PRESENT POSITION OF PALEOZOIC BOTANY 
SCOTT. 881 
vertical series, marked, in a large section of the genus, by the presence 
of prominent ribs. 
Anatomically, the stem of the Lepidodendrez is in all cases mono- 
stelic, with centripetal primary wood, which may extend to the cen- 
ter, or form a ring inclosing a medulla. While in Lepidodendron, 
Bothrodendron, and some species of Sigillaria the primary wood is 
continuous, in other Sigillarie (S. menardi and S. spinulosa) the 
xylem ring is broken up, more or less completely, into distinct 
bundles. These bundles, however, never pass out into the leaves, 
but in all cases the single, usually collateral leaf trace is detached 
from the stele without giving rise to any leaf gap. 
In most species there was a considerable development of centrif- 
ugal secondary wood, consisting of tracheides and medullary rays, 
with a marked radial arrangement of the elements. In a few species 
(e. g., Lepidodendron Harcourtii, the first fossil Lycopod discovered 
with structure preserved) no secondary wood has yet been observed. 
Almost without exception both primary and secondary tracheides 
are of the scalariform type. Although the presence of primary 
phloem can be recognized with certainty, some doubts have been ex- 
pressed as to the production of secondary phloem by the cambium. 
In certain cases (Lepidophloios fuliginosus and Lepidodendron 
obovatum) the secondary xylem may be largely, or even wholly, 
parenchymatous. In all cases, even where secondary vascular tissues 
have not been observed, there was an extensive formation of periderm, 
chiefly in the form of a phelloderm probably produced on the inner 
side of the generative layer. 
The leaves show marked xerophytic adaptations; the vascular 
bundle was surrounded by a sheath of tracheal transfusion tissue, 
and the stomata were commonly sheltered in two deep grooves on 
the lower surface of the leaf. In the curious genus or subgenus 
Sigillariopsis the leaf is traversed through most of its length by two 
vascular bundles, a unique case among Lycopods. According to Re- 
nault, the French species of this genus is further remarkable for the 
occurrence of pitted, as distinguished from Scalariform tracheides. 
An interesting feature in the leaves of the Paleozoic Lycopods 
is the very general presence of a ligule, situated, like that of the 
recent Ligulate, on the upper side of the leaf base and usually 
seated in a deep pit. 
Our knowledge of Stigmaria, which represents the subterranean 
parts of the Lepidodendrew, is still very imperfect, although fossils 
of this nature are among the very commonest Carboniferous speci- 
mens, both as casts and petrifications. The difficulty is that it is 
still impossible to refer the various specimens of Stigmaria to the 
species, or even the genus to which they belonged. Stigmaria has 
been found in connection with the stems both of Sigilaria and 
