644 DR R. KIDSTON AND PROP. W. H. LANG ON OLD RED SANDSTONE PLANTS 
the enlarged ends of the rays. This construction was shown in the single figure 
published, without description, in Dr Mackie’s paper.* The structure of the 
branched shoot-system was primary throughout, no indication of secondary thicken- 
ing having been found even in the largest stems. In addition to the leafy shoots, 
leafless cylindrical axes with a simpler type of central cylinder are met with. These 
rhizomes constituted the subterranean region of Aster oxylon. Their finer branches 
behaved like roots, while the transition between the larger rhizomes and the leafy 
shoots can be clearly followed. There is conclusive evidence, both from continuity 
and from the histology, that the rhizomes, with their fine root-like branches and the 
leafy shoots, were parts of the one plant, Asteroxylon Mackiei , the vegetative organs 
of which are thus pretty fully known.'! 
In two loose blocks of the chert small axes of a peculiar type have been found 
closely associated with the typical remains of the vegetative organs of Asteroxylon. 
In one of the blocks there were in addition dehiscent sporangia containing cuticu- 
larised spores. We regard these structures as possibly, or probably, constituting the 
fertile portion of Asteroxylon. Their description will therefore be given in this 
paper before the general discussion of the morphology of the plant, although in the 
absence of evidence of continuity or histological identity these interesting remains 
must be treated to some extent independently. 
The various parts mentioned above will now be described. It will be convenient 
to deal in order with the leafless rhizomes, the transition from these to the leafy 
shoots, the shoots of various sizes, and lastly with the peculiar axes and the sporangia 
associated with them. 
Rhizomes. 
The leafless axes of various sizes which are met with among the other remains of 
Asteroxylon have a broad cortex and a central cylinder consisting of a simple central 
strand of xylem surrounded by a wide zone of phloem. While the behaviour, 
especially of the finer branches, is very root-like, the axes with this type of con- 
struction, whatever their size, will be spoken of as rhizomes. The main reason for 
this is that, as will be seen in the next section, a gradual transition can be traced 
between certain of the leafless axes and the leafy shoots. Since some of the leafless 
axes were thus undoubtedly rhizomes, and no clear distinction can be drawn between 
these and the more root-like specimens, they seem best regarded as parts of a 
rhizome-system something like that of the existing Psilotales. 
The mode of occurrence of the rhizomes at places in the upper part of bed A" 1 
* Mackie, W., “The Rock Series of Oraigbeg and Ord Hill, Rhynie, Aberdeenshire,” Trans. Edin. Geol. Soc . , 
vol. x, pp. 205-236, pi. xxiii, fig. 6. 
t It may assist the reader in forming an idea of the vegetative organs of Asteroxylon if we note at the outset the 
parallel afforded by the Psilotacese, in which the plant has a leafless, cylindrical, subterranean rhizome of simple 
structure, certain branches of which pass into the more complex aerial leafy shoots. There are also resemblances in 
structure between the leafy shoot of Asteroxylon and that of the Psilotacese, but the most helpful parallel as regards 
the anatomy is found in some species of Lycopodium. 
