430 



NA TURE 



[August 23, 1906 



The elaborate anatomical investigation whicn tneir vege- 

 tative organs had received at the hands of Williamson, 

 Scott, Solms-Laubach, and others showed them to occupy 

 a transitional position between the Ferns and Cycads. 

 In certain respects they showed an advance in the cycadian 

 direction, whilst in others they were wholly fern-like. 

 Their fructifications were unknown, and their nature re- 

 mained an open question. It was for this group, or series 

 of transitional groups, that Potoni^ proposed the appro- 

 priate name of Cycadofiliccs. 



We know now that the Lyginodendreae and Medulloseae 

 bore seeds attached to their fronds. The seeds have been 

 found attached in some cases to reduced fronds consisting 

 of a branching rachis, in others to fronds of the normal 

 filicinean type. Indeed, so far as habit is concerned, these 

 plants may rightly be described as seed-bearing Ferns. 



-As such, indeed, most people will be content to regard 

 them — as forms, that is, having close filicinean relation- 

 ship in which the reproductive method has been profoundly 

 modified, the internal anatomy to a less extent, and the 

 habit hardly at all. Had these Pteridosperms come to light 

 during the lifetime of Hofmeister that master of morph- 

 ology must have pounced upon them as furnishing an 

 important link in his chain. These fossils and the sperm- 

 atozoa which the Japanese botanists discovered in the seeds 

 of Cycas and Ginkgo, indeed, afford the most convincing 

 direct evidence of the soundness of the Hofmeisterian 

 scheme that it is possible to conceive. Nor is that all. 

 For by confirming the indications first revealed by the 

 earlier investigation of the vegetative anatomy, the 

 Pteridosperms have afforded us a striking object-lesson of 

 the value of the anatomical method — of the significance of 

 purely anatomical characters too long ignored by the 

 systematist. 



Not so long ago, when new examples of; these Pterido- 

 sperms were turning up on every hand, some pessimists 

 were inclined to wonder whether, after all, any groups of 

 real Ferns existed in the Palaeozoic rocks. Such sporangia 

 as were known might well be the pollen-sacs of seed-bear- 

 ing plants. All doubts on this score are happily set at rest 

 by the detection of germinating Fern-spores in contem- 

 porary beds. Nor can I think of any more fitting tail- 

 piece to the investigations which lead the way to the 

 Pteridosperms than the discovery, by the same investigator, 

 of the antidote to these rather disturbing views. However, 

 it is needless to dwell further on these matters now, in 

 view of Dr. Scott's address to-morrow upon the Present 

 State of PaljEOzoic Botany. 



But to return to the history of the seed. In the absence 

 of direct evidence, one can only conjecture that some old 

 generalised type of sporangium formed its prototvpe. some- 

 thing substantial, on the lines of a Botryopteris or Zygo- 

 pteris, perhaps. The heterospory that was the precursor 

 of the seed-like condition must have been a transient phase, 

 or else it is lost in the pre-Carboniferous obscurity. Be 

 that as it may, the passage from the dehiscent to the 

 indehiscent monosporal megasporangium finds its analogv 

 in every group of plants. Where there is extreme numerical 

 reduction of the contained structures — be they spores or 

 seeds — a multitude of cases in the Fungi, in the Algs, and 

 the angiospermic flowering plants show that dehiscence 

 tends to become obsolete. The failure to dehisce does not 

 apopar to be directly correlated with any mechanical diffi- 

 culty in ejaculation. It is more probably one of those 

 ob'scure cases of interdependence of phenomena in which 

 the vegetable kingdom abounds. A special investigation 

 directed to the elucidation of this point might be expected 

 to vield interesting results. 



We now come to the consideration of a most character- 

 istic organ of the seed — the pollen-chamber. This cavity 

 arises at the apex of the megasporangium, above the big 

 megaspore, and is found in all the Palaeozoic seeds, with 

 the sole exception, so far as I am aware, of the " seed- 

 like " structures in Lepidocarpon and Miadesmia. The 

 utility of the pollen-chamber is manifest, but its ante- 

 cedents are quite unknown. Upon such a structure as this 

 mav have depended the success of the seed-method at a 

 critical stage in its evolution. In the viviparous 

 Selaginellas, described some years ago in America, the 

 archegonium on the prothallus of the retained megaspore 

 is fertilised by sperms liberated from microspores which 



NO. 192 1, VOL. 74] 



become caught in tne lips of the open megasporangial wall. 

