3 6 4 



THE ANATOMY OF WOODY PLANTS 



ever, on the basis of the organization of primitive organs and 

 regions that the condition of aggregation presented by the genus 

 Ephedra was once present in Gnetum. We may therefore regard 

 the large rays of the genus as comparable with the similar struc- 

 tures in the mature wood of the stem of our northern oaks and 

 derived in a similar manner as the result of aggregation and fusion. 

 The minute organization of the secondary xylem in Gnetum reveals 



in addition to the 

 prominent rays, 

 which have been dis- 

 cussed in the pre- 

 ceding paragraph, 

 other radial struc- 

 tures which are of the 

 uniseriate type and 

 correspond with the 

 similar rays in the 

 Coniferales and with 

 the linear rays of the 

 wood of the oak and 

 allied dicotyledonous 

 types. Parenchyma 

 is likewise abundantly 

 present in the diffuse 

 condition. Last and by no means least, vessels which in most 

 species of the genus present the large caliber characteristic of 

 climbers are present. These will be considered later in a special 

 paragraph. 



In Fig. 262 is shown an older stem of G. scandens. Here the 

 woody cylinder instead of consisting of a single circle of bundles 

 has become polydesmic. This condition cannot be regarded as 

 having any large evolutionary significance, as it is commonly 

 found in the stems of climbers of remote systematic affinities. 

 Its chief significance is in connection with the origin of the type 

 of cylinder presented by the genus Welwitschia and that found 

 in certain cycadean types, living and extinct. Gnetum shows, 

 not only the polydesmic stem often found in woody climbers, 



FIG. 262. Transverse section of an older stem of 

 the same. 



