Beyond leaf projections: the multifunctionality of leaf teeth in Violaceae

The diverse morphology of leaf teeth, including the presence or absence of glands, is a taxonomic attribute. However, the adaptive advantages of these projections remain unclear, and the nature of the glands associated with the margins is commonly disregarded. We selected two species of Violaceae (Pombalia atropurpurea and Anchietea pyrifolia) to describe the morphoanatomy and micromorphology of the leaf teeth and to unravel the nature of the associated glands. Leaves at different developmental stages were collected, fixed, and processed according to standard light and scanning electron microscopy techniques (including SEM-EDS). The guttation process was conducted on young plants of P. atropurpurea. Both species show vascularized teeth bearing a colleter in the apical portion and a hydathode with evident modified stomata, with guttation observed. The colleters are of the standard type and secrete mucilage and proteins in the initial stages of leaf development. In expanded leaves, the colleters are completely collapsed and the hydathodes are active. Crystals occurred along the veins, including the vasculature that irrigates the teeth. These crystals were extravasated through the modified stomata and colleters, a function previously unknown to this gland. We suggest that the leaf teeth of Violaceae perform multiple functions in the plant body, such as (i) protection against desiccation, through the sticky secretion of colleters; (ii) guttation, through modified stomata, and (iii) elimination of excess calcium in the form of crystals.