 This analogy suggests to us that the pollen-chamber cavity 

 may be a relic or modification of the original place of 

 dehiscence. If this conjecture be true, we have here what 

 was once an exit-pore converted to the purposes of ingress, 

 just as we find, in so many Thallophytes, tubes and beaks, 

 once, as it is supposed, the orifices of zoospore discharge, 

 now serving for the reception of male gametes. 



A great feature in the early seed types was the com- 

 plexity of the integument, and this still holds good in 

 recent Cycads and some other Gymnosperms. Protective 

 envelopes are so commonly associated with reproductive 

 organs, and the nutritive conditions are so favourable to 

 their production, that a naked nucellus strikes one as 

 anomalous. If future research confirm the supposition that 

 the ferns which stand in possible relation to early seed- 

 plants were ex-indusiate, like the Marattiaceae, recent and 

 fossil, then no doubt the seed-coat is a new formation, 

 having no true homology with, but merely homoplastic 

 resemblance to, ordinary Fern-indusia. The only case of a 

 naked nucellus that recalls itself is the rather mysterious 

 instance of Lepidocarpon in which Dr. Scott reports thc- 

 not infrequent occurrence of non-integumented mega- 

 sporangia with the prothallus fully developed. 



The robust nature of the seed envelope, which was often 

 drupaceous, is in complete harmony with the whole 

 character of the seed if you regard the habit at its incep- 

 tion as a xerophilous adaptation. And such no doubt it 

 was, an improved method whereby the plant became in- 

 dependent of chance water at a very critical stage in the 

 life-history. Some of the peculiarities of fossil seed-coats, 

 especially the ribbing of the Lagenostomas and several 

 other genera, may be attributed to a multiple origin of 

 this structure, at any rate in some cases. The remarkable 

 circlet of tentacles which surrounds the summit of Lagcno- 

 stoma physoides (best known bv Williamson's earlier name 

 Physostoma elegans) suggests that a number of foliar lobes 

 have been incorporated in the seed, whilst the presence of 

 perimicropylar ridges and the septate canopy in allied forms 

 may be taken as only a less evident indication of the same 

 thing. 



The relation between the integument and sporangial body 

 of recent Gymnosperm seeds is found to be an inconstant 

 character, and the same is true of the fossils. In general 

 character the relationship recalls that which obtains between 

 the ovary and receptacle of an Angiosperm. The Lageno- 

 stomas resemble Cycas and Pinus in having the integument 

 free at the apex only, whilst Taxus, Phyllocladus, and 

 ."Vraucaria are in agreement with the Trigonocarpons and 

 other seeds, which are generally attributed to MeduUosea?. 

 in having an integument which rises freely from tho 

 chalaza. It is interesting to note that the fossil seeds o' 

 the latter group show an additional complexity in the wa!! 

 of the nucellus. For in them a series of tracheal strands 

 or even a mantle of tracheides is found running up fronT 

 the chalaza to the pollen-chamber. It is evident that 

 nothing was spared in these older seeds to ensure adequate 

 access of water to the pollen-chamber where the sperms 

 must have been liberated. 



In due time the protective sheath, or testa, appropriated 

 other functions supplementary to that of protection. Of 

 these the most important must have been the reception of 

 the pollen. A very striking feature in all the Lageno- 

 stomas is the way in which the tip of the nucellus (where 

 the orifice of the pollen-chamber is situated) projects beyond 

 the integument. In these seeds the microspores must have 

 had direct access to the pollen-chamber without first de- 

 scending a micropylar canal. 



In the Medullosean seeds also the nucellus is dis- 

 tinguished by a long beak, as Dr. Scott and Mr. Maslen 

 have shown recentlv for Trigonocarpon, and, as we know, 

 in Stephanospermum, and many other cases. So far as we 

 know, this beak does not extend to the surface, though it 

 engages with the micropylar canal, and is continued some 

 distance up. 



Though it can hardly be supposed that the long beak has 

 been inherited from the ancestral sporangium, its presence 

 may be none the less significant of what took place when 

 the seed method was initiated. The direct pollination in 

 Lagenostoma may well be a survival from the old days 

 when no proper micropyle existed. But when the micro- 



