5XZ THE STRUCTURE OF ECONOMIC PLANTS 



year from seedlings are relatively small. There are reported cases 

 in which large potatoes have been produced in one year from seed; 

 but, ordinarily, normal tuber size for a given variety is not attained 

 for about three years. 



The Primary Root. — The young primary root has a diarch 

 radial protostele. (Fig. 7.) The two protoxylem points abut 

 the pericycle directly, and the metaxylem is differentiated cen- 

 tripetally to form a complete primary xylem strand early in the 

 ontogeny of the primary axis. In some cases, a few fundamental 

 parenchymatous cells may occur adjacent to the metaxylem; and, 

 in others, there may be a small parenchymatous pith region. 

 The first protoxylem elements to differentiate form annular or 

 loosely spiral thickenings while the later ones have scalariform 

 or scalariform-reticulate walls. 



The primary phloem is radially arranged with respect to the 

 protoxylem points, and the protophloem differentiates adjacent 

 to the pericycle. Between the primary xylem strand and the two 

 phloem groups is a zone of fundamental parenchyma in which the 

 cambium is later differentiated. The stele is surrounded by an 

 endodermis which exhibits well-developed Casparian thickenings 

 by the time primary maturation has been completed. The cortex 

 consists of several rows of large parenchymatous cells with inter- 

 cellular spaces, and the cortical cells are limited externally by 

 an epidermis which bears root hairs. 



Origin of Lateral Roots. — Lateral roots arise early in the 

 ontogeny of the primary root, and this is also true of laterals 

 developed from the adventitious roots. They originate before 

 or at about the time that the maturation of the primary xylem 

 has been completed and just prior to the initiation of cambial 

 activity. The first evidence of lateral root formation appears in 

 the radial enlargement of the pericyclic cells which lie directly 

 outside the protoxylem points or slightly to one side of them. 

 This is followed by tangential divisions of the pericyclic cells, 

 and further divisions in three planes result in the formation of the 

 conical growing point of the lateral root. This growth forces 

 the endodermis outward, and it finally ruptures, as do the cortical 

 cells in the line of lateral root extension. The emergence of the 

 root is accomplished in part by mechanical pressure and in part by 

 the digestion of the cortical parenchyma. By the time the lateral 

 root has elongated so as to reach the periphery of the cortex, a 



