Colleters are multicellular secretory structures found on various organs in flowering plants. Colleters on the
adaxial sides of stipules have been hypothesized to play a role in protecting the developing shoot. Rhizophoraceae
is a stipulate family with a broad distribution from mangrove to montane environments, which makes the family
well suited for the examination of this hypothesis, but the colleters of Rhizophoraceae are not well known. We
compared species from all three tribes of Rhizophoraceae, including five inland genera and all four mangrove
genera. In all species, several to hundreds of colleters, sessile or stalked, arranged in rows aggregated in
genus-specific shapes, are found at the adaxial bases of open and closed stipules. Pellacalyx uniquely has additional
colleters at the stipule margins. Colleters are all of the standard type, comprising a central axis of core parenchyma
with large vacuoles and tannins, and an outer palisade-like epidermis with organelles involved in secretory activity.
An exception is Pellacalyx axillaris, in which colleters appear as extremely small epidermal protrusions. Kandelia
obovata has a tracheary element in some colleters. Pellacalyx uniquely has an unusual fleshy outgrowth on the
adaxial stipule base. We propose an evolutionary sequence in which Macarisia has plesiomorphic stipule and
colleter traits and the mangrove Kandelia obovata with colleter vascular traces is most derived. Colleter and
stipule structures are largely concordant with habitat and phylogeny, and show taxonomic value. The strong
alignment of colleter and stipule patterns with habitat is suggestive that colleters have a protective function,
although some components of these patterns may be phylogenetically determined