126 



HARD WICKE'S SCIENCE- G OS SIP. 



trunk and the various forms of its flutings and leaf 

 scars. 



Qur knowledge of its fractification is also extremely 

 limited and uncertain. Mr. Carruthers, F.R.S., one 

 of our great authorities on fossil botan)', thinks that 

 its fruits assumed the form of spores, which were 

 borne at the bases of its long, narrow leaves, but 

 this conjecture, I think, needs further corroboration ; 

 and witli all due respect for so high an authority, I 

 have reasons for thinking that its fruits, when un- 

 doubted specimens are met with, will be found to 

 assume the form of cones, Kke the rest of the 

 lepidodendroid plants. 



In our coal strata I have occasionally found cones 

 ■with long, narrow leaves attached, these being quite 

 different from the ordinary cones (lepidostrobi) of 

 the lepidodendron, and it is not improbable that 

 they may turn out to be the fruits of sigillaria. The 

 lepidostrobus is clothed with the same kind of thick 

 scale-like leaves as its parent, lepidodendron, and 

 in like manner we may expect to find that the fruit 

 of sigillaria will be clothed with the same kind of 

 leaves as the stem. For the present, however, we 

 must leave it an open question until we have obtained 

 raore decided proofs. 



Much diversity of opinion prevails among fossil 

 botanists, concerning the botanical affinities of 

 sigillaria. M. Brongniart, M. Grand'Eury and 

 other French authors and Dr. Dawson of Canada, 

 place it among the Gymnosperms, while Dr. Hooker, 

 Mr. Carruthers, Mr. Binney, Professor W. C. 

 ^Yil]iamson, and other English authors regard it as 

 a cryptogam representing the highest form of lepi- 

 dodendroid type of vegetation. 



These differences of opinion are probably in a 

 great measure due to the very imperfect state of 

 ])reservation in wliich specimens of sigillaria, show- 

 ing structure, have hitherto been found. Even in 

 our coal-balls, where we meet with fragments of 

 .'.igillaria not unfrequently, yet we rarely find any- 

 thing but the epidermal layer, the inner structure 

 being rarely preserved. This is all the more 

 singular, as in the very same material we generally 

 find the lepidodendron in a good state of preserva- 

 tJo.i. 



I\Iuch of this diversity of opinion probably is 

 owing to the different views which obtain among 

 vriters concerning the correlations of the various 

 fiirms of tissue existing in the ancient cryptogams 

 with those existing in modern exogenous plants. 

 Some authors seem to forget that the lepidodendrons 

 r.nd sigillarias and their allies, filled the place in 

 the vegetable world in the coal period, that our 

 modern forest trees do now, and that while in 

 regard to their histological parts, especially tlieir 

 reproductive organs, they undoubtedly belong to the 

 cryptogams, yet in order to meet the extra require- 

 lueiits of their large growtli and tree-like habits they 

 vcrc built upon exogenous principles. Some authors 



deny the existence of medullary rays in these plants, 

 and refuse to recognise the tripartite arrangement of 

 the tissue? of their stems into, central medulla, 

 ligneous cylinder and bark. But Professor W. C. 

 Williamson with, in my opinion, a far better know- 

 ledge and ajopreciation of the homologies of the 

 various tissues existing in these ancient plants and 

 in their modern representatives, following in the 

 footsteps of the illustrious Brongniart, fully recognises 

 the existence of medullary rays, and does not hesitate 

 to employ the terms, medulla or pith, wood or 

 ligneous cylinder, bark and epidermal layer, "be- 

 cause," he says, " I am convinced of their existence 

 in the fossil plants." Following the lead of such 

 excellent guides I shall make use of the same terms 

 in my descriptions of these plants. M. Brongniart 

 was one of the first to describe and figure specimens 

 of sigillaria. He describes the stem of his S/gi/Iaria 

 cicgans as consisting of a central cellular axis or 

 medulla, surrounded by a vascular cylinder, which 

 is invested by a thick cortical layer of cellular tissue. 

 This description almost exactly agrees with one of 

 the stems which Mr. Binney has described under the 

 name of Sigillaria vascidaris. In that memoir Mr. 

 Binney gave figures of two or three different plants^ 

 One of these has been recognized and described by 

 both Mr. Carruthers and by Professor Williamson as 

 the Lepidodendron selaginoides of Sternberg (see 

 fig. 65 of my May j^aper). Another is a similar plant 

 to that from which other figures are taken, which 

 Professor Williamson thinks may be a sigillaria, and 

 he has given this form the name of Diploxyloit 

 zasciildre, for reasons which I shall explain farther 

 on ; for the present, however, I shall retain the 

 older and more familiar name of Sigillaria vas- 

 cularis. This plant is frequently met with in our 

 Halifax coal-balls. It has a central medulla, com- 

 posed of cellular and vascular tissues, surrounded by a 

 woody cylinder, formed of scalariform vessels, which 

 are arranged in radiating lamince, this again being 

 invested with a thick bark composed of cellular 

 tissue and a greater proportion of jDrosenchyma or 

 woody tissue. From the examination of a great 

 number of specimens of this plant, I am able to 

 endorse Professor Williamson's opinion that it is a 

 sigillaria of the diploxylon type. The ordinary 

 form of diploxylon also occurs in our coal-balls, but 

 here, as elsewhere, it is rather rare. 



The centre of a diploxylon stem consists of a 

 vascular cylinder, which is formed of large scalari- 

 form cells and vessels, enclosing a hollow space. 

 Surrounding this vascular or "medullary cylinder," 

 is the ligneous or woody cylinder. The centre of 

 this plant is therefore formed of two woody cylinders; 

 hence its name diploxylon or double wood. Pro- 

 fessor Williamson says that Brongniart's specimen of 

 Sigillaria ehgans belongs to the Favularian type, and 

 that it exhibits a central axis, the structure of which 

 is " nearly identical with " one of his figures of 



