THE CARBONIFEROUS PERIOD. 
77 
the botanist readily distinguishes any particular 
species of Fern by this means, — a birth-mark, as 
it were, by which he detects the parentage of the 
individual. Another indication, equally signifi- 
cant, is found in the tubular structure of the 
wood in Ferns. On a vertical section of any 
well-preserved Fern-trunk from the old forests 
the little tubes may be seen very distinctly run- 
ning up its length ; or, if it he cut through trans- 
versely, they may be traced by the little pores 
like dots on the surface. Trees of this descrip- 
tion are found in the Carboniferous marshes, 
standing erect and perfectly preserved, with 
trunks a foot and a half in diameter, lising to a 
height of many feet. Plants so strongly bitumi- 
nous as the Ferns, when they equalled in size 
many of our present forest-trees, naturally made 
coal deposits of the most combustible quality. 
It is true that we find the anthracite coal of the 
same period with comparatively little bituminous 
matter ; but this is where the bitumen has been 
destroyed by the action of the internal heat of 
the earth. 
Next to the Ferns, the Club-Mosses (. Lycopo - 
diacece ) seem to have contributed most largely to 
the marsh-forests. They were characterized, 
then, as now, by the small size of the leaves 
growing close against the stem, so that the stem 
itself, though covered witk leaves, looks almost 
