September 21, 191 1] 



NATURE 



399 



Amentales and some allied cohorts, and I trust they will 

 be set forth in detail by a better systematist than I can 

 claim to be. My object in bringing the matter forward at 

 all is to point out some of the difficulties which prevent me 

 from accepting a monophyletic origin of the Dicotyledons 

 through the Ranalian plexus. 



One of these difficulties lies in the relationship of the 

 Gnetales to the Dicotyledons. Arber and Parkin have 

 recently made the attempt to gain a clearer insight into the 

 affinities of this somewhat puzzling group by applying to it 

 the " strobilus theory' of Angiospermous descent." The 

 peculiar structure of the flowers of Welwitschia lends itself 

 particularly well to a comparison with those of Cycadoidea, 

 and a gool case can no doubt be made out for a hemiangio- 

 spermous ancestry of this member of the Gnetacese, and by 

 reduction the other members, in many respects simpler, 

 might be derived from a similar ancestor, though probably, 

 so far as Ephedra and Gnetum are concerned, an equally 

 good, if not better, comparison might be made with 

 Cordaites. But even supposing we admit the possibility of 

 a derivation of the Gnetales from an amphisporangiate 

 Pteridosperm, I think the Amentales merit quite as much 

 as the Gnetales to be considered as having taken their 

 origin separately from the Hemiangiosperma;, and not from 

 the Ranalian plexus. I find this view has been put forward 

 also by Lignier 2 in his attempt to reconstruct the phylo- 

 genetic history of the Angiosperms, and I feel strongly 

 that such a polyphyletic descent, whether from the more 

 specialised anthostrobilate Pteridospermae or from several 

 groups of a more primitive Cycado-Cordaitean plexus, is 

 more in accordance with the early differentiation of the 

 Cretaceous Angiosperms, and with the essential differences 

 existing now in the orders grouped together as Archi- 

 chlamydeae. 



Attempts at reconstructing the phylogeny of the Angio- 

 sperms are bound to be at the present time largely specula- 

 tive, I nt we may possibly be on the threshold of the 

 discovery of more certain records of the past history of the 

 higher Spermaphyta, since Dr. Marie Stopes has com- 

 menced to publish her investigations of the Cretaceous fossil 

 plants collected in Japan, and Prof. Jeffrey has been 

 Fortunate enough to discover cretaceous plant-remajns 

 showing structure in America. The former have already 

 provided us with details of an interesting Angiospermic 

 flower, and if the latter have so far only yielded Gymno- 

 sperms, we may at all events learn something of the 

 primitive forms of these plants, the origin of which is still 

 as problematical as is that of the Angiosperms. 



1 trust that the criticisms I have made of the theory put 

 forward by Messrs. Arber and Parkin will not be taken as 

 a want of appreciation on my part of the service they have 

 done in formulating a working hypothesis, but merely as 

 an expression of my desire to walk circumspectly in the 

 vi iv alluring paths by which they have sought to explore 

 the primaeval forest, and not to emulate those rapid but 

 hazardous flights which have become so fashionable of late. 



While the description of new and often intermediate 

 forms of vegetation has aroused such widespread and 

 general interest in Palseobotany, other and more special 

 aspects of the subject have not been without their devotees, 

 and have proved of considerable importance. Morphological 

 anatomy has gained many new points of view, and our 

 knowledge of the evolution of the stele owes much to a 

 careful comparison of recent and fossil forms, even when 

 these investigations have produced conflicting interpretations 

 and divergent views. 



Another promising line of Palaeobotanical research lies in 

 the direction of investigations of the plant tissues from the 

 phvsiological and biological points of view. Happily, the 

 veg< table cell-wall is of much greater toughness than that 

 of animal cells, and in consequence the petrified plant- 

 remains found in the calcareous nodules are often so 

 excellently preserved that we can not only study the 

 lignified and corkv tissues, but also the more delicate 

 parenchymatous cells. Even root-tips, endosperm, and 

 germinating fern-spores are often so little altered by 



1 Arber, E. A. N.. and Parkin. J. : " Studies on the Evolution of Angio- 

 sperms," " The Relationship of the Angiosperms to the Gnetales," Annals 

 of Botany, vol. xxii., 1908. 



