228 PLA NT ANATOMY. 
quite advanced, medullary rays of smaller size are often subse- 
quently developed, which are designated as secondary medullary 
rays. They are met with frequently, for example in the Cin- 
chona barks (Fig. 145), and, according as they are located in 
the xylem or the phloém, are termed secondary xylem (or 
medullary) rays, or bark rays. 
The great variety of special features in the structure of the 
Fic. 146 a.—Transverse section through the peeled root of Rheum Rhaponticum. 
¥, brownish-red medullary rays; v, vessels; 6, crystals of oxalate. 
medullary rays affords very noteworthy and characteristic dis- 
tinctions for many drugs.’ As against roots which have a de- 
cidedly radiate structure (for instance, Rad. Rhapontici, Fig. 
146 a), it may suffice here, for example, to refer to such which 
"In the Conifer, as Essner has shown (Abhandl. d. naturforsch. 
_ Ges., Halle, 1882), the number and height of the medullary rays, as well 
_ asthe form and size of their cells, possess no diagnostical value. 
