Ontogeny, Secretory Process and Senescence of Colleters of Cnidoscolus Pubescens (Euphorbiaceae)

Secretory structures are present in numerous different plant organs. Among the many known secretory structures, colleters are located such that they protect part of plant meristem and developmental tissue. One remarkable characteristic of secretory tissues is the export of exudates; however, the precise mechanism involved in this process is still unclear. To better understand the mechanism of exudate externalization of Cnidoscolus pubescens , it is necessary to understand the ontogenesis, secretory process, and senescence of its petiolar and stipular colleters. Petiolar glands appear at the beginning of leaf development, and they senesce during the initial growth of the leaf blade. Colleters form the epidermis on the inner surface of these cavities and then the cavities open, making the gland appear similar to a pair of stipules. This structure’s outer face and extremities are the first to begin senescence. Stipular colleters are formed by protoderm and fundamental and vascular meristem. These structures have a constriction on their base and exhibit differentiation of a palisade layer only after its development. After differentiation, the cuticle releases from the palisade layer making the colleter flaccid. At the apex, a series of stipules containing colleters, like those of the petiole, overlap with leaf primordia. In stipular colleters, as in the petiolar gland colleters, druse crystals and articulated and anastomosed-type laticifer channels are present. Secretion is composed of polysaccharides, and negative results for other substances show that the principal function of secretion of C. pubescens is protection from dehydration. The secretion is externalized by cuticle disruption. The uniqueness of these petiolar glands and the first description of colleters for Cnidoscolus promise to be of great taxonomic value.