- Lignier, O. : "Essai sur 1'Evolution morphologique du Regne vegetal," 

 Bull, de la Soc. Linncenne de Normandie, 6 ser. , 3 vol., 1909, reimprime 



NO. 2l86, VOL. 87] 



fossilisation that their cells can be as easily studied as if 

 th • sections had been cut from fresh material. It is this 

 excellence of preservation which has enabled us to gain so 

 complete a knowledge of the anatomy of palaeozoic plants, 

 and since the detailed structure of plant organs is often an 

 index of the physical conditions under which the plants 

 grew, we are able to form some opinion as to the habitat 

 of the coal-measure plants. Though a beginning has 

 already been made in this direction by various authors, we 

 have as yet only touched the fringe of the subject, and, as 

 Scott points out in the concluding paragraph of his admir- 

 able " Studies," the biology and ecology of fossil plants 

 offer a wide and promising field of research. Such studies 

 are all the more promising, as we now have material from 

 such widely separated localities as the Lancashire coalfield, 

 Westphalia, Moravia, and the Donetz Basin in Russia. 



Now that it has been definitely shown by Stopes and 

 Watson that the remains of plants are sometimes con- 

 tinuous through adjacent coal-balls, we may safely accept 

 their conclusion that these calcareous concretions were in 

 the main formed in situ, and that the plant-remains they 

 contain represent samples of the vegetable debris of which 

 the coal-seam consists. We have in these petrifactions, 

 therefore, an epitome, more or less fragmentary, of the 

 vegetation existing in palaeozoic times on the area occupied 

 by the coal-seam, and the Stigmarian roots in the under- 

 clay, as well as other considerations, lead us to believe 

 that the seam more frequently represents the remains of 

 the coal-measure forest carbonised in situ. While this 

 seems to be the more usual formation of coal-seams, it is 

 obvious from the microscopic investigations of coal made 

 by Bertrand, and as has recently been so clearly set forth 

 by Arber in his " Manual on the Natural History of Coal," 

 that in the case of bogheads and cannels the seam repre- 

 sents metamorphosed sapropelic deposits of lacustrine 

 origin. In other cases, again, considerations of the nature 

 of the coal and the adjacent rocks may incline us to the 

 belief that some, at any rate, of the deposits of coal may 

 be due to material drifted into large lake-basins by river 

 agency. 



Broadly speaking, however, and particularly when dealing 

 with the seams from which most of our petrified plant- 

 remains have been collected, we may consider the coal as 

 the accumulated material of palaeozoic forests metamor- 

 phosed in situ. What, then, were the physical and climatic 

 conditions of these primaeval forests? TThe prevalence of 

 wide air-spaces in the cortical tissues of young Calamitean 

 roots, as indeed their earlier name Myriophylloides indi- 

 cates, leads us to believe that, as in the case of many of 

 their existing relatives, they were rooted under water or in 

 waterlogged soil. We gather the same from the structure 

 of Stigmaria, while the narrow xerophytic character of the 

 leaves at any rate of the tree-like Catamites and Lepido- 

 dendra closely resembles the modifications met with in our 

 marsh plants. It has been suggested by several authors 

 that the xerophytic character of the foliage of many of our 

 coal-measure plants may be due to the fact that they 

 inhabited a salt marsh. A closer examination of the 

 foliage, however, of such plants as Lepidodendron and 

 Sigillaria does not reveal the characteristic succulency 

 associated with the foliage of most Halophytes, and in 

 view of the absence of such water-storing parenchyma, the 

 well-developed transfusion-cells of the Lepidodendreae can 

 only be taken to be a xerophytic modification such as is 

 met with in recent Conifers. 



The specialisation of the tissues indeed is only such as is 

 quite in keeping" with the xerophytic nature of marsh 

 plants. Moreover, the particular group of Equisetales are 

 quite typical of fresh water, and we should expect that if 

 their ancestors had been Halophytes, some at any rate at 

 the present day would have retained this mode of lif<=. 

 Nor have we at the present time any halophytic Lyco- 

 podiales, while Isoetes, the nearest relative to _ the 

 Lepidodendra, is an aquatic or sub-aquatic form associated 

 with fresh water. 



Among the Filicales, Acrostichum aureum seems to be 

 the only halophytic form, inhabiting as it does the swamps 

 of the "Ceylon littoral, 1 and though, as Miss Thomas has 



1 Tansley, A. G., and Fritsch : " The Flora of the Ceylon Littoral," Ni™ 

 Phytologist, vol. iv., 1905. 



