336 



LYCOPODIALES 



L. selaginoides. It would seem probable that the non-medullated condition, 

 so persistently maintained in the smaller living Lycopods, was the primitive 

 state also for the larger dendroid fossils. The other factor of expansion, by 

 cambial activity, appears to have originated independently of medullation, 

 since it occurs both in medullated and in non-medullated axes. Physio- 

 logically it counterbalanced medullation where both occur together, for it 



Ba 



\t 



B: 



FIG. 176. 



Transverse section of an axis of Lepidodcndron selaginoides, Cj' = centre of the 

 vascular system ; tr= tracheae; /'= vessels of the primary cylinder ',//> = primitive fibres 

 of the primary wood; j5 2 = trac h e i ( l es f tne secondary wood ; r = ray of the secondary 

 wood ;^ 2 = secondary parenchyma; s<r = cambial zone; jL = liber; j = foliar traces detached 

 from the primary cylinder. (After Hovelacque.) 



substituted an enlarging peripheral vascular supply for the reduction in 

 efficiency in the limited central system. This was indeed a necessary 

 condition for dendroid development. 



However large the proportion of pith to the primary wood became in 

 Lepidodendroti) the continuity of the ring was as a rule unbroken, and the 

 leaf-traces were simply inserted upon the primary xylem with the minimum 

 of local disturbance. But in Sigillaria, in which the leaves sometimes 

 attained a very large size, the case is different : though they show in all 



