6o6 



NA TURE 



[November 4, 1922 



The Early History 

 By Dr. D. H. 



I. 



IN these articles the " Early History " of the land 

 flora is understood to cover the Devonian and 

 Lower Carboniferous periods. Before the beginning 

 of the Devonian the records of land plants are too 

 scanty and doubtful to demand much consideration ; 

 on the other hand, the flora of the Upper Carboniferous 

 is so rich and so well known that to include it would 

 unduly extend the limits of this brief survey. 



Concerning the question of the beginnings of a land 

 flora, the position has wholly changed of late, owing 

 chiefly to the Rhyme discoveries. A few years ago we 

 had no clear knowledge of any early Devonian plants, 

 and such imperfect data as we posesssed were commonly 

 ignored or discredited. Nothing definite was known 

 of any really simple fossil land-plants ; it could even 

 be asserted that the Devonian plants, though different 

 in many ways from those of the present day, were 

 about on the same general -level of organisation. 



Now we have learnt, from the Rhynie investigations, 

 that, in the earlier Devonian flora, plants existed of 

 quite surprising simplicity, with a mere thallus, leafless 

 and rootless, like that of some very ordinary seaweed, 

 but yet vascular in structure, and obviously adapted 

 to sub-aerial conditions. 



The crude simplicity of some of the Rhynie plants 

 even suggested the question whether we might not at 

 last be on the track of the original transmigrants from 

 the sea, of those Thalassiophyta whose conquest of 

 the land has been so vividly pictured by Dr. Church. 2 

 No doubt, the Devonian period was altogether too late 

 for the transmigration he describes, but still some of 

 the plants then living might have retained trans- 

 migrant characters. 



The Rhynie fossils are now fairly well known to the 

 botanical reader, and it is perhaps less necessary to 

 insist on their importance than to suggest a warning 

 that we may possibly expect too much from them. 



The flora is extremely limited and local ; our know- 

 ledge of the plants, owing to the unsurpassed skill and 

 judgment with which they have been worked out by 

 Dr. Kidston and Prof. Lang, 3 is exceptionally perfect, 

 but there are very few of them and they are all from 

 one old peat-bed. 



We may shortly recapitulate the leading facts. The 

 Rhynie species of vascular plants are four in number : 

 Rhynia Gwynne-Vaughani, R. major, Hornea Lignieri, 

 and Asterpxylon Mackiei. Rhynia and Hornea con- 

 stitute the family RhyniaceK, remarkable for its 

 extreme simplicity of structure ; Asteroxylon is a 

 much higher plant, and is placed in a distinct family. 



Both the Rhynias are rootless and leafless plants, 

 with a branched underground rhizome, and a vertical 

 aerial stem, also branched; the whole plant was eight 

 inches or more in height. On the rhizome there are 

 absorbent hairs, while the aerial stem possesses a few 

 perfectly typical stomata. Otherwise there is little 



1 Based on a course of lectures given last spring at University College 

 (University of London). 



2 A. H.' Church, " The Thalassiophyta and the Sub-aerial Transmigra- 



: '- : '.I 1 I M< I ! I . I'Jl'J 



J Kidston and Lang, " On Old Red Sandstone Plants, showing Structure, 

 from the Rhynie Chert-bed, Aberdeenshire," Transactions of the Royal 

 Soc. of Edinburgh, Part I., vol. 51, 1917 ; Parts II. and III., vol. 52, 191:0 ; 

 Parts IV. and V., vol, 52, 1921. 



N' '. 2766, VOL. I IO] 



of the Land Flora. 1 



Scott, F.R.S. 

 differentiation between the subterranean and sub- 

 aerial parts. The whole may be called, without hesita- 

 tion, a thallus. Both stem and rhizome are traversed 

 by an extremely simple vascular strand. 



Rhynia major is considerably the larger plant of the 

 two, but R. Gwynne-Vaughani is somewhat the more 

 differentiated, for its aerial stem is studded with 

 hemispherical outgrowths, from which, in some cases, 

 additional branches arose, and often became detached, 

 serving no doubt as a means of propagation. Both 

 outgrowths and adventitious branches are absent from 

 R. major, where the stem is merely forked. 



It was at one time supposed that the outgrowths of 

 R. Gwynne-Vaughani might represent very rudimentary 

 leaves, but later observations have shown that they 

 were developed late in life, usually in connexion with 

 the stomata, and thus formed no part of the original 

 equipment of the plant. They may even have been 

 traumatic in origin. 



The reproductive organs are spore-sacs (sporangia) 

 borne on the ends of branches. In R. major the 

 sporangia are large — nearly half an inch long; they 

 ha\ e ;i fairly complicated wall, and are filled with well- 

 preserved spores, often still grouped in fours, and in all re- 

 spects like those of the Higher Cryptogams now living. 



The second genus, Hornea, has a tuberous rhizome 

 comparable to the protocorm often found in young 

 Club-mosses, but the stem is like that of Rhynia major, 

 on a smaller scale, and just as simple. The sporangia 

 are the most remarkable feature ; they are terminal 

 on the branches, as in Rhynia, but in Hornea each 

 spore-sac has a central column of sterile tissue (the 

 columella), over-arched by the spore-bearing layer, 

 exactly as in the capsule of the Bog-Moss, Sphagnum, 

 at the present day. This moss-like feature is very 

 suggestive and has given rise to a good deal of specula- 

 tion. Another peculiarity of the spore-sac is that its 

 walls are scarcely differentiated from the ordinary- 

 tissues of the branch, and that where the branch forks 

 the sporangium forks too. Here, then, it is evident 

 that the sporangium is not an organ sui generis, as 

 modern botanists have generally taught, but just the 

 end of a branch, set apart for spore-production. 



The Sphagnum-like structure of the spore-sac in 

 Hornea is not without analogy, for just before the 

 Rhynie discoveries, Halle had described, from the 

 Lower Devonian of Norway, a fossil which he named 

 Sporogonites. This is remarkably like the stalked 

 capsule of a Moss in external appearance, and inter- 

 nally (though imperfectly preserved) it proved to have 

 a columella of the same form as that subsequently 

 observed in Hornea. Thus the Rhyniace.s are not 

 only the simplest vascular plants known ; they like- 

 wise suggest analogies with the Bryophytes. They 

 have in fact been placed by different botanists in three 

 different sub-kingdoms : in the Pteridophytes, the 

 Thallophytes, and the Bryophytes, on grounds which 

 will be evident from the facts already given. Possibly 

 they may represent a basal group, related at once to 

 the Vascular Cryptogams and the Moss phylum, while 

 at the same time retaining some of the old characters 

 of an Algal stock. But we cannot regard so interesting 



