THE ANATOMY OF WOODY PLANTS 



is found in the young plant as a passing phase and as frequently 



persisting for many years in that most conservative of plant organs, 



the root. In the Betu- 

 laceae, by contrast, the 

 diffuse type of wood ray 

 prevails, and it is likewise 

 clearly derived from a 

 primitive state of aggre- 

 gation. In the birch, for 

 example, diffuse rays 

 more usually distinguish 

 the stem, while the aggre- 

 gate type is often found 

 present in the case of the 

 root. The general ana- 

 tomical conditions as well 

 FIG. 70. Tangential section of young twig as experimental data in 



^Casuarina equisetifolia showing an aggregate Fagaceae and BetU- 



foliar ray in center flanked on either side by 



uniseriate rays. laceae justify the conclu- 



FIG. 700. Photographs of transverse views of polished stems of Casuarina stricta 

 (right) and Casuarina Fraseri, showing the general topography of diffuse and compound 

 rays. The diffusing rays of C. stricta appear to die out, while the rays of the other 

 species become not only accentuated but also clearly more numerous. 



