ALLIUM CEPA 



187 



sheath of the first foliage leaf which surrounds it. In this manner, 

 the successive leaves are produced and emerge in rapid sequence. 

 The stem axis from which the leaves arise elongates very slightly, 

 and this slow growth, together with the development of the basal 



Fig. 86. A, transection through upper portion of primary root of six-day seedling; B, 

 same, at higher level than in A, showing protoxylem, pxs, consistingof a single cell ; C, same, 

 slightly above B, showing actively dividing cells of cortex (stippled) initiating first adven- 

 titious roots ; D, same, at higher level than C, showing second leaf with first procambial 

 strand (stippled) surrounded by cotyledon : co, cortex; s«, endodermis ; ^/», epidermis ; epi, 

 epidermis of cotyledon ; ^/)2, epidermis of second leaf ; wat, metaxylem ; /»f/, pericycle; ph, 

 phloem; />x, protoxylem ; r, root hair. (After Hoffman.) 



portions of the foliage leaves as storage regions, leads to the forma- 

 tion of the bulb. 



The Primary Root. — The primary root has an exarch, radial 

 protostele which is usually diarch, rarely triarch. At the level 

 of the root hair zone, there are two protoxylem points which al- 

 ternate with the two phloem groups and differentiate centripetally. 

 (Fig. 86, A.') The central portion of the stele is at first parenchym- 

 atous; but, later in ontogeny, metaxylem elements are differen- 



