Primary vascular patterns in root meristems of Pontederia cordata and their relevance to studies of root development

Abstract

There are several files of metaxylem cells in root apices of Pontederia cordata L., each considered to consist of a series of prospective vessels with their ends in contact. Two longitudinally adjacent vessels may be in the same file of cells produced by the root apex or in adjacent files. As the root grows, successive prospective vessels are added to the apical ends of most of the files but not all files are continued. Addition of prospective vessels appears to take place within the "quiescent centre" of the root apical meristem. Where files are not continued there is no immediate readjustment of remaining files. The longitudinal and transverse distribution of components of the vascular system (including protophloem and protoxylem) is discussed in relation to the means by which the pattern of development may be controlled. Rates of production of vessels and the final lengths of the vessels are estimated. The observations and deductions are discussed in relation to other studies of root growth, vascular differentiation, and vascular pattern formation and maintenance.

Cortical development in roots of the aquatic plant Pontederia cordata (Pontederiaceae)

“…Roots in members of this family typically have three or four tiers of apical initials: a tier of rootcap initials (or calyptrogen), one or two tiers of ground meristem-protoderm initials (periblem-dermatogen), and a tier of procambium or stelar initials (plerome). This has been confirmed by Charlton (1980), although Lee (1958) argued that a three-tiered meristem with one layer of initials for the ground meristem-protoderm tip was characteristic of the Pontederiaceae, like Eichhornia, and that all other patterns represented nonmedian sections of the apex. In his study of vascular patterns in adventitious roots of P. cordata, Charlton (1980) described the origin and location of primary and secondary raphide cells in the outer portions of the ground meristem.…”

“…This has been confirmed by Charlton (1980), although Lee (1958) argued that a three-tiered meristem with one layer of initials for the ground meristem-protoderm tip was characteristic of the Pontederiaceae, like Eichhornia, and that all other patterns represented nonmedian sections of the apex. In his study of vascular patterns in adventitious roots of P. cordata, Charlton (1980) described the origin and location of primary and secondary raphide cells in the outer portions of the ground meristem. He also found that a quiescent center extended ϳ70 m behind the junction between the rootcap and the ground meristem-protoderm tip.…”

“…Solms. had a single tier for the tip of the cortex and epidermis, but Charlton (1980) illustrated a root meristem of P. cordata with a separate tier for the epidermal initials. As Byrne and Heimsch (1968) found in species of the dicotyledonous Linum, the number of tiers of initials in the apical meristem can vary.…”

“…To date, a comprehensive developmental and structural characterization of the cortex has not been made in any member of the Pontederiaceae. Root cortical structure in some members (Hasman and İnanç, 1957;Charlton, 1975Charlton, , 1980Tomlinson, 1982) has been described, but these studies need to be extended to include relationships among the endodermis, aerenchyma, and hypodermis, and to describe cell wall compositions and cell patterns of cortical regions. Many species, including those in wetlands, are known to possess an exodermis, i.e., a hypodermis with a Casparian band (Perumalla, Peterson, and Enstone, 1990;Peterson and Perumalla, 1990).…”