CUCURBITA 595 



also have reticulately thickened or pitted walls. (Fig. 311, A.^ 

 Tfie^centrally located metaxylem vessels with the adjacent thick- 

 walled parenchyma may abut the reticulate vessels of the four 

 primary xylem points; or they may be separated from them by 

 two or three layers of vertically elongated thin-walled xylem 

 parenchyma. (Fig. 312., B.) Between the primary xylem and 

 phloem, there is a zone of parenchyma which later gives rise to the 

 cambium. 



As the stele matures, the cortical region is also developing. 

 The inner cortical cells divide periclinally, adding additional 

 layers, and radial divisions occur to compensate for the increased 

 diameter of the stele. Cell divisions cease at about the time that 

 the scalariform elements of the metaxylem develop, and the en- 

 dodermis then forms narrow Casparian strips. 



Lateral Roots. — The lateral and adventitious roots resemble 

 the primary root in structure, but their steles may be triarch or 

 octarch as well as tetrarch. The origin of lateral roots occurs 

 very early in the ontogeny of the primary root when the proto- 

 phloem is differentiating, and prior to the development of the 

 protoxylem. Whiting observed several root tips in which lateral 

 primordia occurred within a millimeter of the apical meristem. 

 There is some disagreement in regard to the tissues involved in the 

 origin of secondary roots in Cucurbita. According to Janczewski 

 (19), in the ontogeny of lateral roots in Cucurbitaceae and Legumi- 

 nosae, the stele of the main root gives rise to the stele of the lateral 

 root; while the endodermis and adjacent cortical cells of the main 

 root form the cortex of the lateral root, at the distal surface of 

 which the meristematic growing point is later developed. Van 

 Tieghem and Douliot (38) rejected Janczewski 's explanation, 

 stating that in C. maxima and C. Pepo, there is definite evidence 

 that the lateral root arises from two pericyclic layers, and that 

 the endodermis and five or six of the inner cortical layers produce 

 a digestive pocket which assists in the emergence and the outward 

 growth of the young root. Whiting's studies tend to confirm 

 the observations made by Janczewski. However, the very early 

 initiation of the primordia of the lateral roots, prior to the matura- 

 tion of the protoxylem and before the endodermal layer of the 

 main root is clearly differentiated, makes it difficult to determine 

 with exactness the location of the tissues from which the primordia 

 arise. 



