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.

An analysis of shoot extension growth of Rhizophora mangle is presented, based on the continuous observation of a small population over a period of two years. This provides phenological information for a tree which fits the general category of "evergrowing" although in South Florida the climate is distinctly seasonal. Environment has a strong governing influence, but the general progression of events is mediated endogenously. Growth is morphologically non-articulate, without obvious flushes, so that the unit of growth is regarded as the single internode. The apical bud has a uniform construction throughout the year, consisting of three leaf primordia and associated stipules, the expansion of the oldest pair of leaf primordia coinciding with the initiation of a new pair. Consequently environmental control of growth is effected through developmental processes in the apical bud. The rate of leaf expansion and leaf fall is at a peak in summer when temperatures and radiation levels are highest. Leaf fall is closely correlated with leaf expansion so that there is a relatively constant number of leaves on a shoot. However, there is some evidence that propagule abscission also affects leaf fall. All lateral organs (vegetative branches, inflorescences, and suppressed axillary buds) are developed within the apical bud, the types of organ differentiated at any one node apparently depending on plastochron length. This is most evident with regard to precocious (sylleptic) branches which are produced periodically in groups of from one to three. Suppressed axillary buds are very reduced and subject to apical dominance. When released from this. by pruning for example, they can reach a size which permits them to grow out as normal apical buds. Under continued

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