840 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
the whole indicated plants of more primitive organisation than those known from 
the Upper Devonian onwards. Without entering into an enumeration or survey of 
these Early Devonian plants from Scotland, Norway, Canada, Bohemia, and elsewhere, 
some of their features may be mentioned. In many of them the stems bore more or 
less evident and usually relatively small leaves, while others resembled fern-fronds 
without laminae. The appearance of most of the Middle Devonian plants was such 
as to show that a distinction of stem and leaf had been established at that period. 
On the other hand such plants as Pseudo sporochnus , and especially the Lower 
Devonian Arthrostigma and Psilophyton, strongly suggested an even more primitive 
organisation on the whole. Many of these remains, though now clearly shown to 
have been vascular land-plants, have at various times been regarded as Algae. Other 
anomalous plants of the same period still imperfectly known may not improbably 
find their place in this latter group, or may have combined characters that are now 
distinctive of Algae and Pteridophyta. 
These indications of plants of simpler and more archaic organisation than any 
Vascular Cryptogams of later periods, and the consequent interest of the Early 
Devonian period for the study of early steps in the land-sporophyte, are not 
inconsistent with more highly organised plants having already existed at that period. 
The information on this point is insufficient, but large stems which bore regularly 
arranged lateral appendages and other stems with suggestively complex internal 
structure are known. It is the undoubted existence at that period of very simply 
organised vascular plants that is the fact of critical importance. 
Though Dawson’s restoration of Psilophyton was used by Lignier as an example 
of such more primitive types of land-plants, it was impossible to overlook the lack of 
critical evidence for some features in the reconstruction. The earlier data, though of 
great interest, were indeed almost always rendered less valuable owing to uncertainty 
regarding essential facts. The study of the Rhynie chert has fortunately added 
definite knowledge of the organisation of certain simple and very completely 
preserved plants of at least a near geological age. 
The information afforded by the study of Asteroxylon, the most complex of the 
Rhynie plants, would not by itself have added fundamentally to what was already 
known or inferred, although it would have rendered this much more complete and 
certain. In Asteroxylon we have a plant-body with shoots consisting of stems 
bearing numerous relatively small leaves. The plant thus conforms, on the whole, 
to the type of shoot known in Tliursophyton ( Lycopodites ) Milleri, Psilophyton 
princeps , and at the present day in Lycopodium and the Psilotacese. In the absence 
of definite roots and the possession of a simply constructed cylindrical rhizome 
continued directly (with the appearance of leaves and complication of the internal 
structure) into the aerial shoots, Asteroxylon shows a simpler organisation than any 
Lycopod. This condition is, however, found in the existing Psilotacese, and it 
presumably held also, as was stated by Dawson, for Psilophyton . While some 
