356 KIDSTON : THE FLORA OF THE CARBONIFEROUS PERIOD. 
The structure of the Sigillarian stem is. in some cases, of the 
same type as occurs in Lepidodendron where the primary wood forms 
a close ring enclosing the pith ; in other examples the primary wood 
foi^ms a circle of distinct, but closely-placed, vascular wedges. These 
two types pass into each other, the passage taking place even in the 
same specimen, by the lateral union of the wedges among themselves. 
As in Lepidodendron, increase to the vascular system takes place by 
the addition of a zone of secondary wood from a cambium layer. The 
Sigillaria is an ex- 
tremely distinct genus 
which comprises many 
species. It is rare in Lower 
Carboniferous times, and 
reaches its maximum de- 
velopment in the Middle 
Coal Measures, where it 
is represented by a con- 
siderable number of species. 
The SigillaricB must 
also have played a most im- 
portant part in the forma- 
tion of coal, and probably 
the spore-bands, to which 
reference has already been 
made, were largely contri- 
buted to by spores from Sigillarian cones. 
Stigmaria Brongniart. Stigmaria (Plate LVL, fig. 3, and Text 
fig. 10), which is the rhizome of the arborescent Lycopods, diverges 
from the base of the trunk in four main branches. Shortly after 
leaving the trunk these again bifurcate, forming eight branches, and 
again, at a distance of a few feet from the previous fork, these 
bifurcate into sixteen branches. They do not again divide, or if so, 
only very rarely. Any terminations I have seen end in a blunt point. 
The outer surface of the cortex bears quincuncially-arranged 
rootlet scars, consisting of a slightly raised rim, containing a circular 
whole is enclosed by a thick bark. 
Fig. 10. — Stigmaria Jicoidts Steriib. sp. 
A, portion of rliizome, natural size ; 
B, rootlet scar — a, vascular cicatrice ; 
h, circular depression. 
