December t, 1898] 



NATURE 



these, nor, I think, aposporous or apogamous growths, throw 

 any direct light upon the story of descent. 



To sum up then, not only do I find that the facts in our pos- 

 session, including the wildest anomalies, are consistent with an 

 antithetic theory, but a comparison of normal forms seems to me 

 to support the opinion that the sporophyte has appeared as the 

 result of gradual elaboration from the zygote, a fresh phase having 

 been thus gradually intercalated in the course of evolution. This 

 idea first clearly stated by Celakovsky in 1868, was developed 

 by him in subsequent writings. I endeavoured to place it on a 

 footing of adaptation to external conditions, in 1890 ; and in 

 1S97 we find Strasburger restating the position in terms almost 

 identical with my own, but upon a basis of nuclear detail which 

 had not been dreamed of when the view was first propounded. 

 Dr. Scott has enthusiastically appreciated the double verification 

 of the forecasts of Prof Pringsheim ; I think that the way in 

 which the antithetic theory is found to work in with the nuclear 

 details recently discovered appeals quite as strongly to my 

 mind. 



In the course of this discussion I have not been anxious to 

 point out such difficulties as beset the homologous view : all I 

 have attempted here has been to set aside some of the difficulties 

 which have been suggested in opposition to an antithetic 

 view, and to show that the latter theory will .adequately cover 

 the facts. 



Returning now to our general inquiry on homology, we see 

 that on the antithetic view the two generations are not hotno- 

 geiielic ; but they may be in a high degree homoplastic, and this 

 homoplasy may be impressed upon the two generations, even in 

 the same species, as in some Lycopods. I have never felt the 

 cogency of the fact that the gametophyte of L, cermium is 

 somewhat similar in lAitline to the young sporophyte. Both 

 generations are exposed to similar circumstances, and may be 

 reasonably expected to have reacted alike. Moreover, the 

 similarity of form of the " leaves " of prothallus and plant 

 is but slight, and is not maintained in allied species. Their 

 arrangement is variable. Between them also lies the essential 

 structural difference, so widespread among Archeg<miate plants, 

 that in the sporophyte stomata and intercellular spaces are 

 present, in the gametophyte they are absent. These are just 

 such differences as point to homoplastic development. More 

 commonly, however, the homoplastic development is only seen 

 in distinct organisms, and in this sense we shall rank the leaf of 

 the Moss as the homoplast, but not the homogene, of the leaf of 

 a Lycopod or of a Fern. 



Theory of the Strobilus. 

 Some years ago I submitted to the Section a theory of the 

 strobilus in Archegoniate plants. Comparisons were drawn 

 between Pteridophytes and Bryophytes, and it was suggested 

 that the origin of the strobilus of thfe former was " from a body 

 of the nature of a sporogonial head." I specially pointed out 

 at the time that my object was not a mere hunt after homologies, 

 but to obtain some reasonable view of the f7iethods of advance in 

 Archegoniate plants. I wish to lay special stress upon this, for 

 some appear to think that by denying an homology which I 

 have not been at pains to maintain, they invalidate this search 

 after the methods of advance. The Bryophytes as we now see 

 them are our best guides in the search after these methods, even 

 though they may not have been in the direct line of descent of 

 Vascular Plants. As regards the comparison of the strobilus with 

 a sporogonial head, I wish to make it clear that a iMoss%^oxo- 

 gonium is not specially indicated. The expression used has 

 been " the origin of the strobilus from a body of the nature of 

 a sporogonial head " — that is simply a part of the sporophyte 

 which bears spores internally as distinct from a lower vegetative 

 region. We see in more than one sequence of Bryophytes how 

 in a sporogonial head, as thus defined, the spore- production 

 becomes restricted in extent, and relegated towards a superficial 

 position by the formation of a central sterile mass. I am ready 

 to join Dr. Scott in his confession of inability to find anything 

 like an intermediate form between the spore-bearing plant of 

 ihe Pteridophyta and the spore-bearing fruit of the Bryophyta, 

 and to .agree that at the best there is nothing more than a remote 

 parallelism not suggestive of affinity ; but none the less I think 

 we should continue to search among the Bryophyta for sugges- 

 tions as to the methods of advance, and to have confidence 

 in transferring these ideas across the gulf, for I believe this to be 

 both a reasonable and a promising method of study. 



NO. I 5 18, VOL. 59] 



DORSIVENTRALl TY. 

