Conceptual Overview

At completion of primary growth (i.e. growth in length), the secondary growth (i.e. growth in thickness) is initiated in roots due to the activity of the vascular cambium. Secondary growth includes the production of secondary phloem and secondary xylem by this lateral meristem. Roots with secondary growth often become woody due to the accumulation of secondary xylem. If we examine older thick roots of trees, we observe annual growth rings in the wood (secondary xylem) resembling those of stems.

The vascular cambium appears first on the inner edges of the phloem strands. While these cambial cells form some secondary elements, the pericyclic cells outside the protoxylem poles also divide. The inner derivatives of these divisions complete the cylinder of cambium by joining the strips located on the inner faces of the phloem strands. The vascular cambium loses its wavy circular outline in transverse sections because on the inner boundary of the phloem, the secondary xylem is deposited earlier than outside the protoxylem. The secondary vascular tissues assume the form of a continuous cylinder and complete enclose the primary xylem. The sieve elements of the primary phloem are crushed, and some of the remaining cells differentiate into fibers. The cambium which arises in the pericycle outside the xylem poles forms wide vascular rays. In a few species, the phloem remains internal to the xylem.

The initiation of phellogen and periderm follows the initiation of vascular cambium and secondary vascular tissues. Phellogen (cork cambium) originates from the pericycle cells located opposite primary phloem. The primary tissues located outside the pericycle (rhizodermis, exodermis, cortex and endodermis) become isolated from the secondary body of the root by periderm formation and die.

The secondary phloem in roots consists of axial elements such as sieve tubes with companion cells, phloem parenchyma cells and phloem fibers, and radial elements, i.e. ray cells. The axial elements of secondary xylem which are deposited by fusiform initials include wood parenchyma, fibers and vessels of various widths with bordered pits arranged in reticulate or scalariform patterns in their lateral walls. The rays are usually wide in herbaceous dicots and as narrow as in stems in woody life-forms. The root secondary xylem typically contains more parenchyma elements than the stem wood.

Periclinal divisions in the pericycle that are not involved in the formation of the vascular cambium occur not only outside the xylem poles,but spread around the circumference of the root. Such divisions are preparatory for the formation of the periderm. A phellogen arises among the outer cells of the proliferated pericycle which forms cork tissue to the outside and phelloderm towards the inside. The phelloderm is difficult to distinguish from the parenchyma derived from the pericycle.

Some dicotyledonous roots may retain their cortex for a period of time during secondary development. Such roots may develop an exodermis or a superficial periderm, and the cortex later becomes lost.

In some cases, the endodermis of the parent root takes part with the pericycle in forming branch roots. Sometimes it forms several cell layers by undergoing both anticlinal and periclinal divisions. In a few cases (e.g. Cucurbitaceae and certain water plants), the innermost cortical cell layers are also involved in the formation of the lateral root by contributing cells to the lateral primordium.

Adventitious roots may be formed in young organs or in older tissues that have not quite lost their meristematic properties. Most adventitious roots arise endogenously, sometimes from primordia laid down previously and remaining dormant until stimulated to growth. Usually the adventitious roots arise from the pericycle, but may also arise from the cambial zone.

Roots in both primary and secondary states of growth may undergo modifications in their structure due to the acquisition of special functions. In this sense, storage, aerial, mycorrhizal and contractile roots as well as roots modified by the interaction with parasitic plants are specially considered in this unit of study.

Many roots may form a symbiotic association with fungi termed mycorrhizae. In ectomycorrhizae, the fungal hyphae grow as a mantle around root endings and between the cells of the rhizodermis and cortex, while in endomycorrhizae the hyphae invade the root cells and the mantle is poorly developed.