THE ANATOMY OF THE ROOT 51 



and dispersion of the primary xylem elements so that the original 

 configuration of the protostele does not persist in the mature root 

 (Pastinaca). (Fig. 19.) 



In addition to the usual methods of secondary thickening out- 

 lined above, certain anomalous cases occur in which the process is 

 modified by the supplementary activity of both primary and second- 

 ary tissues. Among economic plants, this occurs in Beta and Ipo- 

 moea. In the former, further thickening is accomplished by the 

 differentiation of secondary cambiums. These arise in rapid suc- 

 cession in the pericycle and phloem parenchyma, producing a series 

 of concentric rings of vascular and parenchymatous tissue which lie 

 outside the secondary xylem and phloem tissues formed by the 

 primary cambium. (Chap. IX.) In Ipomoea, secondary thicken- 

 ing involves the formation of secondary cambiums which surround 

 the protoxylem points. These may develop throughout the 

 central portion of the axis, forming numerous zones adjacent to the 

 secondary xylem elements or even occasionally in the secondary 

 phloem. (Chap. XVI.) 



Lateral Root Formation. — In angiosperms, lateral roots usu- 

 ally arise in the pericyclic tissue at loci directly outside the proto- 

 xylem points although there are exceptions to this generalization. 

 They are differentiated early in the ontogeny of the axis, usually 

 prior to the initiation of secondary thickening. The first evidence 

 of lateral root formation consists in the radial enlargement of two 

 to several pericyclic cells adjacent to a protoxylem point. This is 

 followed by tangential and radial divisions of the lateral root 

 initials, and a conical growing point is formed. As a result of 

 further growth, the lateral root primordium enlarges and elongates, 

 forcing the endodermis and overlying cortical tissues forward so 

 that eventually they are ruptured or laterally displaced. 



In some roots, as in many cucurbits, legumes, and flax, the endo- 

 dermal cells adjacent to the active pericyclic cells also enter into 

 the early stages of lateral root formation, and may contribute to the 

 formation of the peripheral cells of the newly formed growing 

 point. In flax, the endodermis forms a single layer of meristematic 

 cells overlying the root cap, and this remains active until the lateral 

 root has pushed through the cortex of the primary root. In such 

 cases, the pericyclic region differentiates the stelar portion of the 

 lateral axis, and the endodermis and adjacent cortical cells of the 

 primary root form the outer zones of the root primordium. A 



