5 o8 



NATURE 



[Sept. 20, 1883 



what is essential to the matter, viz. the existence of a cambium- 

 layer producing "centrifugal growth of secondary vascular 

 tissue." 



As already pointed out in the case of the Calatnites, the vas- 

 cular axis of these Lepidodcndra was purely an afipareil con lucteur, 

 unmixed with any wood cells ; hence the appareil dc souticn had 

 to be supplied elsewhere. This was done, as in the Calamites : 

 a thick, persistent, hypodermal zone of meristem 1 developed a 

 layer of prismatic prosenchyma of enormous thickness, 2 which 

 incased the softer structures in a strong cylinder of self-support- 

 ing tissue. We have po-itive evidence that the fructification of 

 many of these plants was in the form of heterosporous strobili. 

 Whether or not such was the case with all these Lepidostroln we 

 are yet unable to determine. But the incalculable myriads of 

 their macrospores, seen in so many coals, afford clear evidence 

 that the heterosporous types must have preponderated vastly over 

 all others. 



Gymnosperms. — Our knowledge of this part of the Carbon- 

 iferous vegetation has made great progress during the last thirty- 

 years. This progress began with my own discovery 3 that all our 

 British Dadoxylons possessed what is termed a discoid pith, such 

 as we see in the white jasmine, some of the American hickories, 

 and several other plants ; at the same time I demonstrated that 

 most of our objects hitherto known as Aiiisias and Sternbergias 

 were merely inorganic casts of these discoid medullary cavit es. 

 Further knowledge of this genus seems to suggest that it was 

 not only the oldest of the true Conifers in point of time, but also 

 one of the lowest of the coniferous types. 



Cycads. — The combined labours of Grand Eury, Brongniart, 

 and Renault have revealed the unexpected predominance in some 

 localities of a primitive but varied type of Cycadean vegetation. 

 Observers have long been familiar with certain seeds known as 

 Trigonocarpons and Cardiocarpons, and with large leaves to 

 which the name of Noeggerathia was given by Sternberg. All 

 these seeds and leaves have been tossed from family to family at 

 the caprice of different classifiers, hut in all cases without much 

 knowledge on which to base their determinations. The rich 

 mass of material disinterred by M. Grand-Eury at St. Etienne, 

 and studied by Br ingniart and M. Renault, has thrown a flood 

 of light upon some of these objects, which now prove to be 

 primaeval types of Cycadean vegetation. 



Mr. Peach's discovery of a specimen demonstrating that the 

 Antholithes Pitcairnia ' of Lindley and Hutton was not only, 

 as these authors anticipated, "the inflorescence of some plant," 

 but that its seeds were the well-known Cardiocarpons, was the 

 first link in an important chain of new evidence. Then followed 

 the rich discoveries at St. Etienne, where a profusion of seeds, 

 displaying wonderfully their internal organisation, was brought 

 to light by the energy of M. Grand-Eury, which seeds M. Brong- 

 niart soon pronounced to be Cycadean. At the same time 1 was 

 obtaining many similar seeds from Oldham and Burntisland, in 

 which also the minute organisation was preserved. Dawson, 

 Newberry, and Lesquereux have also shown that many species 

 of similar seeds, though with no traces of internal structure, 

 occur in the coal-measures of North America. 



Equally important was the further discovery by M. Grand- 

 Eury that the Antholithes, with their Cardiocarpoid seeds, were 

 but one form of the monoclinou-. catkin-like inflorescences of 

 the Noeggerathia, now better known by Unger's name of Cor- 

 daites. These investigations suggest some important conclusions : 

 1st. The vast number and variety of these Cycadean seeds, as 

 well as the enormous size of some of them, is remarkable, 

 showing the existence of an abundant and important Carbon- 

 iferous vegetation, of most of which no trace has yet been 

 discovered other than these isolated seeds. 2nd. Most of the 

 seeds exhibit the morphological peculiarity of having a large 

 cavity (the "cavite pollinique" of Brongniart) between the 

 upper end of the nuceile and its investing episperm, and imme- 

 diately below the ruicro|ile of the seed. That this cavity was 

 destined to have the pollen grains drawn into it, and be thus 

 brought into direct connection with the apex of the micelle, is 

 shown by the various examples in which such grains are still 



* " Memoir " ix. PI. xxv. Figs. 93, 94, 98, 99, 100, and 101. 



2 '• Memoir" xi. PI. xlviii. Fig. 4 fj '. "Memoir" ii. PI. xxix. Fig. 

 42 £. "Memoir" iii. PJ. xhii. Fig. 17. 



3 "On the Structure and Affinities of the Plants hitherto known as 

 Sternbergias," "Memoirs of the Literary and Philosophical Society of 

 Manchester," 1851. M. Renault, in his ".Structure comparee de quelques 

 Tiges de !a Flore Carbontfere" p. 285, has erroneously attributed this 

 discovery to Mr. Dawes, including my illustration from the Jasminium and 

 Juglans. Mr. Dawes' explanation was a very different one. 



