SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 
673 
In a recent paper Halle has discussed the Early Devonian flora on these lines. H6 
surveys the assemblages of plants described from various localities, and indicates the 
general distinction between those of Lower and Middle Devonian strata, and the 
contrast of both with the plants of the Upper Devonian. In his general botanical 
conclusions he points out the prevalence of plants with relatively small spine-like 
leaves (Arthro stigma, Psilophyton) in the Lower Devonian, and suggests that from 
such plants we can pass to the undoubted Lycopodiaceous forms of the Upper 
Devonian. He also points out the prevalence in the Lower Devonian of remains 
like the branched rachises of fern-fronds, but with no indication of pinnules or 
laminae. He suggests that the points of comparison of these, on the one hand with 
such a form as Psilophyton Goldschmidti, and on the other with fern-fronds, enables 
us to think of the Lycopsida and Pteropsida as connected in origin from a common 
form that was already vascular. Halle regards the fructification of Psilophyton as 
described by Dawson (which, however, he separates under the name of Dawsonites 
arcuatud) as the best evidence of the beginning of the Filicinean phylum in the 
Older Devonian flora. He points out the prevalence in succeeding ancient Palaeozoic 
strata of large sporangia borne on special branched fronds without a lamina. The 
natural interpretation is that these fertile fronds represent the primitive state, 
and that the flattened pinnules are a later development by cladodification as 
suggested by Lignier, the sporangia being pre-existent in respect to the laminae 
of the pinnules. 
We have summarised the views of Lignier instead of discussing the question 
afresh, because, without accepting his theory in all details, we find ourselves in 
general agreement with his conception of a divergent progression from the primitive 
land-plants towards the more specialised Lyeopodiales on the one hand and towards 
the Filicinean type of plant on the other. Halle’s survey of the Devonian 
floras in the light of these views also appears to us to be a fair statement of the 
present bearing of the imperfectly known facts. It seems advisable to keep the 
general conception on broad lines, in the hope that as our knowledge of the historical 
data increases the details will emerge naturally. The interest and significance of 
the more generalised plants, the external form and internal structure of which are 
becoming better known, will remain even if it should prove that higher types 
coexisted with them. 
The characters of Asteroxylon are consistent with, and support, the conception 
of the general course of the differentiation of early land-plants outlined above. 
Asteroxylon appears to agree with Psilophyton in possessing in a generalised and 
archaic form characters that are definitely specialised in the Psilotales, Lyeopodiales, 
and Filicales. The Geological age and succession of the Early Devonian plants are 
on the whole consistent with the origin of the various groups of Vascular Cryptogams 
from a common source. If we think of Psilophyton as giving a fair idea of the 
characters of this type, Asteroxylon shows indications of divergence in the direction 
