714 



NATURE 



[August 5, 1920 



the Psilophytales with those of Stauropteris, 

 Botryopteris, the Ophioglossaceae, Osmunda, and 

 the Schizaeaceae, gives a sequence which sketches 

 in broad Hnes, though not monophyletically, a 

 probable origin of marginal sporangia for the 

 Ferns. It is accompanied by reduction of size and 

 spore-number in the later and derivative types, 

 which is continued on to the most advanced of 

 living Ferns. A reduction of the distal branchlets 

 to a single plane, and the webbing of them laterally 

 together, would give a type of sporophyll and 

 fructification known in certain primitive Ferns. 

 But if this were the real course of their evolution, 

 the sporophyll so constructed would be a different 

 thing from the "leaves" seen in Asteroxylon. 

 This was the vision of the prophetic Lignier, who 

 has not lived to see his ideas tested by these new 

 discoveries. But such comparisons still leave in 

 doubt the origin of the axis in fern-like types. It 

 is not- clear yet how near the truth for them my 

 suggestion of 1884 may be: that "the stem and 

 leaf would have originated simultaneously by 

 differentiation of a uniform branch-system into 

 members of two categories." Nevertheless, the 

 important new fact, which now gives reality to 

 this theory, is that a uniform branch-system has 

 been shown to have existed in these early vascular 

 plants. A sympodial development of it, after the 

 manner shown in the leaves of living Ferns, would 

 provide at least one type of foliar appendage, 

 which would bear a relation to the axis similar 

 to that of the pinnae to the phyllopodium or Pachis 

 of the leaf. 2 



On the other hand, comparison of the Bryo- 

 phytes will leave little doubt that the sporangium 

 of the Psilophytales and the sporogonium are 

 kindred structures. If this be so, then we shall 

 see linked together by comparison with these new 

 fossils, not only the sporogonia of Bryophytes and 

 the sporangia of Ferns, but even the pollen-sacs 

 and ovules of Flowering Plants. Long ago it was 

 remarked that the widest gap in the sequence of 

 plants was that between the Bryophytes and the 

 Pteridophytes. It is within this gap that the 

 newly discovered fossils take their natural place, 

 acting as synthetic links, and drawing together 

 more closely the whole sequence of land-living, 

 sporangium-bearing plants. We still await with 

 interest the considered comparisons of the authors 

 of these notable memoirs, though they have 

 already pointed out several fertile lines. But 

 those who have been deeply engaged in compara- 

 tive morphology may be excused for stating how 

 these new facts strike them. Clearly the morpho- 

 logy of land-living plants is again in the melting- 

 pot. It will emerge strengthened by new and 

 positive facts, and refined by comparisons which 

 can now be based upon solid data, and less than 

 before on mere surmise. 



The new facts are thus seen to link the Bryo- 

 phytes and the Pteridophytes more closely to- 

 gether than ever before. It may be that these 

 two great phyla of land-living plants have them- 

 selves diverged from some common source still 



« Phil. Trans., 1884, p. 565. 



NO. 2649, VOL. IOi;"l 



unknown. But that source is reflected more 

 nearly in these early Devonian plants than in any 

 other known forms. If that be so, whence may 

 these still more primitive plants have sprung? 

 The view has always been entertained that the 

 Algae preceded land-living plants. For long the 

 fresh-water green Algae were believed to have 

 provided the source. Latterly from the Continent, 

 but notably also here at home, at the instance of 

 Lang and of Church, the belief has swung round 

 towards marine forms. Highly specialised Algae 

 flourish on every rocky shore. Some of these show 

 alternation. All are rootless. Some have a dif- 

 ferentiation of their branch-system which pre- 

 figures the relation of leaf and axis. Not a few 

 of the Red Seaweeds have spore-tetrads borne 

 internally, and located in the ends of specialised 

 branches called stichidia. These are not alto- 

 gether unlike sporogonia, or the large sporangia 

 of the early Devonian plants. We may well 

 regard it as improbable that any direct transition 

 of such specialised types to a land-habit took 

 place, though this has been hinted at more than 

 oncfe. But at least corresponding features of ex- 

 ternal differentiation and of spore-production are 

 present in both. Homoplasy may be the real 

 explanation of the likeness, but still the similarity 

 exists. • 



From what has been said it is clear that during 

 the years of war plant morphology entered 

 upon a new phase. The problems of origin of 

 root and axis and leaf and sporangium have been 

 propounded afresh in terms of the new dis- 

 coveries. The day is past of that vague surmise 

 on these points which bulked so largely in 

 the discussions of recent decades. It was the 

 paucity of facts that kept opinion in suspense, 

 hovering between rival arguments rather than 

 settling on assured data. Looking back upon the 

 history of that branch of botanical science which 

 is called comparative morphology, there is only 

 one period that can rival the years from 1913 to 

 1920 in point of positive advance. It is the period 

 which led up to the great generalisations of Hof- 

 meister sixty years ago. In the glories of that 

 work Britain had no direct share, though it 

 was carried out at the very time when Lyell, 

 Darwin, Wallace, Hooker, and Huxley were lay- 

 ing the theoretical foundations which gave their 

 real significance to the discoveries then made by 

 Hofmeister. In the words of Sachs : " ^\' hen 

 Darwin's theory was given to the world — the 

 theory of Descent had only to accept what genetic 

 morphology had already brought to view." 

 Science, it is true, is cosmopolitan, and should 

 always be held as such. But still we in Britain 

 may feel a legitimate satisfaction that in these 

 recent discoveries, which have transformed the 

 problems of morphology, the material, the ob- 

 servations, and the arguments based upon them 

 are mainly of British origin. The channel of 

 publication of the results, so largely derived by 

 Scottish workers from Scottish material, has 

 naturally been the Transactions of the Royal 

 Society of Edinburgh. 



