LACTUCA SATIVA 639 



six to eight layers of parenchymatous cells that are round in 

 transection and two or three times as long as broad, with regular 

 intercellular spaces. 



The third histogen, the dermatogen-calyptrogen, gives rise to 

 both epidermis and root cap. The manner of its division has been 

 concisely described by Port, 



"The cells of this layer by periclinal divisions perpetuate the histogen 

 and form successive layers of root cap cells which may subsequently 

 divide in one or more planes to keep pace with the growth of the 

 root axis. The cells which form the proximal margin of the dermat- 

 ogen-calyptrogen layer undergo a final periclinal division and each 

 inner daughter cell then functions as an epidermal initial thereafter 

 dividing only in anticlinal planes to form the epidermis. In this 

 manner a uniseriate epidermis develops and the initiating layer is 

 maintained." 



At maturity, the primary root has a diarch radial protostele 

 in which the two primary phloem groups lie on the flanks of the 

 primary xylem strand and are separated from it by fundamental 

 parenchyma which later functions as a procambial zone. The 

 protoxylem points abut the pericycle which is surrounded by the 

 uniseriate or partially biseriate endodermis. The cortical paren- 

 chyma is limited externally by an epidermis and there are numerous 

 root hairs. (Fig. 335.) 



Secondary Thickening. — Secondary thickening of the root is 

 initiated very early in ontogeny, in the zone of fundamental paren- 

 chyma lying between the primary phloem and xylem, and the 

 production of secondary vascular tissues may occur in seedlings 

 approximately eight days old. The secondary xylem consists of 

 parenchyma and large vessels that are laid down in single or 

 frequently double radial rows. The rows are separated by ray 

 parenchyma and the diarch character of the primary xylem is 

 evident even in older roots because of the development of two 

 prominent pericyclic rays that extend toward the periphery on the 

 same radii as the two protoxylem points. The vessel segments are 

 three to four times as long as broad, the walls being densely pitted 

 with oval or elliptical pits that have their longest dimension in 

 the transverse plane. The elongated xylem parenchyma cells 

 surrounding the vessels are at first thin-walled, but become lignified 

 in mature roots. The ray parenchyma cells are isodiametric or 

 sometimes elongated in the radial dimension. 



