6 3 8 



NA TURE 



[November i i, 1922 



of experiment, I can entertain no doubt that they 

 do so hold far enough to afford assurance that we 

 can transport (with fuel for a considerable journey 

 and at speeds high enough to make us independent 

 of ordinary winds) weights many times greater than 

 that of a man." And 



" I desire to add as a final caution, that I have not 

 asserted that planes such as are here employed in 

 experiment, or even that planes of any kind are the 

 best forms to use in mechanical flight, and that I 

 have also not asserted, without qualification, that 

 mechanical flight is practically possible, since this 

 involves questions as to the method of constructing 

 the mechanism, of securing its safe ascent and descent, 

 and also of securing the indispensable condition for 

 the economic use of the power I have shown to be 

 at our disposal — the condition, I mean, of our ability 

 to guide it in the desired horizontal direction during 



transport, — questions which, in my opinion, are only 

 to be answered by further experiment, and winch 

 belong to the inchoate art or science of aerodromics, 

 on which I do not enter." 



The problems of Langley are still problems, and we 

 have very much to learn about the control of aero- 

 planes. An interesting commentary on Langley's work 

 is provided by the fact that on October 19 the world's 

 record for gliding flight was obtained on a replica of 

 the Langley machine and not by a glider following 

 the modern conventional aeroplane. It would be 

 wrong, I think, to argue superiority of type for the 

 successful glider, but it is a not unwelcome reminder 

 of the enormous progress made by a scientific pioneer 

 at a time when science in aviation is at a very 

 low r ebb. 



The Early History of the Land Flora. 1 



By Dr. D. H. Scott, F.R.S. 



II. 



WHEN we reach the Upper Devonian flora we 

 find ourselves in the midst of a comparatively 

 familiar vegetation. A few of the early forms may 

 have survived, but the bulk of the plants were highly 

 organised Vascular Cryptogams or Spermophytes. 

 While in the Early Devonian no true Ferns have been 

 found, a branched, naked rachis being the nearest 

 approach to a frond, the later vegetation has been 

 called the Archffiopteris flora, after the magnificent 

 ferns or fern-like plants of that genus, of which the 

 famous A . hibernica is the type. We do not, however, 

 know for certain whether these fine plants were really 

 Ferns, or fern-like seed-plants. The presence of true 

 Ferns is more surely attested by Dawson's Astero- 

 pteris, from the State of New York, which has the 

 structure of a Zygopterid, a group well known from 

 Carboniferous rocks. Lycopods had attained a very 

 high development, as shown especially by the genus 

 Bothrodendron, of which the large heterosporous cones 

 are known. 



The now extinct group of the Sphenophyllums, 

 characteristic of Carboniferous times, had also made 

 its appearance in the Upper Devonian flora ; the 

 whorled leaves of these early forms were deeply cut, 

 not wedge-shaped as in most of the later representa- 

 tives. Nathorst's genus Hyenia, which already appears 

 in the Middle Devonian, may probably have been a 

 precursor of the Sphenophylls. 



Another family, represented by Pseudobornia, of 

 Nathorst, from Bear Island, is only known from the 

 Upper Devonian. It was a large plant, with whorled 

 leaves, palmately divided, and further cut into narrow 

 segments, while the long cones are believed to have 

 produced spores of two kinds. Pseudobornia is at 

 present quite isolated ; its affinities may be either 

 with the Sphenophylls or the Horsetails. Apart from 

 this case, the Equisetales do not appear to be repre- 

 sented among our present Devonian records, for the 

 evidence for the occurrence of Archffiocalamites at that 

 period seems to be inadequate. The group, however , 

 was so well developed in Lower Carboniferous times 

 that there can be no doubt it had appeared long before. 



1 Continued from p. 607. 



NO. 2767, VOL. I IO] 



The best proof of the presence of seed-plants in the 

 Upper Devonian is to be found in the occurrence of 

 petrified stems, which, from their organisation, must 

 presumably have belonged to advanced Gymnosperms. 

 The genus Callixylon, apparently allied to the Lower 

 Carboniferous Pitys, has a peculiar and beautiful 

 structure in the secondary wood, the pits being localised 

 in definite groups. The wood appears more highly 

 differentiated than that of most living Conifers. 



Thus the main lines of subsequent evolution were 

 already well laid down in Upper Devonian times. We 

 know practically nothing of their origin. Some 

 botanists believe that the higher plants may have had 

 a common source in some group, already vascular, 

 such as the Psilophytales, while others hold that the 

 main phyla have always been distinct, from the Algal 

 stage onwards. The existence of these rival mono- 

 phyletic and polyphyletic hypotheses, both maintained 

 by able protagonists, shows how little definite know- 

 ledge of the evolutionary history we possess. 



The Lower Carboniferous flora bears a close general 

 resemblance to the Upper Devonian, but is much better 

 known. The wealth of forms is, indeed, so great, that 

 only the merest outline of the main features can be 

 given here. 



The Lycopods were abundantly developed. Many 

 species of Lepidodendron and Lepidophloios are known, 

 not only by external characters, but often by anatomical 

 structure. While the primary ground plan of their 

 anatomy was not unlike that of some of the simpler 

 Lycopods of our own day, most of the old forms de- 

 veloped a considerable zone of secondary wood, and a 

 massive periderm. They were, in fact, adapted to play 

 the part of forest trees. The genus Sigillaria, however, 

 so important in the Upper Carboniferous flora, was still 

 scantily represented. 



As regards their fructification, the Lower Carbonifer- 

 ous Lycopods had attained the highest level which the 

 class ever reached. Not only were their cones con- 

 stantly (so far as observed) heterosporous, with an 

 extreme differentiation of the two kinds of spore, but 

 some of them even developed a kind of seed, a structure 

 quite unknown among Club-mosses of later than 

 Carboniferous age. In the seed-like fructification 



