STIGMARIA. 283 



But it must be remembered that subterranean organs may also be rhizomes, 

 and in that case the objection falls to the ground. Brongniart 1 , who had 

 similar difficulties, was much more cautious. But when Unger goes on to 

 maintain that it is impossible to reconcile the anatomy of Stigmaria with 

 that of Sigillaria he raises an objection which deserves all attention and 

 requires to be properly examined. In comparing the two together, we 

 encounter really two important points of difference. First there is the 

 growth in thickness of the trace-bundles of Stigmaria, and we find nothing 

 that resembles growth of this kind in Sigillaria ; but the more important 

 point is that in Sigillaria, as we have seen, there is no layer of primary 

 wood (bois centripete) differentiated into irregular bundles in the periphery 

 of the pith, such as we observed in Stigmaria on the inner side of the 

 secondary growth. We are therefore obliged to assume that the central 

 cylinder in Stigmaria was purely parenchymatous, with a network only in 

 its periphery of weak, probably normally disposed, tracheal strands, with 

 which the growth in thickness connects at once and immediately, as in the 

 stem of Botrychium. This would explain the fact, that we find it impos- 

 sible or difficult to distinguish the wedges of wood from the primary strands, 

 that the structure of the youngest extremities is essentially the same as that of 

 the older parts. In Stigmaria augustodunensis, though its connection with 

 the group is not perfectly assured (see p. 273), the entire central strand, 

 which is parenchymatous in Stigmaria ficoides, is composed of tracheides. 

 How the one form of structure passed irjto the other at the base of the 

 upright stem we cannot tell, nor is it likely that we shall ever find this 

 portion of the plant with the structure preserved. But the difficulties in 

 this case are certainly much less important than those which would arise in 

 reconciling the structure of stem and root in our living plants, if we had 

 only unconnected fragments of them before us. 



A number of stems with Stigmaria-roots attached were described in 

 the years 1 846, 1 848, and 1 849 by Richard Brown 2 , from the Carboniferous 

 formation of the island of Cape Breton in Nova Scotia. In none of them 

 could there be any doubt with regard to the nature of the diverging root- 

 system. The stems, it is true, which were described in Brown's first pub- 

 lication, could not be certainly determined ; but those of the third work 

 were shown to be genuine Sigillariae (Fig. 37 C). Those of the year 

 1848 were described by Brown as Lepidodendrae ; but judging by the 

 figures I can only see the impression of Dictyoxylon-structure, and as 

 this occurs also in the rind of Sigillaria, Lyginodendron, Heterangium, and 

 other forms, its presence cannot well be employed to prove that Lepido- 

 dendrae also had Stigmaria-roots. If the view expressed by Rich. Brown 



1 Brongniart (2), p. 105. s Brown, Rich. (1), (2), and (3). 



