297 
the Periderm of Fossil Lycopodiales. 
seems more likely that this condition is due to the vicissitudes incidental 
to petrifaction, rather than to preservation at a time of inactivity of the 
meristem. 
The Periderm. 
(i) General Description. 
The periderm forms a most striking feature of the stems and Stig- 
mariae of fossil Lycopods. It may attain a thickness of seven or eight 
centimetres, and, in petrifactions of many specimens, periderm and xylem 
alone are preserved, while detached pieces of the tissue are also extremely 
common. Always easily recognizable, even in young specimens, by the 
character of its elongated cells and their arrangement in regular radial 
files, it may readily be distinguished near the periphery of the organ, 
where it forms a cylindrical or wavy band of tissue, often rather dark in 
colour, and very frequently showing some sort of zonation. 
Generally the periderm presents a homogeneous appearance in 
transverse section, but, as has been mentioned, the outer layers of cer- 
tain Stigmariae are very conspicuously extended tangentially, while in 
Sigillaria spinulosa and other species of its type the outer portion forms 
a network, of which the strands consist of typical periderm cells, while 
the meshes are filled with thin-walled parenchymatous tissue. 
(2) Size and Shape of the Cells. 
The characteristic cells of the periderm, which have frequently been 
described, are prosenchymatous, often many times as long as broad, with 
parallel sides and pointed, somewhat gabled, ends. 
In transverse section they are generally smaller than the cells of the 
outer cortex, and vary considerably, both in size and shape, even in adjacent 
files (see Text-fig. 7, &c.). On the whole they are radially elongated, 
except the outer cells near the phellogen, which are frequently tangentially 
extended. In the second type of Stigmarian periderm mentioned on p. 294, 
the tangential extension of many of the cells of the outer part of the tissue 
is most striking (PI. XXIV, Fig. 2). It is not merely ‘a necessary result of 
the position of the phellogen on the internal edge of the tissue and of the 
increasing girth of the axis but depends on the actual nature of the wide- 
celled portion, of which there are so few files compared to the number 
formed on the inner side of the supposed phellogen. In type 1 the exten- 
sion is hardly noticeable (PI. XXIV, Fig. 1). 
In radial section the appearance of the periderm is very characteristic 
on account of the parallel sides of the cells, and of all the members of one 
file being of uniform length, so that the end walls form almost straight lines. 
The variations in appearance produced, owing to most sections being more 
1 Seward ( 28 ), p. 243. 
