666 DR R. KlDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
Comparative Discussion. 
The organisation of Asteroxylon, though more complex than that of Rhynia and 
Hornea, appears to be in some respects simpler than that of most Vascular Crypto- 
gams. The uncertainty as to the fertile region of the plant makes full comparison 
difficult. On the other hand, there is the great advantage of a fairly complete 
knowledge of the anatomy of the vegetative organs of Asteroxylon. 
On the grounds both of external organisation and of internal structure it is 
possible to make more or less close comparisons with some existing types of 
Pteridophyta and with some extinct archaic forms of this group. In the present 
imperfect, though growing, knowledge of the plants of Early Devonian times the 
recognition of points of comparison in various directions appears more desirable than 
attempts at a hypothetical completion of the facts. We propose in this discussion 
to confine speculation within definite limits, though it cannot be altogether avoided. 
The position arrived at in the preceding section of this paper must be borne in 
mind in the following comparisons. The connection in one plant of the rhizomes, stems, 
and leaves of Asteroxylon is established on satisfactory evidence. That the small axes 
of peculiar structure and the sporangia constituted the fertile region of Asteroxylon 
is regarded as a probable presumption on the grounds of their close association with 
this plant and their absence in relation to the other vascular plants of the bed. 
It should further be pointed out that, close as are some of the resemblances to 
be traced, Asteroxylon stands apart in certain respects from all other Vascular 
Cryptogams. One striking difference concerns the tracheides, those of the meta- 
xylem as well as of the protoxylem having a peculiar type of spiral thickening. 
On account of this and other differences, and because of the imperfection of our 
knowledge of the plant as a whole, it is well to avoid drawing conclusions as to 
actual relationships. 
Comparisons. 
It will be convenient to deal first with the undoubted vegetative organs of 
Asteroxylon , and then to extend the comparisons with other plants to the presumed 
fertile structures. 
1. The organisation of the plant of Asteroxylon into leafless rhizomes and leafy 
aerial shoots finds a close parallel in the existing Psilotales. The absence of hairs 
even from the most root-like branches of the rhizome of Asteroxylon is a remarkable 
but minor point of difference. The resemblance holds even in points of detail as 
regards the absence of definite roots, the gradual passage of branches with the 
structure of the rhizome into aerial shoots, and the appearance of leaves, at first as 
small scales without leaf-traces, on the region of transition from the rhizome to the 
shoot. The comparison is not inconsistent with the very root-like behaviour of 
many of the branches of the rhizome of Asteroxylon. Something similar is shown, 
