674 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
of the Lycopods, while the characters of Stauropteris (though this plant is in its own 
way more advanced and specialised) may suggest the connection of the fern type. 
Partly because of the insufficiency of our knowledge of the plants, and partly 
because of the divergent tendencies towards more specialised groups which they 
exhibit, it must be a matter of difficulty to define the characters of the group to which 
Asteroxylon and Psilophyton belong. We have reason for regarding these plants as 
differentiated into rhizomes, branched aerial shoot-systems with relatively small 
leaves, and leafless branches bearing dehiscent sporangia. Such plants have a more 
complex organisation than the Rhyniacese, with which they will be compared below. 
The present state of our knowledge regarding them would appear to be fairly, though 
tentatively, expressed by distinguishing a more complex family of the Psilophytales, 
which may be termed the Asteroxylacese. 
Comparison of Asteroxylon with the Rhyniacese. Conclusion. 
Asteroxylon must, in conclusion, be compared with the still simpler type of 
Vascular Cryptogams represented by the Rhyniacese. 
The simple structure of the rhizome and basal region of the plant in Asteroxylon 
is clearly comparable with that found throughout the plants of the Rhyniacese. 
There are also indications of a similarly simple construction in some ultimate distal 
divisions' of Asteroxylon. In the leafy middle region of the shoot of Asteroxylon a 
complication in internal structure accompanies that in the external form. 
The small dehiscent sporangia which are presumed to belong to Asteroxylon 
appear to be of an advanced and specialised type as compared with those of Rhynia 
and Hornea. 
Asteroxylon differs from Rhynia and Hornea in the presence of undoubted leaves 
on the aerial stems. The only structures in the Rhyniacese which suggest a 
comparison with leaves are the peculiar hemispherical projections on the stems of 
Rhynia Gwynne- Vaughani, and it is possible to regard these as affording a clue to 
the leaves of Asteroxylon. On the other hand, we are acquainted in the Algse with 
a number of independent examples of the integration of originally similar members 
of a thalloid branch system into a new whole in which the continued axis of growth 
appears as a relatively main “ stem” bearing the subordinated branches or “ leaves” ; 
the relation of the shoot in Asteroxylon to the thalloid type of plant-body in the 
Rhyniacese may alternatively be regarded in some such fashion. 
To enter more fully into the meaning of the differences between the organisation of 
Asteroxylon and that of the Rhyniacese would involve a discussion of the nature of the 
leafy shoot. We shall not enter upon the consideration of the general question here. 
The Vascular Plants which grew together in the Rhynie peat have fortunately 
preserved for us the structure of examples of both the simple and the more complex 
types of the archaic plants which we associate as the Psilophytales. They do not 
