(zea 
cal features, which are superficially very similar throughout, as 
are the leaves, the often large cones with two kinds of spores, and 
the structures which answer to roots and are known as Stigmaria. 
A Stigmaria 1s shown at the base of the Sigillaria windfall in the 
lower right hand corner of the transparency, and these stigmarias, 
with their rootlets, are among the commonest Carboniferous fos- 
sils, especially in the clays underlying the coal seams and in the 
roofing shales immediately above the coal. 
In both Sigilaria and Lepidodendron the zone of secondary 
us me- 
— 
wood is thin and weak, and they sought to make up for t 
chanical weakness by a greatly thickened and complicated cortex, 
which, since most of them grew in wet soil, necessitated rather 
elaborate devices for getting air inside. The geometrical patterns 
of leat cushions with their leaf-scars, which ornament the surface 
of the trunks, are of great importance for classification, since so 
much of the fossil material consists of trunks or flattened frag- 
ments of cortex. 
In Lepidodendron the leaves were borne on spirally arranged, 
crowded leaf-cushions or bolsters, which were always longer than 
wide, with sharp angles at the ends, and rounded sides. These 
bolsters show a subcircular leaf-scar with various subordinate scars 
of the central vascular strand, a pair of scars marking the strands 
— 
of aerating tissue, and one above of the ligule (a membranous 
structure associated with the leaves). 
In the sigillarias the scars are one above the other, the bolsters 
y elevated, and are wider than long, with the 
— 
are usually not great 
angles at the sides, and rounded above and below. A large group 
of sigillarias have the bolsters contiguous, forming prominent ver- 
tical ribs. The actual leaf-scars are not very different from those 
of Leptdodendron, but the leaves are frequently much longer. 
30th, aiter death, tend to have the cortex separate into layers, and 
with the decay of each iayer the appearance of the stem changes. 
The older paleobotanists thought each represented a different genus 
for which they proposed names that are still useful as descriptive 
terms. 
The sigillarias were much more sparingly branched, and seem to 
have varied more in habit than the lepidodendrons. Some pieces 
of stems of columnar species have been found preserved for a 
