Secretory structures on the leaf rachis of Caesalpinieae and Mimosoideae (Leguminosae): Implications for the evolution of nectary glands

Abstract

Cup- or sometimes slit-shaped nectary glands on the rachis are a widespread trait in the legume subfamily Mimosoideae, especially in derived tribes. Their spotty occurrence in genera that appear to be basal has led to uncertainty about when in the mimosoid radiation this character evolved. Until now, specialized rachis glands were unknown in caesalpinioids thought to be related to ancestral mimosoids. We report here the occurrence of rachis glands in seven of the ten species of the Paleotropical genus Erythrophleum, a member of the Dimorphandra group of caesalpinioids thought to include the sister group(s) of mimosoids. The histological structure and location of Erythrophleum glands suggest homology with those of mimosoids; these glands are simpler structurally than rachis glands of any known mimosoid. The Erythrophleum glands differ from those of most mimosoids in the following respects: (1) they are smaller than glands of mimosoids; (2) the secretory surface is sunken in a pit capped by a small round pore rather than exposed on a broad concave or flat surface; (3) a smaller number of cells are involved in production and secretion of nectar; (4) vascular supply to the nectary is less extensive; and (5) mechanical support tissue (sclerenchyma) is less extensive and less organized. Rachis glands appear to be absent in the nine other genera included in the Dimorphandra group. We also report the occurrence of other secretory structures (patches of glandular trichomes) on the rachis of some Caesalpinieae and Mimoseae that lack specialized nectary glands and suggest that these patches of trichomes are primitive homologues of more organized glands. We discuss the significance of these glands and of the patches of trichomes for understanding relationships among primitive mimosoids and related caesalpinioids, and for understanding the origin of ant-guard defenses typical of many mimosoids.