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 to the relative importance of light and air in affecting anatomical differentiation. The over-all morphology of the root system is discussed in relation to its likely function in aeration of subterranean parts.