i54 THE STRUCTURE OF ECONOMIC PLANTS 



sinuous walls, and numerous stomata occur with equal frequency 

 in the upper and lower surfaces. 



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

 protostele; and, at the time of complete maturation of its primary 

 tissues, consists of an epidermis, cortex, and stele. In the apical 

 region, many of the epidermal cells develop as root hairs; while 

 in the transition zone above the piliferous area, the surface 

 is glabrous and cutinized. The cortical region is comprised of 

 several rows (usually three to seven) of parenchymatous cells 

 with large intercellular spaces, the innermost ones being smaller 



and more regular in shape 

 A than those in the peripheral 



rows. (Fig. I2.X.) The cor- 

 tex is limited inwardly by the 

 endodermis, which is single- 

 layered and without inter- 

 pif-par cellular spaces. 



px The development of Cas- 

 parian strips and the later 

 suberization of the walls of 

 the endodermal cells has been 

 investigated by Plaut (15) and 



Fig. 12.1. Transection of primary root at the Ruggeberg (l6), whose find- 



maturation of primary tissues: co, cortex, ■ j^^^^ ^^^^ Confirmed by 



en, endodermis; ep, epidermis; mx, metaxylem; o j 



par, interstitial parenchyma; pel, pericycle; Artschwaget (l). Except for 



/>/-, phloem; /-at, protoxylem. (Redrawn after ^^^ ^^.^j. f^^ millimeters of 

 Artschwager, Jour. Agr. Res.) . 



the root tip, the endodermal 

 cells are characterized by Casparian strips on the radial and 

 end walls which are detectable with the use of stain techniques 

 at about the time that the metaxylem vessels mature. This 

 secondary phase of the endodermis extends axially for approxi- 

 mately 3 cm., at which level the cells are in the tertiary stage 

 of wall development characterized by the deposition of a suberin 

 layer over the entire surface. This occurs later in the cells 

 lying opposite the protoxylem points than in those in other 

 sectors of the endodermis. In the lower hypocotyl, the endo- 

 dermal wall-thickenings are in the secondary phase; and, at 

 its upper limits, no well-defined Casparian strips are laid down. 



The development of the root axis and the differentiation of 

 the primary vascular tissues has been investigated by Lyle (11) 



