158 THE STRUCTURE OF ECONOMIC PLANTS 



parenchymatous tissue with small intercellular spaces at the 

 angles of the cells. The endodermis is thin-walled, except for 

 the Casparian strips; but, later in ontogeny, the radial, end, and 

 inner tangential walls become much thickened, the latter ones 

 appearing to be stratified. 



The pericycle is single-layered, with cells of almost uniform 

 size and dimension, except outside the protoxylem points, where 

 they are smaller. As the root matures, and the cortical cells 

 disintegrate, the walls of the pericyclic cells may become some- 

 what lignified and sclerotic. The protoxylem consists of a few 

 small annular or spiral vessels; and centrad to them is a single 

 large pitted metaxylem element. Alternate with the protoxylem 

 strands are small groups of phloem cells, usually two to four in 

 number; and between the phloem and xylem are fundamental 

 parenchyma cells. When the epidermis and cortical tissues are 

 finally shed, the stelar tissues remain functional, being protected 

 by the thick-walled endodermal and pericyclic cells, as well as by 

 the fundamental parenchyma, which also becomes thick-walled. 

 The character of the phloem and xylem cells remains unchanged. 



The ontogeny of the root is similar to that of corn, barley 

 and other grasses in which the root meristem is differentiated into 

 three histogens. The outermost one is the calyptrogen, from 

 which the root cap is differentiated by successive periclinal divi- 

 sions; and, since the peripheral layers of the root cap are succes- 

 sively abraded, it never becomes very massive. (Fig. 68.) The 

 plerome gives rise to the stelar structures; and, between the 

 plerome and calyptrogen, a single layer of meristematic cells 

 produces the cortical and epidermal tissues. Periclinal divisions 

 of the cells of this intermediate layer produce daughter cells, the 

 outermost ones dividing only anticlinally to form the epidermis; 

 while the inner daughter cells, by subsequent divisions in all 

 planes, give rise to the cortical parenchyma and endodermis. 

 The other seminal roots resemble the primary one in the essential 

 details of structure and ontogeny. 



The origin of lateral roots is pericyclic, occurring in sectors 

 of this layer directly outside of the phloem groups and between 

 two adjacent protoxylem strands. The pericycle becomes multi- 

 layered by successive periclinal divisions, a conical mass of meri- 

 stematic cells is formed, and the histogens develop which give 

 rise to tissues as described for the primary root. 



