400 SPHENOPHYLLALES.—A. SPHENOPHYLLEAE 
early, so that the unaltered primary state is seen only in small twigs. 
Here a protostelic structure is seen, without any pith or conjunctive 
parenchyma. The primary xylem is of triangular form, the groups of 
protoxylem, either single or double, being at the projecting angles; or 
the angles may be duplicated, and a hexarch form be attained. The 
vascular system is strictly cauline: it passes through the nodes without any 
appreciable change of structure, a point of interest for comparison with 
the Equisetal structure as interpreted by Gwynne-Vaughan.! <A peculiarity 
of some importance for further comparison is shown in the primary wood 
4 =a leaf-whorl of Sphenophyllum cunetfolium, and one leaf of it somewhat enlarged. 
=a leaf-whorl of Sphenophllyum tenerrimum. C= Sphenophyllum cverticillatum. 
"speciosa, 
B 
(From Potonie’s Lehrbuch der Pflansenpaldontologie.) D=“ Trizygia’ 
Royle, from the Glossopteris-facies, (after O. Feistmantel.) 
of the ancient species, S. zvsigne, from the calciferous sandstone: here a 
canal is formed at each of the three angles of the primary wood, pre- 
sumably by disorganisation of the protoxylem, as in the Equisetales? 
(Fig. 217). The cambial activity commences immediately outside the 
primary wood, and results in a broad zone of secondary wood, which 
completely surrounds the primary: it is traversed by continuous medullary 
rays in S. signe, but in the later species these are represented only by 
little groups of thin-walled cells, which form, nevertheless, a continuous 
system. Outside the wood lie the phloem and the cortex, the latter 
showing a formation of periderm, which may be repeated, resulting in 
a scaly bark. 
‘Compare Williamson and Scott, Phel. Trans., vol. clxxxv., part. i1., p. 922. 
“Scott, Stedies, p. 88. 
