THE CLUB-MOSSES 179 



The root-system, which seems to have been 

 similar in the two genera, is characteristic. 

 From the base of the trunk four great roots di- 

 verged, running horizontally or obliquely through 

 the soil; each of the four forked at least twice; 

 these underground organs reached an immense 

 size; a root measured by Williamson was 37 ft. 

 4 ins. long and 2 ft. 8 ins. hi diameter at its 

 base. The main roots bore numbers of rootlets, 

 which likewise forked, penetrating the soil hi all 

 directions. j 



The internal structure of the stem in the Lepi- 

 dodendreae was at first somewhat like that in 

 the simpler species of Selaginella, but it varied 

 considerably in the different species, and in most 

 cases became profoundly changed hi advancing 

 age by the occurrence of secondary growth. The 

 stem always contains a single vascular cylinder; 

 hi some Lepidodendrons the central column of 

 wood is solid; in most species there is a pith, and 

 in some Sigillarias the surrounding zone of wood 

 is more or less completely broken up into sepa- 

 rate strands. So far as the primary wood is con- 

 cerned, the development was always from with- 

 out inwards, as in most recent Lycopods; in most 

 of the Lepidodendrons, however, and in all 

 known Sigillarias, a considerable thickness of 

 secondary wood was added by a cambium; there 

 was also an extensive development of secondary 

 tissue in the cortex. 



