The Development, Structure and Function of Dendroid Colleters in Psychotria kirkii Hiern (Rubiaceae)

The dendroid colleters of the leaf nodulated Rubiaceous shrubPsychotria kirkii Hiern. have been studied with respect to theirdevelopment, structure and function. The colleters, which arisefrom the adaxial surface of stipules of apical and lateral shoots,secrete a protein/carbohydrate mucilaginous substance in whichis maintained a colony of leaf nodule bacteria. The colletersare multicellular and multiseriate, consisting of a two to fourcell thick stalk from which radiate up to 70 elongate secretorybranch cells. Cuticle envelops both stalk and branch cells inearly developmental stages and as secretory activity increasesthis cuticle is largely lost in two different ways. The majorpart is forced off the branch cell surface through the passageof a largely carbohydrate component of the mucilage which surroundsthe colleters: a second method of cuticle loss involves thepassage from the cell of small electron-translucent bodies whichbecome coated with cuticle as they exit the cell. The mucilagein which the bacterial cells are found provides a vehicle wherebythe bacteria are able to enter the leaves, thus leading to theinitiation of leaf nodules. Psychotria kirkii Hiern., secretory dendroid colleters, symbiosis, ultrastructure, trichome development