Colleters in Casearia (Salicaceae): a new interpretation for the theoid teeth

Salicaceae possess a range of leaf teeth types, many characterized as nectariferous salicoid or violoid teeth. One large genus, Casearia , deviates from this generalization in having theoid teeth. Although taxonomic descriptions of Casearia have included reports of glands at the apex of teeth, there have been no descriptions of their anatomy or functional role. Here, we aim to describe the anatomy of the theoid teeth of 43 Casearia spp. Leaf samples from herbarium specimens were processed for light and scanning electron microscopy. Leaves of C. sylvestris were collected and fixed in the field for histochemical tests, revealing only polysaccharides in secretory epidermal cells and outside of these cells. The glands have precocious development and are composed of a non‐vascular central axis covered with a palisade‐like secretory epidermis. During senescence, the cells of the palisade‐like secretory epidermis and central axis appear to lose their typical form, with some cell walls disintegrating. In the mature leaf, an abscission zone at the base of the gland causes detachment of the gland. The placement, anatomical structure, precocious activity and polysaccharide secretion (in C. sylvestris ) allow the recognition of the glands associated with the marginal teeth in Casearia as colleters. Similar studies are needed for other genera in order to clarify how colleters have evolved in Salicaceae and how they are functionally and anatomically related to other teeth types in the family.