September 27, 1900] 



NATURE 



539 



therefore, have been derived from them ; but the fact that the 

 Cordaitacere exhibit certain cycadean affinities, and the dis- 

 covery of the Cycadofilices, suggest that what may be termed 

 the cycadean phylum of Gymnosperms (including the Cordait- 

 ace;^, Bennettitaceee, CycadacecK, and perhaps the (jinkgoacese) 

 had its origin in a filicineous ancestry, of which, it must be 

 admitted, no forms have as yet been recognised. 



Turning to the Pteridophyta, the origin of the Ferns is still 

 quite unknown : the one fact which seems to be clear is that the 

 eusporangiate forms (Marattiacete) are more primitive than the 

 leptosporangiate. With regard to the Equisetinre, the Cala- 

 mariaceoe were no doubt the ancestors of the existing and of 

 the fossil Equisetums. Similarly, in the Lycopodinte, the 

 palreozoic Lepidodendraces were the forerunners of the existing 

 Lycopodiums and Selaginellas. The discovery of the Spheno- 

 phyllaceix; seems to throw some further light upon the phylogeny 

 of these two groups, inasmuch as these plants possess characters 

 which indicate affinity with both the Equisetinoe and the 

 Lycopodinw, thus suggesting the possibility that they may have 

 sprung from the same ancestral stock. 



To complete the geological survey of the vegetable kingdom 

 I will briefly allude to the Bryophyta and the Thallophyta. 

 Owing no doubt to their delicate texture, the records of these 

 plants have been found to be very incomplete. So much is this 

 the case with the Bryophyta that I forbear to make any statement 

 concerning them. The chief point of interest with regard to 

 the Fungi is that most of those which have been discovered in 

 the fossil state were found in the tissues of woody plants on 

 which they were parasitic. In this way it has been possible to 

 ascertain, with some probability, the existence of Bacteria and 

 of mycelial Fungi in the Pala30zoic period. The records of the 

 Algi^; are more satisfactory ; they have been traced far back 

 into the Pah^ozoic age, where they are represented by siphon- 

 aceous forms and by the somewhat obscure plants known as 

 Nematophyciis and Pachytheca. 



In a general way the study of Paleobotany has proved the 

 development of higher from lower forms in the successive 

 geological periods. Thus the Tertiary and Quaternary periods 

 are characterised by the predominance of Angiosperms, just as 

 the Mesozoic period is characterised by the predominance of 

 Gymnosperms, and the Palaeozoic by the predominance of 

 Pteridophyta. And yet, as I have been pointing out, we are 

 not able to trace the ancestry of any one of the larger groups 

 of plants. The chief reason for this is that the geological 

 record, so far as it is known, has been found to break off with 

 such surprising abruptness that the earliest, and therefore the 

 most interesting, chapters in the evolution of plants are closed 

 to us. After the wealth of plant-forms in the Carboniferous 

 epoch there is a striking falling-off in the Devonian, in which, 

 however, plants of high organisation, such as the Cordaitacere, 

 the CalamariacecB and the Lepidodendracese, still occur. In 

 the Silurian epoch vascular plants are but sparingly present — 

 but it is remarkable that any such highly organised plants should 

 be found there— together with probable Algoe, such as Nema- 

 tophyciis and Pachytheca. The Cambrian rocks present nothing 

 but so-called " Fucoids," such as Eophyton, Sec, some of 

 which may be Algae. The only known fossil in the oldest strata 

 of all, the Archaean, is the much-discussed Eozoon canadense, 

 probably of animal origin ; but the occurrence here of large 

 deposits of graphite seems to indicate the existence of a con- 

 siderable flora which has, unfortunately, become quite undeter- 

 minable. Thus, whilst there is some evidence that the primi- 

 tive plants were Algae, there is at present no available record of 

 the various stages through which the Silurian and Devonian 

 vascular plants were evolved from them. 



Morphology. 

 If int^uiry be made as to the cause of the great advance in the 

 recognition of the true affinities of plants, and consequently in 

 their classification, which distinguishes the nineteenth century, I 

 would refer it to the progress made in the study of morphology. 

