214 THE EVOLUTION OF PLANTS 



The group is an attractive one; the decorative 

 form of the plant is beautifully shown in impres- 

 sions, and the structure no less perfectly preserved 

 in the petrified specimens. The stem was trav- 

 ersed by a single vascular column; the wood, 

 more or less triangular in section, was solid, with no 

 pith, and its development was from without in- 

 wards, as it is in most Lycopods. This applies to 

 the young condition; subsequently a zone of 

 secondary wood and bast was added by means of 

 a cambium, and at the same time a bark of some 

 thickness was formed, displacing the original 

 cortex. The roots grew in thickness in the same 

 way. The leaves of the wedge-like form, with their 

 numerous vascular strands, somewhat resembled 

 the leaflets of a Fern (fig. 24); when cut into 

 narrow segments the structure was simpler, with 

 one strand only in each. 



The fructification was very characteristic. The 

 cones, when well defined, bore a general resem- 

 blance to those .of the Calamites. The sporangia 

 were borne on organs sometimes much like the 

 fertile scales of that family, but there was much 

 more variety. In a very complex cone of the 

 Sphenophyll family named Cheirostrobus, from 

 the Lower Carboniferous rocks of Scotland, the 

 sporangium-bearing organs have four sporangia 

 each, and are almost exactly like those of a Gala- 



