CYCADOFILICALES AND CYCADALES 



297 



and its absence in the foliar traces show that secondary growth 

 is a comparatively recently acquired feature in the genus under 

 discussion. The sclerotic nests in the pith appear to vouch for 

 the extra-stelar origin of the medullary region. The situation 

 as a whole is like that found in the case of the Osmundaceae, 

 except for the complication introduced by the presence of the 

 secondary growth. In the genus Botrychium among the Ophio- 

 glossaceae secondary tissues, however, are often well developed, 

 and it has long 

 been realized that 

 the occurrence of 

 secondary activity 

 in the fibrovascular 

 tissues is not an 

 extremely impor- 

 tant criterion from 

 the evolutionary 

 standpoint. It 

 was at one time 

 thought that the 

 genus Lyginoden- 

 dron was the proto- 

 type of the 

 Cycadales, but this 

 opinion has been 

 given up in favor 

 of a derivation of 

 the cycadean type from forms like the Medulloseae, for which the 

 evidence is extremely strong. 



We may now turn our attention to the anatomical organization 

 of that group of gymnosperms which still persists as the Cycadales. 

 This family was well developed in Mesozoic times, but there is 

 no evidence that it was represented in the Paleozoic age. The 

 stem of the cycadean gymnosperms, in its modern representatives 

 at any rate, is soft and parenchymatous and is characterized by a 

 very large medulla. Both cortex and medulla contain numerous 

 mucilage canals. The woody cylinder is ordinarily very thin 



FIG. 211. Transverse section of primary and second- 

 ary wood in Lyginodendron. 



