536 



SCIENCE. 



[Vol. II., No. 3T. 



large-ribbed Sigillariae is still very imperfect. Paleon- 

 tologists who have urged the separation of the Sigil- 

 lariae from the Lepidodendra have attached weightto 

 the difference between the longitudinally ridged and 

 furrowed external bark of the former plants, along 

 which ridges the leaf-scars are disposed in vertical 

 lines, and the diagonally arranged scars of Lepido- 

 dendron. They have also dwelt upon the alleged 

 absence of branches from the sigillarian stems. I 

 think that their mistake, so far as the branching is 

 concerned, has arisen from their expectation that 

 the branches must necessarily have had the same 

 vertically grooved appearance and longitudinal ar- 

 rangement of the leaf-scars as they observed in the 

 more aged trunks: hence they have probably seen 

 the branches of Sigillariae without recorjnizing them. 

 Personally, I believe this to have been the case. I 

 further entertain the belief, that the transition from 

 tlie vertical phyllotaxis, or leaf-arrangement, of the 

 sigillarian leaf -scars, to the diagonal one of the Lepi- 

 dodendra, will ultimately be found to be effected 

 through the subgenus Favulaiia, in many of which 

 the diagonal arrangement becomes quite as conspicu- 

 ous as the vertical one. This is the case even in 

 Brongniart's classic specimen of Sigillaria elegans, 

 long the only fragment of that genus known, which 

 preserved its internal structure. The fact is, the 

 sliape of the leaf-scars, as well as their proximity to 

 each other, underwent great changes as lepidoden- 

 droid and sigillarian stems advanced from youth to 

 age. Thus Presl's genus Bergeria was based on 

 forms of lepidodendroid scars which we now find 

 on the terminal branches of unmistakable lepido- 

 dendra.i The phyllotaxis of Sigillaria, of the type 

 of S. occulata, passes by imperceptible gradations 

 into that of Favularia. In many young branches the 

 leaves were densely crowded together; but the ex- 

 ogenous development of the interior of the stem, and 

 its consequent growth both in length and thickness, 

 pushed these scars apart at the same time that it in- 

 creased their size and altered their shape. We see 

 precisely the same effects produced upon tlie large, 

 fruit-scars of Ulodendron by the same causes. The 

 carboniferous lycopods were mostly arborescent; but 

 some few dwarf forms, apparently like the modern 

 Selaginellae, have been found in the Saarbriiclven 

 coal-fields. Many, it not all, the arborescent forms 

 produced secondary wood by' means of a cambium- 

 layer, as they increased in age. In the case of some 

 of them,- this was done in a very rudimentary man- 

 ner; nevertheless, suflicienlly so to demonstrate what 

 is essential to the matter, viz., the existence of a 

 cambium-layer producing ' centrifugal growth of sec- 

 ondary vascular tissue.' 



As already pointed out in the case of the Calamites, 

 the vascular axis of these Lepidodendra was purely 

 an 'appareil conducteur,' unmixed with any wood- 

 cells: hence the 'appareil de soulien' had to be sup- 

 plied elsewliere. This was done as in the Calamites: 

 a thick, persistent, hypodermal zone of meristem' 



' See Memoir xii., pi. xxxiv. 

 2 E. g. L. Ilarcoui'tii, Memoir 

 » Memoir ix., pi. xxv., lifjs. 93 



developed a layer of 'prismatic prosenchyma of enor- 

 mous thickness,^ which incased the softer structures 

 in a strong cylinder of self-supporting tissue. We 

 have positive 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 

 the Lepidostrobi, 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. 



Gymnospernis. — Our knowledge of this part of the 

 carboniferous vegetation has made great progress dur- 

 ing the last thirty years. This progress began with 

 my own discovery^ 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 Artisias and Sternbcrgias were merely 

 inorganic casts of these discoid medullary cavities. 

 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 conif- 

 erous types. 



Cycads. — The combined labors of Grand-Eury, 

 Brongniart, and Renault, have revealed the unexpect- 

 ed predominance in some localities of a primitive but 

 varied type of cycadean vegetation. Observers have 

 long been familiar with certain seeds known as Trig- 

 onocarpons 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 differ- 

 ent classifiers, but, in all cases, without much knowl- 

 edge on which to base their determinations. The 

 rich mass of material disinterred by M. Grand-Eury 

 at St. Etienne, and studied by Brongniart and M. 

 Renault, has thrown a flood of light upon some of 

 these objects, which now prove to be primeval types 

 of cycadean vegetation. 



Mr. Peach's discovery of a specimen demonstrating 

 that the Antholithes Pitcairniae " of Lindley and 

 Button 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 fol- 

 lowed the rich discoveries at St. Etienne, where a 

 profusion of seeds, displaying wonderfully llieir inter- 

 nal organization, was brought to light by the energy 

 of M. Grand-Eury; which seeds M. Brongniart soon 

 pronounced to be cycadean. At the same time I 

 was obtaining many similar seeds from Oldham and 

 Burntisland, in which, also, the minute organiza- 



' Memoir xi., pi. xlviii., lig. iff; Memoir ii., pi. xxlx., fig. 

 42 * ,• Memoir iii., pi. xliii., fig. 17. 



= On the slructure and afUnities of the plants liitherto known as 

 Sternberglas. Memoirs of the literary and philosophical society 

 of Manchester, 1851. M. Eenaiilt, in his Structure compnrie de 

 quelques tiges de la flore carboniffere, p. 285, has erroneously at- 

 tributed this discovery to Mr. Dawes, including my illuslralion 

 from the jasmine and juglans. Mr. Dawes' explanation was n 

 very difterent one. 



■■' Fossil flora, p. 82. 



