74 



THE PLANT CELL. 



stems and roots may be protective in nature, since the outer 

 walls are often very thick (see Figs. 55, 56). The endodermal 

 cells do not always contain starch, as the latter is being constantly 

 used up by the cambium, &c. 



ii. The pericycle is a meristematic zone of cells occurring in 

 the root, just internal to the endodermis ; it may be several cells 

 in thickness, and from it are produced, (a) the secondary or 

 lateral roots, (6) a ring of cork, and (c) the pericyclic fibres; 

 the latter being elongated thick-walled sclerenchymatous fibres, 

 not unlike bast-fibres, and often taking the place of these. The 

 pericycle may consist of one layer of cells only, and may even be 

 absent altogether in some roots, and at times it may arise from 

 the outermost cells of the central cylinder. 



iii. The medulla or pith is formed in stems and roots of more 

 than one year's growth by the remains of the central groimd- 



Fig. 57. — YouxG Cells of the RroiMENTARY Pith or Lupulus hurmilns.— 

 a, Cell-wall ; p, cytoplasm ; n, nucleus. 



tissue of the younger stem. Near the apex of the shoot the 

 cells of the centrally situated portion of the ground-tissue are 

 young undifferentiated elements possessing protoplasmic contents 

 (see Fig. 57). Older cells of the medulla are, except in some 

 succulent herbaceous stems, devoid of protoplasm, and contain 

 only air; and their walls are at times very thick, and per- 

 forated by numerous simple pits (see Figs. 58, 59). A 

 medullary sheath formed of somewhat rectangular thick-walled 

 elements, in the walls of which either simple or small bordered- 

 pits are sometimes seen, may occasionally be present. This so- 

 called sheath is only a slightly modified layer of the outermost 

 cells of the medulla. 



