188 PROFESSOR BALFOUR ON CERTAIN VEGETABLE ORGANISMS 



ous vegetable impressions, particularly of Sigillaria and Stigmaria. These plants 

 appear to have been concerned in the formation of this coal, and specimens in 

 the Edinburgh Botanical Museum seem to prove this. That these plants do fre- 

 quently form coal, has been long believed by geologists and fossil botanists ; but this 

 opinion has been lately called in question by Mr Quekett, who, as the result of his 

 histological researches, states that " such plants rarely, if ever, form coal." This 

 startling statement appears to me to be founded on very slender data. We un- 

 doubtedly meet with separate specimens of these plants converted into carbona- 

 ceous matter, and if so, why should they not occur in a similar condition in mass, 

 so as to form coal beds ? 



Sigillarias are perhaps the most important plants in the coal formation, form- 

 ing a conspicuous feature in almost every field, appearing in all the strata, and 

 very generally distributed both in the Old and New World. There are upwards 

 of sixty species described. The plants appear to have been Acrogenous, and to 

 have had a lax tissue, allied to the succulent ferns belonging to the suborders 

 Marattiacese and Danseacese. Their roots, denominated Stigmarias, are also 

 very abundant, and are common in the underclay of coal beds, as well as in the 

 coal itself. 



The tissue of these plants has in some instances been well preserved; in 

 other cases it has been much compressed, and so altered as to show scarcely any 

 structure under the microscope. J. D. Hooker remarks, " Considering the exceed- 

 ingly lax and compressible tissue of the ordinary coal plants, it is not wonderful 

 that instructive specimens are rare. Plants whose tissues are so loose as to be 

 convertible after death into a mass of such uniform structure as coal, evidently 

 could not retain their characters well during fossilization." The singularly 

 succulent texture, and extraordinary size of both the vascular and cellular tissue 

 of many Sigillarias indicate possibly a great amount of humidity. The vascular 

 tissue of Sigillarias consists chiefly of scalariform and dotted vessels ; the former 

 marked by bars more or less complete, and the latter by dots or pits on the walls. 

 (Plate II., figs. 6-11.) 



The absence of other parts of plants, and indeed of any plants but the roots 

 of Sigillarias, in the underclay, seems to indicate that the soil was not fitted for 

 the growth of other vegetables. It is probable that the decay of those plants 

 whose roots struck into the underclay would produce a uniform bed of peat, 

 adapted to the growth of the ferns and other plants which are fossilized in the 

 superincumbent shales. Sigillarias also occur in the shale above the coal, and in 

 many cases their Stigmaria roots appear to have been incorporated with the coal 

 below * Quekett, on the other hand, maintains that coal is not formed, in any 

 instances, from plants with a lax tissue, but in all cases from Coniferse ; and 



* J. D. Hooker, on the Plants of the Coal Measures, in Report of Geological Survey, vol. ii. 