 The earlier botanists regarded all the various parts of plants as 

 "organs" in relation to their supposed function ; hence their 

 description of plants was simply " organography." The idea of 

 regarding the parts of the plant-body, not in connection with 

 their functions, but with reference to their development and 

 their mutual relations, seems to have originated with Jung in the 

 seventeenth century (1687); it was revived by C. F. Wolff 

 about seventy years later (1759), but it did not materially affect 

 the study of plants until well on in the nineteenth century, after 



NO. 1613, VOL. 62] 



Goethe had repeatedly written on the subject and had devised 

 the term " morphology " to designate it. For a time this some- 

 what abstract mode of treatment led to mere theorising and 

 speculation, so much so that the years 1820-1840 will always l>e 

 stigmatised as the period of the " Naturphilosophie." But 

 fortunately this time of barrenness was succeeded by a veritable 

 renascence. Robert Brown and Henfrey in England ; 

 Brongniart, St. Hilaire and Tulasne in France ; Mohl, 

 Schleiden, Naegeli, A. Braun, and, above all, Ilofmeister in 

 Germany, led the way back from the pursuit of fantastic will-o'- 

 the-wisps to the observation of actual fact. Instead of evolving 

 schemes out of their own internal consciousness as to how plants 

 ought to be constructed, they endeavoured to discover by the 

 study of development, and more particularly of embryogeny, 

 how they actually are constructed, with the result that within a 

 decade Hofmeister discovered the alternation of generations in 

 the higher plants ; a discovery which must ever rank as one of 

 the most brilliant triumphs of morphological research. 



With the knowledge thus acquired it became possible to 

 determine the true relations of the various parts of the plant- 

 body ; to distinguish these parts as " members " rather than as 

 "organs"; in a word, to establish homologies where hitherto 

 only analogies had been traced — which is the essential difference 

 between morphology and organography. 



The publication of the "Origin of Species" profoundly 

 affected the progress or morphology, as of all branches of 

 biological research : but it did not 'alter its trend ; it confirmed 

 and extended it. We are not satisfied now with establishing 

 homologies, but we go on to inquire into the origin and 

 phylogeny of the members of the body. In illustration I may 

 briefly refer to two problems of this kind which at the present 

 time are agitating the bocanical world. The first is as to the 

 origin of the alternation of generations. Did it come about by 

 the modification of the sexual generation (gametophyte) into an 

 asexual (sporophyte) ; or is the sporophyte a new formation in- 

 tercalated into the life-history ? In a word, is the alternation of 

 generations to be regarded as homologous or as antithetic ? I 

 am not rash enough to express any opinion on this controversy ; 

 nor is it necessary that I should do so, since the subject has 

 twice been threshed out at recent meetings of this Section. The 

 second problem is as to the origin of the sphorophylls, and, 

 indeed, of all the various kinds of leaves of the sporophyte in 

 the higher plants. It is suggested, on the one hand, that the 

 sporophylls of the Pteridophyta have arisen by gradual sterilisa- 

 tion and segmentation from an unsegmented and almost wholly 

 reproductive body, represented in our day by the sporogonium 

 of the Bryophyta ; and that the vegetative leaves have been de- 

 rived by further sterilisation from the sporophylls. On the 

 other hand, it is urged that the vegetative leaves are the more 

 primitive, and that the sporophylls have been derived from 

 them. It will be at once observed that this second problem is 

 intimately connected with the first. The sterilisation theory of 

 the origin of leaves is a necessary consequence of the antithetic 

 view of the alternation of generations ; whilst the derivation of 

 sporophylls from foliage-leaves is similarly associated with the 

 homologous view. Here, again, exercising a wise discretion, I 

 will only venture to express my appreciation of the important 

 work which has been done in connection with this controversy — 

 work that will be equally valuable, whatever the issue may 

 eventually be. 



I will conclude my remarks on morphology with a few illus- 

 trations of the aid which the advance in this department has 

 given to the progress of classification. For instance, Linnteus 

 divided plants into Phanerogams and Cryptogams, on the 

 ground that in the former the reproductive organs and pro- 

 cesses are conspicuous, whereas in the latter they are obscure. 

 In view of our increased knowledge of Cryptogams this ground 

 of distinction is no longer tenable ; whilst still recognising the 

 validity of the division, our reasons for doing so are altogether 

 difterent. For us. Phanerogams are plants which produce a 

 seed ; Cryptogams are plants which do not produce a seed. 

 Again, we distinguish the Pteridophyta and the Bryophyta irom 

 the Thallophyta, not on account of their more complex struc- 

 ture, but mainly on the ground that the alternation of genera- 

 tions is regular in the two former groups, whilst it is irregular or 

 altogether wanting in the latter. Similarly, the essential distinc- 

 tion between the Pteridophyta and the Bryophyta is that in the 

 former the sporophyte, in the latter the gametophyte, is the 

 preponderating form. It has enabled us further to correct in 

 many respects the classifications of our predecessors by altering 



