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 
Transverse section of an axis of Lefidodcndron selaginoides. Cyv=centre of the 
vascular system ; ¢-=tracheae ; J"=vessels of the primary cylinder ;_/4=primitive fibres 
of the primary wood; S2=tracheides of the secondary wood ; »=ray of the secondary 
wood ; f2=secondary parenchyma; sc=cambial zone; L =liber ; s=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 
Lepidodendron, 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 Szg//arda, in which the leaves sometimes 
attained a very large size, the case is different: though they show in all 
