W. Carruthers — The Forests of the Coal Period. 299 



regions. So also the spores of Equisetum and Calamites agree in size, 

 as may be seen in Plate II., Figs. 3, 4, and 9, where the spores of the 

 two genera are magnified to the same extent. And a similar com- 

 parison of the macrosj)ore and microspore of Triplosjiorites with those 

 of Selaginella, and of the microspore of Lepidostrobus with that of 

 Lycopodium, exhibits a similar agreement. This is made apparent 

 by the drawings of the two kinds of spores of Selaginella on Plate 

 III., Figs. 3 and 4, with those of Triplosporites, Figs. 8 and 9, which 

 are drawn to the same scale. ^ 



The fossils represented by the gi-oup of stems known under the 

 name of Lepidodendron, and by the three fruits described, agree in all 

 essential characters with the living Club-mosses, the only difference 

 of importance being that the stem of the fossil has a higher organiza- 

 tion suited to its arborescent habit. The vascular tissue continued 

 to increase with the growth of the plant somewhat like an exogenous 

 stem. In all the living vascular cryptogams, the vascular tissue is 

 produced at once in its full extent except in Isoetes, which has a 

 cambium layer surrounding the cylinder of wood in which as the 

 plant grows new vascular tissue is developed. The zone of thin- 

 walled spherical cells which surrounds the woody cylinder in 

 Lepidodendron, and which is so rarely preserved, has been a true 

 cambium layer like that in Isoetes. But for the existence of this 

 small water-plant, the large trees of the coal-forests would present 

 in the growth of their stems an inexplicable anomaly. 



Sigillaria, a very abundant Carboniferous fossil, is a member of 

 the same family as Lepidodendron. Its stem is rarely preserved so 

 as to exhibit structure, the only specimen hitherto described being 

 S. elegans, Brongn. ; ^ but its roots are frequently found in a very 

 perfect condition. The name Stigmaria was given to the roots at a 

 time when they were supposed to be independent plants. Their 

 relation to Sigillaria was suggested by Prof. Brongniart from the 

 correspondence in their structure, by Sir W. Logan from the position 

 the two fossils occupied in the beds in which they occur, and the 

 matter was finally set at rest when Mr. Binney observed the roots 

 and stems in actual continuity. 



As the structure and arrangement of corresponding parts in the 

 same plant are uniform, as of the root, stem, branches, and axis of 

 the cone, we may supply the want of information regarding the stem 

 by that which can be obtained from the root. 



The root is composed of a central medulla surrounded by a cylinder 

 of scalariform tissue, and this again is invested by a large cellular 

 layer. The vascular cylinder is broken up by meshes through which 

 passed the vascular bundles to the rootlets. There are no traces 

 whatever of medullary rays in the wood. The supposed medullary 

 rays which have been described in Sigillaria are the accidental re- 

 sults of desiccation in particular specimens. The internal structure 



1 I have given the precise measurement of these spores in a Notice of some Plant 

 Eemains from Brazil. Geol. Mag., Vol. VI. (1869), pp. 153, 154. 



2 Observations sur la Structure interieure du ISigillaria elegans. Par A. 

 Brongniart. Archives du Mus., Vol. i. (1831) p. 405. 



