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

Aerial roots in Rhizophora rise 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. Since the aerial roots of Rhizophora re one of the most striking features of the genus, they have occasioned frequent comment, as early as 305 B.C. by Theophrastus (Bowman 1917). Scientific investigations in more recent times have included those of Warming (1883), Karsten (1891), Schenck (1889), and Boergesen and Paulsen (1900). Most interest has centered on the likely function of aerial roots in the aeration of the attached subterranean roots which are typically anchored in anaerobic soils (Jost 1887, Karsten 1891). Direct demonstration f this role in aeration remains to be done, as indeed it does with the root systems of most trees. In fact, the aerating function has remained difficult o demonstrate convincingly (e.g., Kramer et al. 1952 for Taxodium). Evidence is largely circumstantial, but the most recent comprehensive investigation by Scholander et al. (1955), using Rhizophora nd Avicennia, seems conclusive. Mention should be made, however, of the interpretation byTroll and Dragendorff (1931) of the root system of Sonneratia caseolaris as a mechanism for adjusting to soil accretion and not as an aerating system. In our present study we are concerned more with the dynamic morphology of the root system than with its function, but we emphasize that a detailed knowledge of structure must precede any discussion of physiology. Further, the root system of Rhizophora provides one among many examples of aerial root systems in tropical plants in which there are striking morphogenetic responses to abruptly changing environments (Gill and Tomlinson 1975). Our present-study includes little experimental work; we have largely relied upon natural variation of roots in different substrates to provide evidence for causal responses. This information has provided insight into the ecological adaptability of the Rhizophora root system, since so much previous description has referred to an invariate root environment which is tide-washed and has a soft, anaerobic substrate. In South Florida, at least, Rhizophora habitats are often varied, and the response of root systems to this variation must be considered along with aeration. BIOTROPICA 9(3) 145-155 1977 145