THE FLORA OF THE COAL FORMATION. 467 



lying separately, in other cases grouped in bands half an inch or more 

 in diameter, and embedded in a loose sort of mineral charcoal. When 

 treated with nitric acid, each bundle resolves itself into a few scalari- 

 form vessels surrounded with a sheath of woody fibres, often minutely 

 porous. This structure is precisely that of macerated fern-stipes ; but, 

 as already stated, there may have been some other coal-plants whose 

 leaves presented similar bundles. As stated in my former paper " On 

 the Vegetable Stx'uctures in Coal," this kind of tissue is especially 

 abundant in the coarse and laminated portions of the coal, which we 

 know on other evidence to have been made up, not of trunks of trees, 

 but of mixed herbaceous matters (Fig. 175, C). 



c. Scalariform vessels. — These are very abundant in the mineral 

 charcoal, though the coarser kinds have been crushed and broken in 

 such a manner that they usually appear as mere debris. The sca- 

 lariform vessels of Lepklodendron., Lepidophloios, and Stigmaria are 

 very coarse, and much resemble each other. Those of ferns are finer, 

 and sometimes have a reticulated structure. Those of Sigillaria 

 are much finer, and often have the aspect of wood-cells with trans- 

 versely elongated pores like those of Cycas. Good examples of 

 these are figured in the paper already referred to (see also Fig. 175, 

 A and D). 



d. Discigerous wood-cells. — These are the true bordered pores 

 characteristic of Sigillaria, Calamodendron, and Dadoxylon. In the 

 two former genera the discs or poics are large and irregularly ar- 

 ranged, either in one row or several rows ; but in the latter case they 

 are sometimes regularly alternate and contiguous. In the genus 

 Dadoxylon they arc of smaller size, and always regularly contiguous 

 in two or more rows, so as to jDresent an hexagonal areolation. Dis- 

 cigerous structures of Sigillaria and Calamodendron are very abun- 

 dant in the coal, and numerous examples were figured in my paper 

 above cited. I have indicated by the name Reticulated Tissue certain 

 cells or vessels which may either be reticulated scalariform vessels, 

 or an imperfect form of discigerous tissue. I believe tliem to belong 

 to Stigmaria or Calamodendron (Figs. 162 and 175, A.) 



e. Epidermal tissue. — This is a dense cellular tissue representing 

 the outer integuments of various leaves, herbaceous stems, and fruits. 

 I have ascertained that the structures in question occur in tiie leaves 

 and stipes of Cordaifes and ferns, and in the outer coat of Carpolitcs 

 and Sporangitcs. With this I may include the obscure and thick- 

 wallcd cellular tissue of the outer bark of Sigillaria and Lepidoden- 

 dron and other trees, which, though usually consolidated into com- 

 pact coal, sometimes exhibits its structure. 



