: 
4 
* 
7 
q 
sweet 
K.— BOTANY. 185 
been made to Professor Halle’s position. After speaking of the possible 
relation of the Psilophyton type to Lycopods on the one hand and Ferns 
on the other, he adds: * From this point of view the whole pteridophytic 
stock would be monophyletic, the Lycopsida and the Pteropsida being 
derived from a common form already vascular. It would not thus be 
necessary to assume a parallel evolution of a similar vascular system 
along two different lines.’ (Halle, l.c., p. 39.) 
He does not refer to the Articulate, of which, it is true, there are 
only the most doubtful indications in the Lower Devonian rocks. Halle, 
too, accepts Ligmier’s view of the twofold origin of the leaf, from 
emergences in the Lycopsida, from thallus-branches in the Pteropsida. 
Kidston and Lang, in the light of their Rhynie discoveries, regard 
Halle’s survey as ‘ a fair statement of the present bearing of the imper- 
fectly known facts.’ They lay great stress on the synthetic nature of 
their genus Asterorylon, which they say ‘appears to agree with Psilo- 
phyton in possessing in a generalised and archaic form characters that 
are definitely specialised in the Psilotales, Lycopodiales, and Filicales.’ 
They add: ‘ The Geological age and succession of the Karly Devonian 
plants are, on the whole, consistent with the origin of the various 
groups of Vascular Cryptogams from a common source.'t We have 
already referred to the Bryophytic features, which have been recognised 
in the Rhyniacee. Kidston and Lang make use of these to extend their 
tentative conclusions to the Bryophyta. In concluding their third 
memoir they say: ‘In Rhynia and Hornea we have revealed to us a 
much simpler type of Vascular Cryptogam than any with which we 
were previously acquainted. This type suggests the convergence of 
Pteridophyta and Bryophyta backwards to an Algal stock. The know- 
ledge of Asteroxylon confirms and enriches our conception of a more 
complex but archaic type of the Vascular Cryptogams, which supports 
the idea of the divergence of the great classes of Pteridophyta from a 
common type, and links this on to the simpler Rhyniacee’ (l.c., 
p. 675.) The monophyletic view, though stated with appropriate caution, 
could not be more clearly expressed. It is fully maintained in these 
authors’ later statements. 
It is evidently impossible to decide between the two theories in 
the present state of our knowledge; we are now only beginning to 
acquire some conception of the vegetation of Karly Devonian times. The 
discovery, however, of the existence at that period of an unexpectedly 
simple race of vascular plants to some extent favours a monophyletic 
interpretation, even though we accept with some reserve the wonderful 
- synthesis of characters which Asterorylon appears to exhibit. To some 
minds, too, the important points in which all existing Pteridophyta, how- 
ever diverse, agree will still suggest a common origin not too remote. 
Among such common characters may be mentioned the alternation of 
generations with the sporophyte predominant ; the development both of 
the spores and the sexual organs; and the histology, especially of the 
11 On Old Red Sandstone Plants, showing structure, from the Rhynie Chert 
4 Bed, Part III., p. 673. In Part IV. this conclusion is further emphasised, 
and it is suggested that the Rhyniacee are really too simple morphologically 
to suit the views of either Lignier or Church. 
1921 Mi 
