Relation of Pteridosperm Anatomy to that of Cycads. 43 
Considering first Scott’s theory with regard to the derivation 
of Cycadean anatomy from the Lyginopteridean type :— 
Heterangium (19; 14, p. 401) is the simplest known member of 
this alliance. It is a genus having a considerable geological range, 
from the Scottish Calciferous Sandstone at the base of the 
Carboniferous, through the Coal Measures to the Permian beds of 
France. Heterangium Grievii, the oldest and best known species 
may be taken as typical of the genus. The specimens come from 
the Calciferous Sandstone of Pettycur, and the reference to the 
Lyginopteridese is based mainly on vegetative characters, although 
it is supported by the association of the stems with seeds 
(Conostoma ) of the same type as those of Lyginopteris (8, p. 111.) 
The stem (Fig. 1, a) has a single vascular cylinder, that is, it is 
monostelic. The stele is solid, consisting of a central mass of 
grouped tracheides (the primary wood) and strands of conjunctive 
parenchyma; the tracheides are, as a rule, much more abundant 
than the parenchyma, and there is never any attempt, on the part 
of the cellular tissue, to form a pith. Definite clusters of tracheides 
are arranged peripherally and contain the spirally-thickened 
mesarch protoxylem elements, with which are associated a few 
parenchymatous cells (Fig. 1, b). These clusters of tracheides are 
continuous with the leaf-trace bundles. The primary xylem 
elements, apart from the protoxylem, typically possess multiseriate 
bordered pits on all walls. Most specimens show a development of 
secondary wood, the secondary tracheides bearing numerous 
bordered pits on their radial walls only. A ring of phloem 
surrounds the xylem and is succeeded by pericycle. 
The leaf-trace bundles are of large size, and are usually 
without any secondary tissues. They are at first collateral, but 
become concentric before entering the leaf-bases. The traces in 
Heterangium Grievii usually remain undivided throughout their 
course, each petiole being traversed by a single vascular bundle of 
concentric mesarch structure. Other Heterangiums have double 
leaf-traces, this being the case in Heterangium tiliceoides, but here 
it appears that the two bundles unite in the petiole (19, p. 763). 
The anatomy of Heterangium does not of itself indicate a 
connection with that of the Cycads. It is in the next type, clearly 
derivable from Heterangium , that the similarity between the recent 
and fossil types is seen. 
Lyginopteris 1 (19; 14, p. 357) represents the next known stage 
' Potonie’s name Lyginopteris is adopted here, as it is undoubtedly more 
correct than Lyginodendron, a name which has been used for quite different 
types. See Potonie, Lehrbuch der Pflanzenpalseontologie, p. 171, 1899. 
