LEPIDODENDRON 141 



xylem-cylinder 2 cm. or more in diameter. Cambial 

 growth here went on vigorously, producing a zone of 

 wood nearly 3 cm. thick in the larger trees, some of 

 which were a couple of feet in diameter. In one par- 

 ticular specimen the state of preservation was very 

 remarkable. To quote Williamson's words : — " At 

 Laggan Bay [Arran] the bases of thirteen large stems 

 stood erect and closely aggregated. Further investiga- 

 tion showed that twelve of these were merely cylinders 

 of outer cortex, all their more internal tissues having 

 disappeared and been replaced by volcanic ash, with 

 which the trees had been destroyed and buried. The 

 decay of the softer portions of the bark had loosened 

 all the harder vascular structures compassing their 

 several steles, and allowed them to float out when the 

 area became submerged. But the exceptional stem 

 had met with different treatment. In the first instance, 

 it also had lost all its vegetable contents, which had 

 evidently floated out upon the neighbouring waters. 

 Directed by some fortunate current, a quantity of the 

 floating debris had been washed back into and filled 

 the vacant cavity of the thirteenth stem. Further 

 examination of this debris showed that it consisted of 

 fragments of bark and of Stigmarian rootlets, including a 

 fragment of a vascular axis of a Stigmaria ; 1 but what 

 was still more important, we found in it the entire and 

 fully developed steles of no less than five of the remaining 

 trees, which had been tumbled into this single one." 2 



1 We shall see further on that these Stigmariae were the rhizomes or 

 roots of the Lepidodendra themselves, and of the allied Sigillariae. 



2 Williamson, "Growth and Development of the Carboniferous 

 Arborescent Lepidodendra," Mem. and Proc. Manchester Lit. and Phil. 

 Soc. ser. iv. vol. ix. p. 45, 1895. 



