176 



JIARDWICKE'S SCIENCE-GOSSIP. 



A SKETCH OF THE FLORA OF THE 

 COAL PERIOD. 



By Rev. J. Magens Mello, M.A., F.G.S., &c. 



[Coniinued/rom page 159.] 



r^IGILLLARIA.—^G may turn next to the 

 w3 Sigillaria. Trees of this genus were also 

 abundant in the swamps in the Carboniferous times, 

 and a very large number of species have been 

 described. The Sigillaria together with Lepido- 

 dendra and Calamites are considered by Professor 

 Wilhamson to form an exogenous division of the 

 vascular cryptogams. The structure of a Sigillaria 

 is very peculiar and anomalous, but it is not often 

 sufficiently well preserved for detailed examination. 

 Some information is nevertheless obtained by means 

 of its roots which were long thought to be the 

 remains of independent plants and were accord- 

 ingly named Stigmaria. Indeed the fragmentary 

 character of so much of the Carboniferous vegeta- 

 tion has caused much confusion in this way, different 

 generic names having been frequently assigned to 

 portions of one and the same plant. Although the 

 Sigillaria has now been frequently found in situ, 

 standing erect with its roots attached, the name 

 Stigraaria is still retained for the latter for convenience, 

 and besides this some Stigmariae have certainly been 

 the roots of other plants than Sigillaria, such as 

 Knorria and Lepidodendron. 



The structure of the root of a plant affords some 

 clue to the structure of its stems, and so, could we be 

 sure that in the case of a given Stigmaria we were 

 dealing with the root of a Sigillaria, we should be 

 able to arrive at some notion more or less definite 

 regarding that plant. Fortunately the Stigmarian 

 roots are often very well preserved, and good micro- 

 scopical sections of them can be obtained. They are 

 seen to be composed of a central pith of large poly- 

 gonal vessels, longitudinally arranged, surrounded 

 by a cylinder of scalariform tissue, this again is sur- 

 rounded by a large cellular layer. The vascular 

 cylinder is broken up by meshes through which passed 

 the vascular bundles to the rootlets. No traces of 

 medullary rays are found in the wood. This is 

 Mr. Carruthers' description, yet he shows that the 

 structure of the stem is precisely the same as in Lepi- 

 dodendron, in which, however, as has been observed, 

 Professor Williamson believes he has detected an 

 appearance of medullary rays ; these are supposed by 

 jMr. Carruthers to be simply the vascular bundles 

 supplying the rootlets or leaves as the case may be. 

 Outside comes the bark, a thick cellular layer the 

 external cells of which are smaller than those of the 

 interior, and arc fusiform. The whole of the vascular 

 tissue of Stigmaria is composed of scalariform vessels, 

 a clear proof that the true position of these plants is 

 amongst the Cryptogams, although very various 



opinions have at different times been held upon the 

 subject. By some writers they have been classed with 

 the Coniferffi and by others with the Cycads, The 

 leaves of Sigillaria have been described as linear in 

 form. The trunks are fluted and marked with the 

 long oval or pentangular leaf scars, which are more 

 or less spirally arranged. 



From the present condition of most of the pre- 

 served specimens it would appear that the inner parts 

 of the trunk decayed far more rapidly than the outer, 

 so they became hollow whilst standing, and flattened 

 when they fell, so that we now find the bark of two 

 opposite sides usually converted into bright coal, 

 forming a double layer from half to one inch in 

 thickness. Cylindrical portions of Lepidodendrous 

 stems are far more frequently found than of Sigillaria, 

 but when these latter occur in a more or less vertical 

 position in the strata the circular shape is retained, 

 and the bark having got filled up with sand or mud, 

 gives us a perfect cast of the interior, whilst oc- 

 casionally affording specimens of contemporary 

 animal life. 



The fruit of Sigillaria has been described and 

 resembles very closely the Flemingites already men- 

 tioned, the only difference being that " the small 

 • sporangia are scattered in an irregular patch over the 

 dilated base of an ordinary leaf, this leaf is moreover 

 inserted into the stem almost vertically instead of 

 horizontally, as in the case of the Lepidodendroid 

 fruit ; " and the Sigillariostrobiis, as Schimper calls the 

 fruit cone, bears both macrospores and microspores. 

 Large quantities of the macrospores are not unfre- 

 quently found in great abundance in the beds con- 

 taining Sigillarian and Stigmarian remains, they are 

 also sometimes found in the interior of the trunks. 

 The spores and sporangia of Sigillaria and Lepidoden- 

 dron occur in such quantities in many coals that beds 

 of considerable thickness are formed of but little else. 

 Mr. Binney has described a seam some six feet in 

 thickness almost entirely composed of macrospores. 

 They have been found in splint coal in Fife, together 

 with Stigmarise and Sigillaria, in the Parrot coal of 

 Armiston, and in Boghead coal ; indeed, in each of 

 the leading varieties of coal they abound. They 

 occur in the Tasmanian resiniferous shale, forming 

 from forty to fifty per cent, of the rock. Mr. Carruthers 

 has suggested a way in which such enormous deposits 

 of spores may have been formed by reference to the 

 so-called sulphur showers produced by the shedding 

 of the pollen in the Scotch and Norwegian pine 

 forests. 



To the extinct group of arborescent Lycopodiaceae the 

 fossil forms named Ulodendra, Halonia, Dadoxylon 

 and others have been assigned. Halonia already men- 

 tioned as being possibly the fruit-bearing branch of a 

 Lepidodendron has also been spoken of as the root of 

 the same plant, although as the two have never been 

 found in connection the question of its affinity must 

 remain uncertain. 



