ZEA MAYS 



III 



, rt hr 



exarch 



until after that of the protoxylem. As they mature, the vessel 

 segments become somewhat longer, the end walls are resorbed, and 

 the secondary thickening of the lateral walls results in a narrowly 

 reticulate or pitted type. The phloem elements are elongated and 

 thin-walled. 



The number of protoxylem points is variable, ranging from xo to 

 40, and there are commonly two or three protoxylem strands to 

 each large metaxylem vessel. 

 (Fig. 47.) The groups of 

 primary phloem are on radii 

 alternate with the protoxylem 

 and abut the pericycle. The 

 parenchymatous cells which 

 lie between the xylem points 

 and surround the phloem be- 

 come thickened and lignified, 

 forming a continuous zone of 

 thick-walled connective tis- 

 sue as the root matures. The 

 pericycle consists of a single 

 layer of thin-walled cells 

 which may become thick- 

 walled later in ontogeny; and 

 lateral roots originate in this 

 region. The endodermis is „ _ . , r ,. • 



o Fig. 47. Transection or a sector or the pri- 



alsO a single layer; and, at maryroot: «, cortex; g«, endodermis; ep,epideT- 



maturity its cells have the U- "^'^' ""^^ metaxylem; pd, pericycle; ph, phloem; 



•^ ' r 1 • 1 • • ?'' P'"^hi P^' protoxylem; rt hr, root hair. 



shaped type of thickening in (After Avery.) 

 which the end, radial, and 



inner tangential walls are reinforced. The passage cells have no 

 secondary thickening except the Casparian strip, and they usually 

 occur on the same radii as the protoxylem points. (Fig. 14.) The 

 cortex is comprised of several layers of parenchymatous cells 

 bounded externally by a more or less ephemeral epidermis. It pro- 

 duces an abundance of root hairs immediately back of the zone of 

 root elongation; but disintegrates later, and the outer cells of the 

 cortex form a lignified and suberized exodermis. 



The ontogeny of the seminal roots arising above the scutellar 

 node, and of the adventitious roots that develop later in the basal 

 intercalary meristems of higher internodes, is essentially like that 



