MORPHOLOGY OF HIGHER PLANTS. 



345 



strand of the former is concentric, wliereas in the latter it is 

 radial. The primary structure of Dicotyledonous roots is much 

 the same as in Monocotyledonous roots. The primary structures 

 of Dicotyledonous stems resemble the primary structure in Dicoty- 

 ledonous roots except that the fibrovascular bundles of the former 

 are of the collateral type. The secondary structures of both roots 

 and stems of Dicotyledons are oractically alike. The characteristics 



Fig. 199- Section of a four-year-old stem of a pine cut in winter; q, view in trans 

 verse section; 1, radial-longitudinal section; t, tangential-longitudinal section; f, spring 

 wood; s, fall wood; m, pith; i, 2, 3, 4, successive years' rings of growth in which is shown 

 the dividing line; ms, medullary rays in transverse section; ms^, ms'', medullary rays 

 in radial-longitudinal section; ms^, medullary rays in tangential-longitudinal sectionr 

 c. cambium; b, bast; h, resin-canals; br, bork. — After Strasburger, 



distinguishing the primary and secondary structures of Dicotyle- 

 donous stems may be summarized as follows : 



Primary Structures. — Epidermis, hypodermis, primary cor- 

 tex, endodermis, pericambium or pericycle, stele consisting of 

 collateral fibrovascular bundles and pith. 



Secondary Structures. — Periderm derived from phellogen ; 

 secondary cortex, consisting of parenchyma and occasionally 

 stone cells or secretory cells or vessels ; phloem consisting of 

 sieve, accompanying cells and sometimes bast fibers ; cambium in 



