THE CARBONIFEROUS AGE. 129 



type, in which the fronds were borne in pairs on 

 opposite sides of the stem, leaving when they fell two 

 rows of large horseshoe-shaped scars marking the sides 

 of the trunk. Botanists, who have been puzzled 

 with these plants almost as much as with the Stig- 

 maria, have supposed these scars to be marks of 

 branches, of cones, and even of aerial roots; but 

 specimens in my collection prove conclusively that 

 the stem of this genus was a great caudex made up of 

 the bases of two rows of huge leaves cemented toge- 

 ther probably by intervening cellular tissue. As in 

 the Devonian and in modern times, the stems of the 

 tree-ferns of the Carboniferous strengthened them- 

 selves by immense bundles of cord-like aerial roots, 

 which look like enormous fossil brooms, and are known 

 under the name Psaronius. 



We have only time to glance at the vast brakes of 

 tall Calamites which fringe the Sigillaria woods, and 

 stretch far seaward over tidal flats. They were allied 

 to modern Mares' Tails or Equisetums, but were of 

 gigantic size, and much more woody structure of stem. 

 The Calamites grew on wet mud and sand-flats, and 

 also in swamps ; and they appear to have been espe- 

 cially adapted to take root in and clothe and mat 

 together soft sludgy material recently deposited or 

 in process of deposition. When the seed or spore 

 of a Calamite had taken root, it probably produced 

 a little low whorl of leaves surrounding one small 

 joint, from which another and another, widening in 

 size, arose, producing a cylindrical stem, tapering to 



K 



