THE ANATOMY OF THE ROOT 



43 



a well-organized root with a root cap and partially differentiated 

 stele (Zea and Lactuca). Even in the former instance, in the very 

 early stages of germination, the peripheral cells of the meristem 

 divide rapidly; and, before the emergence of the root tip from the 

 seed coat, a root cap (calyptra) is organized. 



The root cap persists throughout the life of the root. As growth 

 and development proceed, it is reduced outwardly by the abrasion 

 and disintegration of the outer cells as the root penetrates the soil, 

 and is renewed from within by the addition of new cells from the 

 meristem. Thus, it maintains a relatively uniform size. In a 



Fig. 14. A sector of the polyarch stele of the corn root showing the thick-walled endo- 

 dermis with passage cells, the pericycle, protoxylem points with thin-walled phloem elements 

 lying on alternate radii to them, the large metaxylem vessels surrounded by thick-walled 

 connective tissue, and the pith. 



median longisection of a root tip, it appears as a more or less conical 

 cap several layers thick at its apex which decreases in thickness 

 laterally. 



After considerable growth and elongation, the primary root is 

 divisible into several regions, in addition to the root cap and the 

 growing point it surrounds. Directly back of the growing point is 

 a zone known as the region of elongation, in which the newly formed 

 cells are increasing in size, chiefly by the elongation of the com- 

 ponent cells in the axial direction. This region is relatively short 

 and merges insensibly into what may be termed the Zpne of differ- 

 entiation, in which many of the cells may continue to enlarge and 

 some still divide, but the majority begin to develop the special 

 characteristics of the several tissues which constitute the primary 



