289 
the Periderm of Fossil Lycopodiales. 
leaf-base. In Lepidodendron selaginoides and an undetermined Lepidoden- 
dron species these areas appear to be quite irregular in position (Text-fig. 1, A 
and b), but in another young twig they show a tendency to start from the 
middle of some of the leaf-bases (Text-fig. 2, a ). In Bothrodendron 
mundum the secondary tissue is more regular in development, arising 
almost all round the stem at the same time (Text-fig. 2 , b). 
Text-fig. 2. Diagrammatic transverse sections to show first beginnings of periderm formation. 
A, Lepidodendron sp. {selaginoides X). U. C. L. Coll., A on. B, Bothrodendron mundum . 
pd , periderm ; st, stele ; x 2 , secondary wood. U. C. L. Coll., A 014. x 12. 
The developing meristem gradually extends until a complete cylinder 
is formed, but one or two gaps may remain for some time, as in Lepidoden- 
dron Hickii 1 and L. fuliginosum, and as a result of its irregular origin the 
secondary tissue produced is, at any rate at first, also irregular in thickness. 
Thus, in the specimens shown in Text-fig. 1, ten layers of periderm cells 
have already been formed in one place in A, and in B fourteen layers. 
1 Noticed first by Williamson ( 42 ), p. 7. 
