668 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
immersed or mesarch protoxylem* in the ends of the rays, and the position of the 
phloem between and around the rays of the stellate xylem. The leaf-traces in both 
Lycopodium and Asteroxylon depart in more than one vertical series from each ray 
of the stellate xylem. They consist of a strand of xylem, with the smaller 
protoxylem elements in a central position, surrounded by a zone of soft tissue or 
phloem. The anatomical parallel between Asteroxylon and Lycopodium extends to 
the differentiation and zoning of the cortex, although the trabecular condition of 
the inner cortex is never so marked in the latter genus. 
The termination of the leaf-trace in the leaf-base of Asteroxylon, so that the 
simple leaf-blade has no vascular bundle, is a noteworthy point of difference from 
Lycopodium. Other differences concern the type of thickening of the tracheides 
of the metaxylem, already referred to, and the absence of definitely specialised sieve- 
tubes in the phloem of Asteroxylon. In both these respects the stele of Asteroxylon 
appears to be less differentiated than that of Lycopodium. 
A more distant comparison can be made between the stele of Asteroxylon with 
its stellate xylem and, the stellate stem-steles of some Zygopteridese, such as Astero- 
chlasna and Asteropteris. The points of difference as regards both stele and leaf- 
traces are perhaps more important than those of agreement. The discovery in 
Asteroxylon of an ancient plant with a stele so like that of Lycopodium tends 
to strengthen the tentative comparisons that have been made between the 
Zygopteridean and Lycopodium steles, t 
Another comparison that requires mention is that between the anatomy of 
Asteroxylon and Stauropteris. The vascular structure of the latter is clearly more 
fern-like. It would be dangerous to go further at present than to recognise 
that there are certain similarities in the arrangement of the tissues of the stele 
between Stauropteris and the stems of Asteroxylon. These features in the structure 
of Stauropteris have excited interest and proved a source of difficulty since the plant 
was first recognised.]; 
4. The various comparisons made above were between the vegetative organs of 
Asteroxylon and other plants. The following comparisons concern the small axes 
with peculiar anatomy, and the associated sporangia which we regard as probably 
the fertile region of the same plant. 
{a) The type of association of the plant remains in the block of chert which 
we owe to Dr Gordon is suggestively like the appearance presented by sections 
across the massed remains of Stauropteris oldhamia. It has been pointed out 
* The protoxylem of Lycopodium is usually exarch, as it may also he in Asteroxylon. Where larger leaf-traces 
join the stem-xylem in some species of Lycopodium a mesarch structure is shown. (Cf. Sinnott; “On Mesarch 
Structure in Lycopodium ,” Bot. Gazette, 48, pi. x, fig. 5.) 
f Cf. Bertrand, P., Progressus Rei Botanicae, Bd. iv, p. 276. 
t The vascular structure of Stauropteris will evidently be regarded differently according to whether it is assumed 
that it is a specialised Zygopteridean frond (Bertrand, P., Etudes sur la Fronde des Zygopteridees , Lille, 1909), or that 
it is a plant of more archaic type than the Zygopterideas, though tending in their direction (Lignier, Bull. Soc. 
Bot. de France, t. 59 (1912), p. 1). 
