IPOMOEA BATATUS 495 



lateral extension of the cambium from the points of initial activity 

 ultimately involves the pericyclic cells which lie outside the pri- 

 mary xylem points. This results in the formation of a complete 

 cambial cylinder which becomes more and more regular in outline 

 until it is finally symmetrical. 



The relation of the activity of the centrally located parenchyma 

 to the differentiation of the primary xylem elements has been 

 differently interpreted by McCormick (10) and Artschwager (i). 

 The former states that "in the young root there is usually a solid 

 arrangement of vessels with no thin-walled parenchyma between 

 them"; and when a root starts to thicken, parenchyma increases in 

 amount and separates the protoxylem points from the centrally 

 located metaxylem until each primary xylem point consists of a 

 separate strand of one to several vessels. Artschwager, on the 

 other hand, finds that parenchymatous activity "always precedes 

 the differentiation of xylem on the inner face of the protoxylem 

 groups." The activity of the primary cambium and the conse- 

 quent enlargement of the axis results in a tangential stretching of 

 the endodermal cells, and the parenchymatous cells of the cortex 

 increase in number for a time, keeping pace with the growth of the 

 root. Later, the intercellular spaces become larger, the tissue of 

 the cortex disintegrates, and the epidermal cells are ruptured. 



(Fig. 155.) 



Tertiary Thickening of the Root. — Up to this point, the 

 development of the axis follows the usual sequence found in fleshy 

 roots which thicken secondarily; but at this time, certain anoma- 

 lous developments occur which complicate an interpretation of the 

 ontogeny unless successive stages are considered in order. The 

 first of these is the formation of secondary cambiums which origi- 

 nate in the parenchyma of the central portion of the axis surround- 

 ing each of the protoxylem points. (Fig. X56.) This is followed 

 by the origin of additional secondary cambiums which may develop 

 extensively throughout the central parenchymatous zone of the 

 axis. These cambiums frequently occur as cylinders which sur- 

 round groups of secondary xylem elements that have matured 

 centrad to the primary cambium. (Fig. i56.) Coincident with 

 the formation of the secondary cambiums, an active meriste- 

 matic zone forms around the central metaxylem vessel, and this 

 may increase in width until a region of considerable size is 

 formed. 



