302 



THE ANATOMY OF WOODY PLANTS 



A 



condition is paralleled in the reproductive axes of the cones of 

 the living Cycadales and in the seedling of their most primitive 

 genus, Cycas. 



As regards anatomical organization the foliar traces in their 

 cortical course, and even sometimes in the bases of the petioles, 

 show an interesting resemblance to the bundles of the stem in 

 Medullosa. Fig. 215 illustrates the structure of one of the leaf 



traces of Cycas re- 

 valuta as it passes 

 through the cortex. 

 The bundle is 

 clearly concentric 

 and consists of a 

 central core of 

 primary wood sur- 

 rounded by regu- 

 larly seriate 

 secondary xylem 

 and traversed by 

 wood rays. If the 

 figure under discus- 

 sion be compared 

 with that of a stem 

 bundle of Medul- 

 losa (Fig. 208), it 

 becomes evident that the only striking difference is presented by 

 the much smaller size of the strand of the living genus. 



The root in the Cycadales presents little that is of interest, 

 since the feature of the presence of centripetal wood is equally 

 exemplified by all roots as a characteristic connected with their 

 extremely conservative organization. It is only between Mesozoic 

 and modern types in which the evolutionary importance of the 

 centripetal wood has passed into the background that the anatom- 

 ical structure of the root comes into the phylogenetic foreground 

 in connection with the doctrine of descent. A feature not of 

 evolutionary interest presented by the cycadean root is the frequent 

 presence of root tubercles. 



FIG. 215. Cortical strand of Cycas revoluta 



