358 REMAINS OF STEMS OF DOUBTFUL AFFINITY, ETC. 



The next forms to be mentioned are the genera Lyginodendron, Will., 

 and Heterangium, Corda, which seem to stand in a relation to one another 

 similar to that of Poroxylon Boysseti to P. Duchartrei. Lyginodendron 

 Oldhamiamum, Will., a full account of which we owe to Williamson 1 , occurs 

 frequently in the calcareous nodules of the coal-fields of Lancashire and 

 Yorkshire, and also in those of Langendreer and Orlau. A transverse 

 section of a stem in a nodule from the latter locality is figured by Stur 2 . 

 These remains were first discovered by Binney, and were called Dadoxylon 

 Oldhamium. The name Lyginodendron adopted by Williamson was in- 

 vented by Gourlie, who used it to express the peculiar impressions of rinds 

 with Dictyoxylon-structure mentioned above on pp. 8 and 218. As the name 

 is no longer necessary for this purpose, Williamson has transferred it to our 

 plant, and not without reason, since some of these impressions do certainly 

 come from it, and it also shows Dictyoxylon-structure in the rind. A full 

 account of the history of this genus will be found in Williamson. 



At first sight the transverse section presents essentially the appearance 

 of a stem of Cycadeae. We find a well-developed and rather thick ring of 

 secondary wood divided by many broad medullary rays into numerous 

 plates a few cell-rows in breadth, quite after the manner of Cycadeae. The 

 elements of this wood appear to be entirely pitted tracheides. Williamson 

 states that zones have been observed which look like annual rings, but I do 

 not find any in my specimens ; they are said to occur particularly on old 

 and very thick specimens, such as the one figured by Williamson 3 . The 

 space inclosed by the ring of wood contains a central mass of parenchyma, 

 which in many cases however is entirely destroyed, and this is surrounded on 

 all sides by irregularly shaped bundles of varying length and breadth. These 

 bundles immediately adjoin the inner surface of the ring of wood ; the 

 elements of smallest transverse section lie on their outer side. The bundles 

 stand in no regular relation to the segments of the ring of wood. They are 

 usually more or less pushed out of their original position, owing to the 

 disappearance of the central parenchyma and the collapse of the stems; 

 but where these are preserved intact, as in one of Williamson's figures 4 , 

 there the bundles appear less as single separate bundles than as fragments 

 of a ring inclosing the central parenchyma, as in Sigillaria spinulosa (see 

 p. 252-3 and Fig. 49). This would be in favour of the explanation that 

 they are axile bundles with a parenchymatous centre, such as we found 

 in Lepidodendrae. Further investigation will be required to determine 

 the truth of the one or the other view. 



The rather thick rind separates into three layers. The innermost layer is 

 the bast in the form of wedges, which are the direct continuation of the rays 



1 Williamson (1), iv. * Stur (2). 3 Williamson (1), IV, t. 22, f. 4. 



1 Williamson (1), iv, t. 22, f. 2. 



