CYCADOFILICALES AND CYCADALES 



295 



by the presence of more numerous large concentric fibrovascular 

 strands than those shown in Fig. 208. In such instances the con- 

 centric strands often show a more or less complete degeneracy of 

 the secondary xylem on their inner surfaces. This situation has 

 been regarded with reason as of significance in foreshadowing the 

 type of organization found in the axes of the Cycadales, living 

 and extinct. It will be seen from the account of Medullosa here 

 supplied that the genus presents some marked features of ana- 

 tomical resemblance to 

 ferns, the main contrasts 

 in organization being due 

 to the appearance of 

 secondary growth in con- 

 nection with the strands 

 belonging to the stem 

 proper. In accordance 

 with the canons of anat- 

 omy formulated in an 

 earlier chapter of the pres- 

 ent work, the secondary 

 activity has not yet pene- 

 trated into the traces which 

 pass out into the more con- 

 servative foliar structures. The Medulloseae are undoubtedly of 

 great interest from the standpoint of the evolution of cycadean 

 forms, and there can be little question that these types so common 

 as charred remains in the Carboniferous coals of Europe and 

 America came very near to being the actual ancestors of our 

 living Cycadales. 



As a third illustration of the Cycadofilicales we may take the 

 genus Lyginodendron, the organization of which has been so admir- 

 ably described by English anatomists. Fig. 210 illustrates the 

 structure of the stem in this genus. The same sclerified ribbing 

 is observed as in the other two genera discussed above. The 

 central cylinder, however, presents a marked contrast to that of 

 either Heterangium or Medullosa, for it consists of collateral strands 

 arranged in a closed circle. The pith is occupied by sclerotic 



FIG. 209. Foliar bundles of Medullosa anglica 



