September 29, 192 1] 



NATURE 



15 7 



remains of which Palaeopitys Milleri, from the 

 Middle Old Red Sandstone of Cromarty, is the 

 type. We need much further investigation of 

 these higher forms of Early Devonian vegetation, 

 but we know enough to impose caution on our 

 speculations. 



The Rhyniacese, at all events, were leafless and 

 rootless plants. In one species of Rhynia and in 

 Hornea the aerial stems are entirely without any 

 appendages, while in the other Rhynia there are 

 hemispherical swellings, which have been identi- 

 fied by Arber with certain states of the spines in 

 Psilophyton. The emergences of R. G-wynne- 

 Vaiighani have been interpreted as nascent leaves, 

 but more recent- observations, showing their late 

 histological origin, have rendered this hypothesis 

 very doubtful. 



In Asteroxylon, a higher plant altogether, the 

 stem is clothed with quite distinct leaves, though 

 they are somewhat rudimentary as regards their 

 vascular supply. Have we, in these plants, and 

 others of contemporary date, the first origin of 

 the leaf from a mere non-vascular emergence, or 

 had reduction already begun, so that in 

 Rhyniacese, for example, the leaves were in the 

 act of disappearance? In the former case we 

 should be assisting at the birth of Lignier's 

 phylloids, the microphylls of the Lycopod series. 



But the opposite view may also be tenable. 

 We have already seen that these plants have been 

 referred both to the Pteridophytes and the Thallo- 

 phytes ; they also show signs of Bryophytic 

 affinities, and I understand that it has even been 

 proposed to include them in the Bryophyta, in 

 which case every possible view will be repre- 

 sented. The Sphagnum-like structure of the 

 columellate sporangium or sporogonium of 

 Hornea and Sporogonites may justify the Bryo- 

 phytic attribution, and it is then, of course, easy 

 to extend it to Rhynia. If we were to adopt this 

 opinion we should probably have to regard these 

 simple Devonian plants as representing stages in 

 the reduction of the sporophyte to a sporo- 

 gonium, the leaves being already nearly or quite 

 lost, while the branched thallus was still much in 

 excess of the simple seta of the modern Moss or 

 Hepatic. Naturally we know nothing of the 

 gametophyte, so that the material for compari- 

 son is limited. Kidston and Lang, however, have 

 recently pointed out that the presence of spore- 

 tetrads clearly indicates the existence of a 

 gametophyte. 



I make no attempt to decide between these 

 views. There can be no reasonable doubt that 

 the Psilophytales generally represent an earlier 

 phase of cormophytic life than any of the groups 

 previously recognised. But we must not assume 

 that all their characters were primitive. It has 

 been pointed out that the Rhyniaceae were peat 

 plants, and that the peat flora is apt to be peculiar. 

 Under such conditions it is not improbable that 

 a certain amount of reduction may have already 

 been undergone, though this is not the view taken 

 by the investigators. 



NO. 2709, VOL. 108] 



The recent work on the Early Devonian flora 

 has wide bearings. It has. long been noticed that 

 among the fossils of that period no typical fern- 

 fronds are found. Those remains which are most 

 suggestive of fern-like habit consist merely of 

 a naked, branched rachis. It used to be assumed 

 that the absence of a lamina might be explained 

 by bad preservation. But, as Prof. Halle 

 points out, the chief reason for condemning the 

 preservation as bad was the fact that a lamina was 

 absent ! 



The evidence really seems to indicate that the 

 so-called fronds of that age did not possess a 

 leaf-blade. As Prof. Halle says: "In the 

 Lower Devonian, finally, we find frond-like struc- 

 tures bearing sporangia, but no fronds with 

 developed laminae. One can hardly escape the 

 conclusion that the ' modified ' fertile fronds may 

 represent the primitive state in this case and that 

 the flattened pinnules are a later development, 

 as suggested by Prof. Lignier. " These naked 

 fronds may, in fact, be regarded as the 

 little-differentiated branches of a thallus. 



The evidence, as at present understood, seems 

 to suggest that, in the earlier Devonian flora, 

 ferns, properly so called, may not yet have been 

 in existence. The predecessors of the ferns were 

 there, no doubt, but not, so far as we know, the 

 ferns themselves. Yet it seems that highlv 

 organised stems of a gymnospermous type were 

 already present at about the same period. Thus 

 the evidence from the older Devonian flora, so 

 far as it goes, materially supports the opinion that 

 the seed plants cannot have arisen from ferns, 

 for the line of the Spermophyta seems to have 

 been already distinct at a time when true ferns 

 had not yet appeared. 



The idea, which I once advocated, that the 

 Gymnosperms were derived, through the Pterido- 

 sperms, from the ferns must, I think, be given up, 

 on grounds which were stated two years ago at 

 the Bournemouth meeting of the Association. It is 

 safer to regard the Pteridosperms, and therefore 

 the seed plants generally, as a distinct stock, prob- 

 ably as ancient as any of the recognised phyla of 

 vascular Cryptogams, and derived from some 

 unknown and older source. At the same time the 

 striking parallelism between the Pteridosperms 

 and the true ferns must be recognised. These 

 views are essentially in agreement with those pre- 

 viously expressed by my friend Dr. Kidston. 



The significance of the Pteridosperms has per- 

 haps been somewhat misunderstood. It now 

 seems that they do not, as some of us once 

 imagined, indicate the descent of the seed plants 

 from ferns, but rather show that the seed plants 

 passed through a fern-like phase ; they ran a 

 parallel course with the true cryptogamic ferns, 

 and, like them, sprang from some quite earlv 

 race of land plants, such as Rhynie has revealed 

 to us. But the phylum was never any more fern- 

 like than the Pteridosperms themselves. 



On our hypothesis, the Upper Palaeozoic phvla, 

 with which we have to reckon, are the Pterido- 



