170 



THE ANATOMY OF WOODY PLANTS 



known as leaf or branch gaps, as the case may be. In the lower 

 forms, with super-addition of secondary to primary woody struc- 

 tures, the primary cylinder of the siphonostele still maintained to 

 a large degree its individuality. This is, for example, true in 



Fig. i22a representing the organiza- 

 tion of a stem in a lepidodendrid, 

 an arboreal club moss of the Pale- 

 ozoic age. The lepidodendroid 

 arboreal club mosses include forms 

 referred to the general cognomen 

 of Sigillaria. In lycopods of this 

 type, particularly in the Permian 

 age and toward the end of their 

 term of existence as an element of 

 our earth's flora, the primary wood 

 became much reduced in amount. 



FIG. 122 



FIG. 122 a, b, and c. Diagrams to illustrate the effect of the degeneracy of the 

 primary wood on the development of the secondary xylem. 



Not only did this situation express itself in the thinning down of 

 the primary woody cylinder, but there was also present a lack of 

 continuity due to the complete elimination of tracheids in certain 

 segments. This phase is revealed in b. Here the primary wood 

 constituting a continuous cylinder in a is interrupted by intervals 

 resulting from the scanty development of the primary xylem. 

 The gaps between the resulting slender isolated strands are per- 



