the Periderm of Fossil Lycopodiales . 315 
In Bothrodendron , Stigmaria type 1, and Stigmaria type 2 the outer 
primary cortex is persistent with a development of 10, 40, and 80 rows of 
periderm cells, respectively. 
The lack of cork and the nearness to the surface of a delicate layer like 
the phellogen may be mentioned as lending support to Seward’s suggestion, 
owing to the absence of annual rings in the wood, that these plants were not 
exposed to seasonal change but grew under uniform climatic conditions ; 1 
nor could the conditions have been extreme in character. 
(2) The Outside Sculpture. 
It has been seen above that there is no evidence in the fossil Lycopods 
of the presence of bark, in the sense of dead and dried-up tissues which 
have been cut off by the production of a zone of cork ; but the question still 
remains whether the leaf-bases, and subsequently the outer layers of the 
periderm, were thrown off during life as the natural sequence of increase in 
age and girth of the. organ. This has been taken as the normal course of 
events by many writers. Among others, Hovelacque stated that in Lepido - 
dendron selaginoides decortication would take place along the zones in the 
periderm, 2 and Renault along the zones between the meshes in the Dictyo- 
xylon types, 3 while Williamson spoke of the outer tissues being thrown off 
in Lepidodendron Wunschianum , though not during life in Stigmaria , 4 
More recently, Seward has described the stretching and gradual obliteration 
of the leaf-cushions, the production of longitudinal fissures, and later the 
exfoliation of the outer tissues both in Lepidodendron and Sigillaria , 5 so 
that in Lepidodendron Wunschianum , for instance, * the bark presents 
a fissured appearance like that with which we are familiar on an old Oak 
or Elm stem.’ 6 On the other hand, Potonie commented on there being no 
formation of bark, as the epidermis is retained and follows the growth in 
thickness of the stem. 7 
In considering this question it must be remembered that specimens are 
found showing every stage of decortication with the corresponding differences 
in external appearance, which originally led to the foundation of the false 
genera, Bergeria , Aspidiaria , Knorria , &c. The difficulty is to determine 
how far these specimens represent the condition of the plant during life. 
Stems with furrowed outer layers have certainly been found, 8 while the 
great prevalence of detached pieces of periderm, or of periderm and the 
tissues outside, might be adduced as pointing to some of these having been 
cast off during life. 
1 Seward (28), p. 269. 2 Hovelacque (15), p. 158. 
3 Renault (21), p. 176 ; Renault and Roche (23), p. 8. 
4 Williamson (40), p. 497; (43), p. 26. 6 Seward (28), pp. 94, 105, 118, &c. 
6 Loc. cit., p. 170. 7 Potonie (18), pp. 219, 249. 
8 Described by Binney, Dawson, Williamson, Seward, &c. 
