EASTERN BORDERS. 295 



deposited. As fossil plants are generally fragmentary and imperfect, 

 and but rarely present tlie essential organs of fructification, the 

 inquiry is difficult ; it has nevertheless been prosecuted with ability 

 and zeal by eminent naturalists on the continent and in our own 

 country ; and, though several problems in Fossil Botany yet remain 

 unsolved, the structure and characters of many carboniferous plants 

 have been successfully determined. 



Fossil plants occur chiefly in the shales and sandstones associated 

 with coal ; not unfrequently they are found in nodules of ironstone, 

 and occasionally in limestone, as at Howick, North Sunderland, and 

 Scremerston. They consist of the stems and branches of trees, of 

 roots, fronds and leaves, and rarely of fruit. Most commonly they 

 are impressions or casts, the original vegetable matter being replaced 

 by sand or mud. In shales, the stems are flattened, but in sandstone 

 they are frequently round, and retain their natural shape ; usually 

 they are broken and prostrate, but in some cases they stand 

 perpendicular to the line of stratification, apparently rooted on 

 the spot where they grew. Coarse mud or sand has not, however, 

 in all instances replaced the vegetable matter ; for minerals, such 

 as silica and carbonate of Ume, have, in a state of solution, penetrated 

 the tissues of the plant and preserved its structure ; and, when in 

 this condition, the fossil admits of as exact an examination as a 

 modern vegetable, and contains vrithin itself the evidence by which 

 its nature and relationship can be determined. But in order to 

 understand the nature of this evidence, and also the references which 

 may be made to vegetable types, it vrill be of use briefly to explain 

 in what manner the existing Flora can be arranged in great divisions 

 or classes, according to their distinctive internal structures. To one 

 or other of the four following classes all existing plants may be 

 referred : — 



1 . THALLOGENs(from flaXXos, a leaf, and yevyaeiv, to produce), which 



are composed entirely of cellular tissue, and include Lichens, 

 Fungi or Mushrooms, and Algse or Sea-weeds. 



2. AcROGENS (axpos, summit), which have centres either hollow, or 



composed of cellular tissue ; vascular or woody bundles are 

 produced simultaneously near the circumference, and the stems 

 increase upward, and not in breadth; they include Mosses, 

 Lycopodiums or Club-Mosses, and Ferns. 



3. Endogens (evSov, within), which have definite bundles of vascular 



tissue scattered throughout cellular tissue ; the additions in the 

 first instance are towards the centre; they possess no true bark, 

 and no medullary sheath or rays ; Palm Trees and Grasses are 

 included in this class. 



4. ExoGENS (e^w, outward), which possess a distinct pith and bark, 



with medullary rays connecting the two; they increase, year after 

 year, by regular additions of vascular circles between the external 

 bark and the woody zone previously formed; Oaks, Beeches, all 

 our forest trees, and most British plants belong to this class. 



Let us show the application of these structural peculiarities to a 



