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Tree crown growth reflects many aspects of the species’ life history. It was postulated that the potential for a pioneer species to invade a deforested area, or maintain a viable population in a forest under intermediate disturbance regime, is related to the tree trunk growth rates, patterns of crown construction, as well as the ability to regenerate broken branches. We investigated if Mabea fistulifera Mart. (Euphorbiaceae) crown architecture and tree allometry varies in two contrasted environments in a secondary fragment of Atlantic rainforest: forest edge and interior. First, we described the development of crown architecture by following the growth of the crown in 15 young individuals, with special attention to growth and branching models and to the morphologic differentiation of axes. Secondly, allometric differences in crown construction patterns between environments were studied by quantifying 10 traits in 20 individuals from each environment. Nozeran architectural model was found, a process of parenchymatization of the terminal meristem and a tier of branches form distally followed by an orthotropic shoot forming the next relay axis. This results in modular sympodial branching and polyaxial morphology of branches, allowing this tree species to occupy different environments by quick expansion wherever light availability is higher, towards the upper canopy (forest interior) or towards one side (forest edge). Crown asymmetry occurred towards border sunlit spaces or in response to inner canopy openings caused by hilly topography. Crown structure and growth patterns here found are suitable for a pioneer species adapted to intermediate disturbance regime.

Mabea is an important neotropical genus of the Euphorbiaceae, whose secretory structure diversity is still little known. Mabea fistulifera Mart., one of the most representative species in the genus, is a pioneer plant that is widely distributed across Brazil and Bolivia. We investigated the distribution, morphology, histochemistry and ultrastructure of colleters, reported for the first time to M. fistulifera, aiming to elucidate the gland functions, exudate nature and secretory activity dynamics. Samples of vegetative and reproductive apices and of leaves at different developmental stages were processed following usual procedures for analysis in light, scanning and transmission electron microscopies. Colleters are present exclusively on leaves, occurring on the adaxial surface and apical region of stipules and on the leaf blade margin. Colleters have protodermal origin and are functional since early leaf differentiation. In the colleter secretory portion, central-axis cells and surface cells have similar origin and structure, both showing numerous mitochondria, dictyosomes, rough endoplasmic reticulum and ribosomes. The translucid secretion is of mucilaginous nature, being formed by polysaccharides, proteins and alkaloids, which is compatible with the observed cellular machinery. The presence of a hygroscopic exudate in contact with developing vegetative and reproductive organs is important for the protection of young tissues against desiccation, especially in view of the fact that the species occurs in open environments, which are subjected to water stress, elevated temperature and high light incidence.