Rhizophoraceae

This family is now placed in the order Malpighiales, though under the Cronquist system, they formed an order in themselves (Rhizophorales). It is sister group to Erythroxylaceae. The remaining mangrove genera are restricted to the IWP region. == Genera ==

, Plants of the World Online accepted these genera: • Anopyxis • Blepharistemma • Bruguiera • Carallia • Cassipourea • Ceriops • Comiphyton • Crossostylis • Gynotroches • Kandelia • Macarisia • Paradrypetes • Pellacalyx • Rhizophora • Sterigmapetalum == Morphological Characteristics ==

The tribe Macarisieae is characterized by a few plesiomorphies unknown in the rest of the family, such as superior ovary position, the presence of a seed appendage, and the absence of aerial roots. Within Gynotrocheae, Crossostylis is morphologically distinct from other Gynotrocheae in having capsular fruits that split open at maturity and an appendage on a mature seed. Vivipary: The embryo of Rhizophoreae starts germination without dormancy. These wood structures keep the xylem sap in high tension to absorb water, resulting in a high sodium chloride concentration and high osmotic potential. Underground roots, like all plant tissues, require oxygen for respiration. In underground soils of terrestrial habitats, gas exchanges take place at the interstitial pores among the soil particles. In waterlogged soils, the diffusion rate of oxygen is extremely low. Rhizophoreae adapts to the anaerobic soils by having extensive roots above the ground which increases the surface area for gas exchanges. The surface of aerial roots carry numerous gas exchange pores called lenticels, through which oxygen could diffuse into the underground tissues with air-filled spaces. == Evolutionary history ==

The ancestor of Rhizophoraceae experienced two whole genome duplication events. The first duplication event corresponds to the triplication shared among angiosperms. The second duplication event was dated to ~74.6 million years ago (mya). Around 66 mya, the planet underwent the Cretaceous–Tertiary mass extinction. Then around 56.4 mya, the mangrove lineage diverged from its terrestrial relatives. The divergence happened to occur in the time frame with in the extreme global warming event, the Paleocene-Eocene Thermal Maximum (PETM). During this time period, there is a shift from a terrestrial to a marine, potentially anoxic, sedimentary depositional environment, suggesting a sea level rise. After the dramatic global warming period, the mangrove species within Rhizophoraceae diversified within 10 mya, which is relatively short in evolutionary sense. Although the sequence of the events does not suggest an absolute causal relationships between the former and the latter, a reasonable hypothesis for the diversification of Rhizophoraceae could be formulated: The second event of whole genome duplication increased the adaptability of the ancestor of Rhizophoraceae and chances of survival during the Cretaceous-Tertiary mass extinction by generating novel genetic materials for evolution to work on. During the PETM global warming period, the terrestrial ancestors of Rhizophoraceae living close to the shore were forced into the intertidal zone because of a large-scale sea-level rise. This sea level change exerted some selective pressure on the ancestors of Rhizophoraceae and those that were successfully adapted to the intertidal zone diverged from their terrestrial relatives and colonized this new habitat. Eventually, differential habitats within the intertidal zone resulted in the speciation within the mangrove lineage of Rhizophoraceae. ==References==