156 



PRACTICAL BOTANY 



primary xylem in its original position : the further 

 arrangement of the secondary tissues is as in the Scarlet- 

 Runner, but note these points of difference in the root 

 as a whole : 



pr.xy 



FIG. 10. A. Diagram illustrating the disposition of tissues in the young root of a 

 Dicotyledon, before the cambial divisions begin. 



B. The same at a later stage, when the cambium may be clearly recognized. 



C. Diagram of arrangement of tissues in the root, after secondary thickening has 



been in progress for a considerable time : in such a root the cortex, which is 

 seen as a broad band (c) in Figs. A, B, has been completely thrown off, and 

 the section is now limited by tissue (p) derived from the deeper-lying 

 pericycle. 



c = cortex ; p = pericycle; pr.xy = protoxylem ; pr.phl = protophloem ; m = 

 pith ; cb = cambium ; xy" = secondary xylem ; phi" = secondary phloem ; 

 p.mr = primary medullary rays ; mr" = secondary medullary rays. 



1. There is no pith. 



2. The primary medullary rays opposite the groups 

 of primary xylem are relatively small and inconspicuous. 



3. The xylem is marked off into annual rings, as in 

 the secondary xylem of the stem. 



