494 



THE STRUCTURE OF ECONOMIC PLANTS 



stele becomes much enlarged and forms a single metaxylem vessel 

 which may be separated from the adjacent metaxylem and the 

 protoxylem points by a zone of fundamental parenchyma. In some 

 instances, two central metaxylem vessels are formed, one of which 

 is usually larger than the other. The outermost metaxylem ele- 

 ments have loosely reticulate secondary wall thickenings, while 

 the walls of the centrally located vessel are closely reticulate. 



Lateral roots are formed very early in the ontogeny of the primary 

 root; and, before the protoxylem is completely differentiated, the 

 development of laterals may be initiated from the cells of the 

 pericycle lying directly outside the xylem points. (Fig. X54, B.) 

 Tangential and radial divisions of the pericyclic cells result in the 

 formation of a conical growing point of meristematic cells, and the 

 subsequent development of the lateral root is similar to that de- 

 scribed for the primary root. The laterals are usually tetrarch, 

 occasionally triarch. Pentarch, hexarch, and other types that 

 occur in adventitious roots were not observed in the seedling 

 material investigated. 



Secondary Thickening of the Root. — The lateral or adven- 

 titious roots, which are destined to become fleshy, resemble the 

 primary root in the organization of their primary tissues except 

 that they are frequently pentarch or hexarch rather than tetrarch. 

 Likewise, the ontogeny of the fleshy root, up to the time of matura- 

 tion of the primary tissues, parallels that of the primary axis. 

 There is no question but that the development of the primary 

 xylem is exarch and that the arrangement of the primary xylem 

 and phloem is radial, so that it is inaccurate to interpret the fleshy 

 axis as a stem. Artschwager (i), in commenting upon this point, 

 states that 



"the validity of this assumption . . . becomes untenable when young 

 material is studied. The exarch position of the protoxylem decides, 

 without further argument, in favor of the root-structure theory of 

 these organs." 



At about the time that the primary xylem strand reaches 

 maturity, the parenchymatous zone separating the primary xylem 

 and phloem gives rise to a cambium which is irregular in outline. 

 The first-formed secondary elements, arising from this primary 

 cambium, are laid down in the angles between the primary xylem 

 points and centrad to the primary phloem groups. As these 

 angles are occupied by maturing secondary xylem elements, the 



