The Root Apex of Malva Sylvestris. I. Structural Development

A B S T RAe T Embryos and primary roots at various growth stages of Malva sylvestris were treated with Hs-thymidine to determine the presence of a quiescent center by autoradiography. The nearly complete absence of nuclear labeling in cells of the quiescent center at all stages of root growth suggested that it was established prior to germination. The quiescent center increased in size during growth, and the cellular changes in apical organization as identified in a related anatomical study occurred outside of its limits. These anatomical changes were caused by the production of rootcap cells by the initials of the secondary columella and outer cortex. Autoradiography confirmed that the secondary columella initials were not quiescent during all growth stages and the outer cortical initials were not quiescent in those growth stages in which the anatomical changes occurred. The concepts of the promeristem and protomeristem are discussed in relation to the quiescent center.

The Root Apex of Malva Sylvestris. Ii. The Quiescent Center

Byrne

Heimsch

1970

Developmental Anatomy in Roots of Ipomoea Purpurea. I. Radicle and Primary Root

Seago

1971

The developmental anatomy of the primary root of Ipomoea purpurea was studied at several growth stages, beginning with the radicle. The radicle is generally composed of three superimposed tiers of initials, which produce the vascular cylinder, cortex, and columella; and a peripheral band of lateral rootcap‐epidermal initials. The radicular cortex contains 16–19 immature laticifers; none of the tissue regions in the radicle contains mature cells. Following germination and during the first 2–3 cm growth of the primary root the apical meristem and its derivative tissues undergo a series of modifications. Root apical diameter decreases as cells in lateral portions of the rootcap elongate; meanwhile, the columella enlarges vertically. The relationship between cortical and columellar initials changes as fewer mitoses occur in the former while the latter remain active. In longer roots the columellar initials are directly in contact with the vascular initials. Cortical size diminishes during early root growth as cortical laticifers and their associated cells cease to be produced by the outer cortical initials and ground meristem. Early procambium, at the level of vascular pattern initiation, decreases in diameter by cellular reorientation, and the vascular cylinder decreases in overall diameter although the tetrarch pattern remains unchanged.

Developmental Anatomy in Roots of Ipomoea Purpurea. Ii. Initiation and Development of Secondary Roots

Seago

1973

In seedlings of Ipomoea purpurea secondary roots are initiated in the primary root pericycle opposite immature protoxylem. Cells derived from immature endodermis, pericycle, and incipient protoxylem and stelar parenchyma contribute to the primordium. The derivatives of the endodermis become a uniseriate covering over the tip and flanks of the primordium and emerged secondary root; the endodermal covering is sloughed off when the lateral root reaches 1–5 mm in length. A series of periclinal and anticlinal divisions in the pericycle and its derivatives gives rise to the main body of the secondary root. The initials for the vascular cylinder, cortex, and rootcap‐epidermis complex are established very early during primordium enlargement. After emergence from the primary root, the cortical initials undergo significant structural modifications related to enlargement of the ground meristem and cortex, and the rootcapepidermal initials are partitioned into columellar initials and lateral rootcapepidermal initials. Procambium diameter increases by periclinal divisions in peripheral sectors. The mature vascular cylinder is comprised of several vascular patterns, ranging from diarch to pentarch, that are probably related ontogenetically. Cells derived from incipient protoxylem and stelar parenchyma cells of the primary root form the vascuar connection between primary and secondary roots.