SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 667 
though less strikingly, in the plants of Psilotum and Tmesipteris, and is brought out 
very clearly in the young stage of the latter. As Holloway’s * account and figures 
show, one rhizome-like branch of the young plant on the prothallus turns up and 
becomes leafy, while the other extends into the soil in a root-like fashion. This 
second branch, however, in some cases becomes a leafy shoot. 
The close comparison thus possible with Psilotum and Tmesipteris helps us in 
picturing the mode of growth of Asteroxylon , and adds weight to the view that the 
Psilotales have on the whole preserved a primitive type of organisation, and do not 
owe their simplicity to reduction. 
In contrast to the comparison with the Psilotacese, that afforded by the species 
of Lycopodium and Selaginella, which show a distinction of subterranean rhizomes 
and aerial shoot-systems, is more superficial. The rhizomes in the Lycopodiacese 
bear small scale-leaves, and are probably to be regarded as modified aerial shoots. 
They also bear true roots. 
2. As regards the external morphology of the leafy aerial shoots of Asteroxylon 
and the relations of stem and leaf, the comparison is perhaps closest to Lycopodium 
among existing plants, though it also holds in all essentials for the Psilotacese. The 
dichotomous and lateral branching also find parallels in the shoot-systems of the 
same plants. 
In this connection the existence in Early Devonian times of a number of plants 
known as impressions, in which the stems were more or less closely clothed with 
small leaves, must be referred to here. A striking example is Thursophyton 
( Lycopodites ) Milleri, from the Middle Old Red Sandstone of Scotland and Norway. 
This and other ancient plants, such as Psilophyton and Arthrostigma, will be con- 
sidered further below. 
3. In its anatomy Asteroxylon is most closely comparable with the Psilotacese 
and with Lycopodium, though some other plants must be mentioned. 
The simple stele of the rhizome of Asteroxylon can be compared with that of the 
rhizome of Psilotum and Tmesipteris. There is a central strand of tracheides, with- 
out evident protoxylem, surrounded by phloem. The transition from this structure 
to the more complex stele of the leafy shoot also finds its closest parallel in the 
Psilotacese. As in these plants, it is associated with the appearance of small scale- 
leaves. 
The stele of the aerial shoot of Asteroxylon corresponds in a striking fashion with 
that of the stem of Lycopodium. That there are also points of resemblance to the 
stele of Psilotum and to the primary structure of Sphenophyllum need only be 
mentioned in passing, since they concern features that are better shown in Lycopodium. 
Outstanding points of comparison between the steles of Asteroxylon and of some 
species of Lycopodium (e.g. L. Selago) are the circular outline of the stele as a 
whole, the stellate outline of the xylem with the ends of the rays often enlarged, the 
* Holloway, J. E., “The Prothallus and Young Plant of Tmesipteris ,” Trans. N.Z. Institute , vol. 1, 1917. 
TRANS. ROY. SOC. EDIN., VOL. LII, PART III (NO. 26). 103 
