THE ORIGIN OF PLANT LIFE ON THE LAND. 107 



broken off they left round marks regularly arranged. These 

 roots are the so-called Stigmarm, so abundant in every coal-field, 

 and especially filling the " under-clays " of the coal-beds, which 

 are the soils on which the plants forming these beds were sup- 

 ported. The true botanical position of the Sigillarice has been 





Fig. 98.— Carboniferous Tree-ferns. 



A, Me^aJ>hyton vtagHificum {Dn.'). c, Palaapterd Hartii (Jin.). 

 D, F, Acadica (Dn.). 



a matter of much controversy. Some of them undoubtedly 

 have structures akin to those of t' 2 tree-like Club-Mosses, as 

 Williamson has well shown, and may have been cryptogamous. 

 Others have structures of higher character, akin to thos . of the 

 modern Cycads, and seem to have borne nutlets allied to 

 those of these plants. Yet the external forms of these diverse 