* " Fossil Flora," p. 82. 



found in that cavity. 1 3rd. M. Grand-Eury has shown that 

 some of his forms of Cordait s possessed the discoid or Stern- 

 bergian pith which I had previously found in Dadoxylon ; and, 

 lastly, these Cordaites prove that a diclinous form of vegetation 

 existed at this early period in the history of the flowering plants, 

 but whether in a monoecious or a dicecious form we have as yet 

 no means of determining. Their reproductive structures differ 

 widely from the true cones borne by most Cycads at the present 

 day. 



Conifers. — It has long been remarked that few real cones of 

 Conifers have hitherto been found in the Carboniferous rocks, 

 and I doubt if any such have yet been met with. Large quan- 

 tities of the woody stems now known as Dadoxylons have been 

 found both in Europe and America. These stems present a true 

 coniferous structure both in the pith, medullary sheath, wood, 

 and bark. 2 The wood presents one very peculiar feature. Its 

 foliar bundles, though in most other respects exactly like those 

 of ordinaiy Conifer-., are given off, not singly, but in pairs. 3 I 

 have only round this arrangement of double foliar bundles in the 

 Chinese Gingko [Saiisburia adiantifolia).* This fact is not un- 

 important when connected with another one. Sir Joseph Hooker 

 long ago expressed his opinion that the well-known Trigone- 

 carports 1 of the coal measures were the seeds of a Conifer allied 

 to this Saiisburia. The abundance of the fragments of Da- 

 doxylon, combined with the readiness with which cones and seeds 

 are preserved in a fossil state, make it probable that the fruits 

 belonging to these woody stems would be -o preserved. But of 

 cones we find no trace, and, as we discover no other plant in the 

 Carboniferous strata to which the Trigonocarpons could with any 

 probability have belonged, these facts affjrd grounds for asso- 

 ciating them with the Dadoxylons. These combined reasons, 

 viz. the structure of the stems with their characteristic foliar 

 bundles, and the Gingko-like character of the seeds, suggest the 

 probability that these Dadoxylons, the earliest of known Conifers, 

 bel inged tithe Taxinea, the lowest of these coniferous types, 

 and of which the living Saiisburia may perhaps be regarded as 

 the least advanced recent form. 



Thus far our attention has been directed only to plants whose 

 affinities have been ascertained with such a degree of probability 

 as to make them available witnesses, so far as they go, when the 

 question of vegetable evolution is sub judice. But there remain 

 others, and probably equally important ones, respecting which 

 we have yet much to learn. In most cases we have only met 

 with detached portions of these plants, such as stems or repro- 

 ductive structures, which we are unable to connect with their 

 other organs. The minute tissues of these plants are preserved 

 in an exquisite degree of perfection ; hence we are able to affirm 

 that, wdiatever they may lie. they differ widely from every type 

 that we are acquainted with amongst living ones. The exogenous 

 stems or branches from Oldham and Halifax which I described 

 under the name of Astromyelon,* and of which a much fuller 

 description will be found in my forthcoming Memoir xii., belong 

 to a plant of this description The remarkable conformation of 

 its bark obviously indicates a plant of more or less aquatic 

 habits, since it closely resembles those of Myriophyllum, Marsilea, 

 and a number of other aquatic plants belonging to various classes. 

 But its general features su :gest nearer affinities to the latter genus 

 than to any olher. Another very characteristic stem is the 

 ffeteranginm Griciii, 7 only found in any quantity at Burntisland, 

 but of which we have recently obtained one or two small specimens 

 at Halifax. This plant displays an abundant supply of primary, 

 isolated, vascular bundles, surrounded by a very feeble develop- 

 ment of secondary vascular tissue. Still more remarkable is the 

 Lyginodendron Oldhamium,* a stem not uncommon at Oldham, 

 and not unfrequently found at Halifax. Unlike the Heterangium, 

 it- primary vascular elements are feeble, but its tendency to 

 develop secondary zylem is very characteristic of the plant. 

 An equally peculiar feature is seen in the outermost layer of its 

 cellular bark, which is penetrated by innumerable longitudinal 

 lamina: of prosenchymatous tissue, which is arranged in precisely 

 the same way as is the hard bast in the lime and similar trees, 



1 " Memoir" viii. PI. ii. Figs. 70 and 72. Brongniart, " Recherches sur 

 les Graines Fossiles Silicinees," PI. xvi. rigs. 1, 2 ; PI. xx. Fig. 2. 



2 Dr. Dawson finds the discoid pith in one of the living Canadian 

 Conifers. 



3 " Memoir" viii. PI. lviii. Fig. 48, and PI. ix. Figs. 44-46. 



4 "Memoir" xii. PI. xxxiii. Figs. 28, 29. 



5 " Memoir " viii. Figs. 94-115. 



6 "Memoir" ix., in which I only described decorticated _ specimens 

 Messrs. Cash and Hick described a specimen in which the peculiar bark was 

 preserved under the name of Astromyclen WiUiamsonis. See " Proceedings 

 of- the Yorkshire Polytechnic Society," vol. vii. part iv. i88t. 



' "Memoir "iii. 8 " Memoir " iii. 



