﻿MCCORMICK— I POMOEA BATATAS 



and was quite green, had an excessive amount of secondary 

 xylem compactly arranged around primary xylem. The structure, 

 such as shown in fig. 2, extends in this particular root about 6 cm. 

 beyond the place of attachment of the root of the stem, but within 

 a short distance from this point 

 thin-walled parenchyma between 

 the vessels becomes active 

 (fig. 3). The parenchyma rapidly 

 increases in amount and sepa- 

 rates the xylem more and more 

 into strands, consisting of one 

 to several vessels in a strand 

 (fig. 4). Around each strand 

 the parenchyma is organized 

 into concentric layers of cam- 

 bium. These secondary cam- 

 biums are capable of forming 

 both xylem and phloem, though 

 become wholly 



primary cambium has not 1 

 inactive, but throughout the 

 growth of the root continues 



ing f 



red 



rids of 



1 and phloem, each of which 

 rn becomes surrounded by / 

 Tyloses are common / 

 and often the vessels are com- 

 pletely filled with them. 



Phloem. — The phloem is rich 

 in sieve tubes with prominent 

 companion cells. The sieve plates 

 are horizontal (fig. 5) or sharply 

 inclined, so that in transverse 

 sections they may be readily over- 

 looked (fig. 6). Interxylary 

 phloem appears very soon after the thin- walled parenchyi 

 becomes active between the vessels, and apparently it may 

 formed before a definite cambium is organized (fig. 5). Even 



