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become phloem. In cross sections the primary xylem is seen as the central 

 tissue of most roots and is often more or less star-shaped, although the 

 so-called radii ( ends of longitudinal ridges alternating with troughs ) may 

 vary in number from 2 to 5 or more in different kinds of plants. However, 

 the roots of some plants, especially monocots and herbaceous dicots, 

 may have a central pith. When the central xylem is star-shaped in cross 

 section, the primary phloem tissue usually alternates with the radii of 



,PRIMARY PHLOEM 



\ PRIMARY XYLEM' 



-SECONDARY PHLOEM 



rt--CAMBIUM- 



SECONDARY XYLEM 



Fig. 124. Diagram of relations of primary and secondary, xylem and phloem in a 



young growing root. 



the xylem tissue. A little later a vascular cambium develops from paren- 

 chyma cells between the xylem and phloem ( Fig. 124 ) . 



The outermost cells of the vascular cylinder may be recognizable as 

 pericycle. This is a highly versatile tissue and of great importance, since 

 from it secondary roots and adventitious stems often develop. It mav, 

 through further growth, add to the thickening of the root. Cork cambiums 

 usually develop from the pericycle in young roots. 



The inner layer of the cortex adjoining the pericycle usually becomes 

 more or less recognizable as an endodermis. The thickness of the cortex 

 varies greatly in roots of different kinds of plants. In very young roots 

 it is usually the most conspicuous part. The first cork cambium in the 

 young roots of some plants may develop from cortical parenchyma. 



Among the last tissues to appear in a young root are the primordia 

 of lateral roots. Unlike the primordia of axillary branches of stems, these 

 root primordia develop not from the young epidermal and cortical cells 

 near the apex but from deeper lying tissues (usually the pericycle or, 

 in older roots, the phloem ) at some distance from the apex. Renewal of 

 cell division in scattered groups of cells in the pericycle results in root 

 primordia that grow outward through the cortex and emerge through 

 the epidermis as secondary roots. Lateral roots are said, therefore, to 

 have an endogenous origin, in contrast to the exogenous origin of the 

 stems that grow as axillary branches. Stem branches may also have an 



