THE LYCOPODIALES 



257 



to those appearing in the cortical region. It may accordingly 

 be stated that even the imperfect evidence supplied by the stem 

 of the lepidodendrids in a condition of obvious degeneracy of 

 the primary stelar tissues when they are first presented on the pages 

 of the geological record does not definitely justify the conclusion 

 that the pith is of stelar origin. In a later chapter it will be made 

 clear that the evidence supplied by the lower Pteropsida, which 

 is at once more 

 abundant and 

 more decisive, dis- 

 tinctly vouches for 

 the extra-stelar 

 derivation of the 

 medulla. 



The degeneracy 

 of the tracheary 

 elements which has 

 already been noted 

 in the case of the 

 protostelic type of 

 cylinder in the 

 lepidodendrids 

 makes itself par- 

 ticularly obvious 

 in the higher 



siphonostelic representatives of the group and especially in the 

 stems included under the Sigillariaceae. The reduction in amount 

 of the primary tissues results in the appearance of gaps in the wall 

 of the stelar tube which are not related to appendages. The 

 central cylinder consequently becomes discontinuous, as has 

 been shown in Fig. 122, page 170. Since the later-developed 

 secondary tissues naturally first appear opposite the framework 

 outlined in the primary wood, the secondary xylem is likewise at 

 the beginning of its formation discontinuous, and continuity 

 appears only after cambial activity has resulted in the formation 

 of a woody cylinder of some thickness. Where the primary struc- 

 ture of the xylem has undergone the extreme degree of reduction 



FIG. 184. Part of the same, more highly magnified 



