of the Fossil Plants of the genus Sigillaria. 9 



the inner layer are elongated in the longitudinal direction of the stem, 

 and arranged parallel to each other ; they have all the appearance of 

 that kind of tissue called Prosenchjma. The cells of the outer layer are 

 not in parallel lines, but in some cases are very regularly arranged, while 

 in others they are less so ; they constitute ordinary cellular tissue or 

 parenchyma. The superficial cells differ from the latter in having a 

 thicker membrane. 



I shall now briefly advert to some points of difference and of resem- 

 blance which Brongniart mentions as existing between Sigillaria elegang, 

 and some other plants, both recent and fossil. Among existing Exogens, 

 the ligneous cylinder is variously constituted. In the Angiospermous divi- 

 sion of this class it is formed of a mixture of woody fibre or fusiform cells, 

 oHothrenchyma, and occasionally of other forms of tissue, and in the Gym- 

 nosperraous division, for example the Conifers and the C^'cases, it is ge- 

 nerally formed of one kind.^' Now, the ligneous cylinder of Sigillaria 

 elegans is formed of an uniform tissue as in the last division. In Stigma- 

 ria, Anabathra,\ and some other allied fossils which vegetated during the 

 same geological epoch, the same uniformity of structure occurs. So far 

 the recent and fossil plants resemble each other. But when a further 

 comparison is attempted, we find that all the points of agreement have 

 been exhausted. In most of the living Conifers and Cycases, it is well 

 known that the ligneous vessels are marked only on those walls which 

 correspond with the direction of the medullary rays ; but in the fossils 

 just mentioned, the whole of the walls are marked. We have here a 

 remarkable difference, to which may be added another. In the fossils, 

 it is the character of the vessels to be marked with transverse and reti- 

 culating bars, but in nearly all Coniferce, and in a great number of Cyca' 

 dacecB, the vessels are characterized with what are generally called disks. 



Respecting the first difference, botanists are acquainted with a few 



♦ It would appear, however, from the observations of Professor D. Don, that, 

 in Cycadaceoe, there is not such a general uniformity in the vessels of their lig- 

 neous system. And with respect to Coniferce^ Lindley mentions the Yew {Taxus 

 haccata) and Abies Douglassi as exceptions; Goeppart, in his Memoir entitled 

 " De Coniferarum Structura Anatomica," has given some longitudinal sections of 

 the former, exhibiting its structure. 



t I defer entering into any particulars respecting these two fossils, until the 

 close of this paper. It is necessary, however, in this place, to extract the fol- 

 lowing from Brongniart's " Observations," with reference to Stigmaria. ^ The 

 internal bundles which form the medullary sheath of Sigillaria elegans, and which 

 are so completely distinguished by the irregular and non-radial arrangement of 

 their constituent vessels, are completely absent in Stigmaria. There would thus 

 appear to be the same difference between these fossils as is observable between 

 the stems or branches of any dicotyledonous plant in [which the ligneous cylinder 

 is accompanied, on its inner side, by the bundles of the medullary sheath, and 

 their roots, which are deprived of these bundles." 



