THE LYCOPODIALES 259 



dence as to the origin of parenchymatous elements in the primary 

 xylem. In the later (Permian) representatives of the Sigillariae the 

 primary cylinder became so much reduced that it was no longer 

 continuous. This topographical condition of the primary wood 

 was responsible, as indicated above, for a resultant discontinuity 

 of the secondary xylem. The processes of the pith extending 

 between the primary bundles and a short distance into the second- 

 ary cylinder are in a certain sense medullary rays, since they take 

 their origin from the medulla; but they have nothing in common 

 with the radial masses of storage tissue resulting from cambial 

 activity which characterize the organization of the secondary 

 cylinder. Further, they should not in any way be confused with 

 foliar gaps, since in the Lycopsida interruptions of this nature in 

 the fibrovascular cylinder do not occur. In the particular case 

 under consideration the leaf traces originate opposite the strands 

 of primary xylem and do not subtend the intervals between 

 them. It is clear that the Lepidodendraceae, although entirely 

 extinct, furnish extremely valuable data for the elucidation of the 

 origin of the parenchyma in the primary wood and for that of the 

 radial storage devices of the secondary xylem. Further, they 

 throw a very clear light on the general morphology of the secondary 

 woody cylinder in vascular plants, since the comparative study of 

 their stems from lower to higher geological levels makes it obvious 

 that the radial parenchymatous bands of the secondary wood 

 cannot appropriately be called medullary rays. They should be 

 called wood rays, as the inward relation to the medulla is neither 

 a primitive nor an essential condition. 



Not only in regard to the parenchymatous structures of their 

 primary and secondary wood, as well as by their great geological 

 age and early culmination, do the lepidodendrids in the large sense 

 show themselves to be primitive representatives of vascular plants, 

 but also by the organization of the fibrous elements of the secondary 

 wood. It has been pointed out in an earlier chapter that a typical 

 element of the primary wood in all plants is the scalariform tracheid. 

 In the secondary xylem of plants in general the scalariform element 

 has given place to the pitted tracheid, which is universal for 

 the various groups of gymnosperms and for the angiosperms with 