 Interesting questions arise in connection with dorsiventral 

 structure. In the Equisetineae, and almost all Lycopodineae, the 

 strobilus is of the radiate type, therein corresponding to the 

 radial structure of typical sporogonia. While certain Ferns are 

 of the radiate type, others are conspicuously dorsiventral, even 

 from their earliest embryonic state. Dorsiventral structure also 

 appears in the vegetative region, and sometimes, though rarely, 

 in the strobilus of Selagine/la. Prof Goebel, in a chapter 

 of his " Organographie," the publication of which may be 

 recognised as the leading event in the morphological studies of 

 the year, discusses the origin of the dorsiventral state in a 

 number of examples, and his results have a most interesting 

 bearing on our theory of the strobilus. 



He shows in the case of Vacciitiiiiit MyrtiUiis how the first 

 shoot of the seedling is orthotropic and radial ; the lateral 

 shoots, formed after the apical growth of this is arrested, are 

 also orthotropic, but the lateral shoots of higher order become 

 plagiotropic with leaves in two lateral rows. He points out 

 the intermediate steps from one condition to the other, and how 

 finally the growing point itself is influenced by the external 

 agency (apparently light), which leads to a change of the leaf- 

 arrangement. This seems to be the case in many other Phan- 

 erogamic plants. 



A particularly interesting account is also given of similar 

 changes in Selaginella. Some eight species are orthotropic, 

 radial, and isophyllous. .S". saiiguinolenta shows a direct 

 response to external conditions, being upright and isophyllous 

 in bright and dry situations, plagiotropic and anisophyllous in 

 damp and shady situations. The bulk of the genus are, how- 

 ever, either plagiotropic and anisophyllous throughout, or some 

 may have an early orthotropic stage. But he concludes that 

 even in " habitually" anisophyllous Selaginellas we have to do 

 with an adaptive character, induced probably by light. 



We see then good evidence that in certain cases the dorsi- 

 ventral shoot is a result of adaptation, and the radial probably 

 the primitive. Was this always so ? We need not discuss the 

 case of the gametophyte, as the problem there is even more 

 varied and difficult, and does not at the moment engage our 

 attention. But the question whether in the sporophyte the 

 radial was in all cases the primitive type is clearly related to our 

 theory of the strobilus. The sporogonia of Bryophytes are, 

 with few exceptions, orthotropic, and almost uniformly radial ; 

 exceptions such as Diphysciiim and Buxbaumia have been 

 shown to have an interesting relation to the incidence of light, 

 and are readily recognised as derivative. The distinctively 

 strobiloid Pteridophytes mcstly maintain this radial structure ; 

 this may be so both in strobilus and vegetative organs, as in 

 Eijiiisettim, Isoetes, in most species of Lycopodiiim, and in some 

 Setaginellas ; or the vegetative region may be dorsiventral, and 

 the strobilus return to the radial type, as in some species of 

 Lycopoditttn and most Selaginellas ; but in some Selagiiiellas 

 even the strobilus may be dorsiventral. 



In the Ferns the case is less obvious ; the large size of the 

 leaves, combined often with a dorsiventral structure of the shoot, 

 makes a comparison with a radial strobilus less easy. Goebel 

 has pointed out that in many dorsiventral Ferns the dorsi- 

 ventrality is already defined in the piincluiii vegetatioiiis, and 

 does not depend upon a subsequent .shifting of the parts. But 

 it should be remembered how many Ferns are orthotropic and 

 radial ; that almost all the large genera include species with 

 simple unbranched leaves. Further, the series of the Ophio- 

 glossaceae, possibly a distinct phylum from the true Ferns, may 

 be held to illustrate a progressive elaboration of the leaf, 

 from smaller le'ved forms which are orthotropic and radial, 

 to larger-leaved forms, which are sometimes orthotropic and radial 

 [Botryihiiim), sometimes plagiotropic, and dorsiventral (He/ 

 iiiiiithostaihys). It is not, I think, improbable that these, and 

 also the true Ferns, are referable in origin to an orthotropic 

 strobiloid type, with radial structure. This opinion w>as in 

 substance suggested in 1894 at Oxford ; these recent observ- 

 ations of Goebel on the derivative nature of dorsiventral shoots 

 strengthen the position then taken up, while they supply us 

 with fresh examples of homoplastic development. 



CONCLUSIO.N. 



This discussion was entered on with a view to finding whither 



phylogeny as a basis of morphology would lead us. However 



unprepared we may be to pursue it with certainty into detail, or 



