PTERIDOPHYTA 53 



the cellular structure is easily observed, while the peculiar 

 ornamentation of the surface of the stem is shown by the ex- 

 ternal molds. In the great majority of specimens the leaf 

 bases only are preserved; but attached leaves have been 

 found in some of the calcified specimens of the English " coal 

 balls," in which whole masses of stems, leaves, and fruits are 

 found in a wonderfully preserved condition. These coal balls 

 are simply concretions of the carbonates of lime and magnesia 

 which formed around certain masses of the peaty vegetation 

 as centers and, through inclosing and interpenetrating them, pre- 

 served them from the peculiar processes of decay which con- 

 verted the rest of the vegetation into coal. In them the min- 

 eral matter slowly replaced the vegetable matter, molecule by 

 molecule, thus preserving the cellular structure to a remarkable 

 degree. Such balls are especially frequent in the coal of certain 

 parts of England (Lancashire and Yorkshire). 



Enormous spreading root-like underground stems occur in 

 the rocks of the Pennsylvanian, apparently in the position in 

 which they grew. Such root-Uke organs, to which the name 

 Stigmaria has been given, were possessed by both Lepidodendron 

 and the allied genus, Sigillaria. That they are not true roots 

 is evidenced by their anatomical structure and their habit of 

 branching, always into two, as has been noted in the stems of 

 Lepidodendron. They bear many small appendages which 

 seem to be true roots. Such root-bearing underground stems 

 are characteristic of living lycopods. These large and spread- 

 ing root-like organs were adapted for growth in wet ground where 

 long roots were not needed for penetrating far into the soil for 

 moisture. 



It is in the fossil cones, that show its manner of fruiting, 

 that Lepidodendron betrays its relationship to the living 

 club-mosses. The cones of both fossil and living lycopods con- 

 sist of scales arranged around an axis, each bearing on its upper 

 side a large spore case containing many spores, all of one kind. 

 These cones of Lepidodendron are known by the collective name 



