?p 47 
K.— BOTANY. 177 
as shown by the presence of a water-conducting tissue and stomata, 
and by the manifestly air-borne spores. These simplest land-plants 
are the Rhyniacee (Rhynia and Hornea), while the third genus, 
Asteroxylon, was more advanced and further removed from any possible 
transmigrant type. 
My friend Dr. Arber was so impressed by the primitive character 
of Rhynia (the only one of these genera then known) that he boldly 
called it a Thallophyte, while recognising, in respect of anatomical 
structure, an intermediate position on the way to Pteridophyta. This 
is not really very different from the view taken by the investigators 
themselves, though they call the plants Pteridophytes, which they 
certainly are, if we go by internal structure rather than external 
morphology. But if, as Kidston and Lang suggest, the Rhyniacewe 
‘find their place near the beginning of a current of change from an 
Alga-like type of plant to the type of the simpler vascular Crypto- 
gams,’* they must have been very primitive indeed and might even be 
regarded as fairly representing the true transmigrants which had not 
long taken to the land. 
It is true that the Middle Devonian is much too late a period for 
the original transmigration (I believe there is some evidence for land- 
animals in the Lower Silurian), but one may argue that some of the 
transmigrant forms may have survived as late as the Devonian, just 
as the Selaginella type seems to have gone on with little change from 
the Carboniferous to the present time. There must have been many 
such survivals of earlier forms in the Devonian period, if Arber was 
right in regarding all the characteristic plants of the Psilophyton Flora 
as ‘much more probably Thallophyta than Pteridophyta.’* Cer- 
tainly some of them, apart from the Rhyniaceew, have an alga-like 
appearance (e.g. Pseudosporochnus) and there is some evidence that 
such plants also were already vascular. There is, in fact, no doubt 
that the earlier Devonian Flora is turning out to have been on the whole 
more peculiar and more unlike the higher plants than we realised a 
few years ago. The Early Devonian plants cannot usually be referred 
to any of the recognised groups of Pteridophytes, and this is not owing 
to our imperfect knowledge, for it is just in those cases where ‘the 
plants are most thoroughly known that their unique systematic position 
is most manifest. Arber called all the plants in question ‘ Procormo- 
phyta ’—an appropriate name. As Kidston and Lang point out in 
their later work, the three groups—Pteridophyta, Bryophyta, and 
Alge—are brought nearer together by the Rhynie fossils. 
And yet there is evidence that about the same period stems with 
the highly organised structure of Gymnospermous trees already 
existed. I refer to remains of which Pale@opitys Milleri, from the 
Middle Old Red Sandstone of Cromarty, is the type. We need much 
further investigation of these higher forms of Early Devonian vegeta- 
tion, but we know enough to impose caution on our speculations. 
4 This view is further developed and expanded in the authors’ fourth 
Memoir, which I have had the privilege of reading in MS. 
5 Devonian Vloras, a Study of the Origin of Cormophyta. Cambridge, 1921, 
