Studies on the Growth of Red Mangrove (Rhizophora mangle L.) 4. The Adult Root System

Aerial roots in Rhizophora arise from stems, branches, and secondarily thickened, anchored, aerial roots, but only after injury from unanchored aerial roots. Lateral subterranean roots are abundantly developed upon penetration of a mud substrate whereupon the histology of the root undergoes marked and abrupt changes so that chlorophyll is no longer developed, trichosclereids no longer differentiate, tannin cells become few, the cortex becomes markedly lacunose, and the sequence of protoxylem differentiation changes. Root systems developed in contrasting environments like water, sand, or mud provide evidence as to the relative importance of light and air in affecting anatomical differentiation. The overall morphology of the root system is discussed in relation to its likely function in aeration of subterranean parts. WE HAVE OUTLINED briefly the general features of the root system of Rhizophora mangle (Gill and Tomlinson 1969), and we now amplify that preliminary statement. In another article in this same series (Gill and Tomlinson 1971a) the distinctive method of growth of aerial roots of red mangrove, which leads to distinctive anatomical features, was described. This present article completes the account of the development of the root system by describing distal anchorage of aerial roots, the overall establishment of the characteristic looping, above-ground root system, changes in the anatomy of roots as they penetrate the substrate, and also discusses the likely physiology of roots in relation to the distribution of gas space within them. An attempt is made to relate these features to the general ecology of the Rhizophora root system in differing environments.