angmjti which we desire to 

 suggest is that they are derivatives of an ancient and 

 ' generaHsed ' (or rather non-specialised) Fern-stock, which 

 already show a marked divergence in the Cycadean direc- 

 tion. Of the two genera Hetcrangmm appears to be the 

 more ancient, and certainly stands nearer to the Filicinean 

 ancestry. Lyginodendron^ while still retaining conspicuous 

 Fern-like characters, has advanced much further on Cyca- 

 dean lines." ^ 



The generic name Lyginodendron was proposed in 1843 

 by Mr. Gourlie for a piece of fossil plant which he regarded 

 as a new form, but did not attempt to describe scientifically, 

 Williamson in 1873 applied Gourlie's term to certain plant 

 fragments which he investigated from the calcareous nodules 

 {" coal balls ") of Lancashire. The plant so named by Wil- 

 liamson had been previously described by Binney in 1866 

 as Dadoxylon Oldhamiiwi. Three years later Williamson 

 proposed a new name, Dictyoxylon, for Binney's plant in 

 place of Dadoxylon, a genus in which the Oldham specimen 

 could not be included. Finally Gourlie's name was adopted, 

 in the belief that this older generic designation had been 

 assigned to a plant identical with the much more perfectly 

 preserved Lancashire examples. 



So far all specimens of Lyginodendron have been in- 

 cluded under one specific name, L. Oldhaniium Will. The 

 stems of this plant may be readily recognised in transverse 

 section by the characteristic structure of the outer cortex, 

 and by the histological features of the central cylinder and 

 leaf-trace bundles. The plant is monostelic, and in a young 

 stem the stele consists of an axial parenchymatous pith with 

 nests or groups of dark and thick walled cells. At the 

 periphery of this tissue occur several detached strands of 

 collateral groups of primary xylem and phloem, the former 

 consisting of scalariform, reticulate, and spiral tracheae, 

 with a very little parenchyma, and the latter of thinner- 

 walled elongated elements and parenchymatous cells, with 



^ Williamson and Scott, p. 769. 



