Botany · Ch 10 — Secondary Growth
Secondary Growth in Dicot Root
Secondary Growth in Dicot Root
Roots undergo secondary growth for essentially the same reason stems do - the aerial parts of the plant need a stronger, thicker anchoring and conducting base as the plant grows - and the overall process (a vascular cambium producing wood and bast, followed by a cork cambium producing periderm) closely parallels secondary growth in the stem. Where root secondary growth differs sharply from stem secondary growth is in how the vascular cambium itself first forms. In the stem, part of the cambial ring (the intrafascicular portion) is already primary in origin, present inside each vascular bundle from early development. In the root, by contrast, the vascular cambium is completely secondary in origin from the start: it arises from a combination of conjunctive tissue lying just below the phloem strands and a portion of the pericycle lying above the protoxylem points, which together form a continuous but initially wavy ring …
What this figure shows. Five diagrammatic transverse sections (a-e) tracing dicot root secondary growth from the primary structure through to the fully secondary condition. The labelled tissues named across the panels include Epidermis, Endodermis, Pericycle, Primary xylem and Primary phloem (the starting primary structure); Cambium and Cambial ring forming from conjunctive tissue below the phloem bundles and part of the pericycle above the protoxylem, first wavy/discontinuous and then becoming circular; and, in the later panels, Secondary xylem, Secondary phloem, Xylem ray, Phloem ray and the Phellogen (cork cambium) that eventually replaces the epidermis. NOTE ON RECOVERY: the exact tissue layout within each individual lettered panel could not be reliably reconstructed from the extracted text, since the panel labels are listed together rather than grouped per-panel in the source; the overall progression described here is accurate, but the precise per-panel detail should …