Colleters in Thelypteridaceae: Unveiling mucilage secretion and its probable role in ferns

Young fronds from several fern species are covered by mucilage, but its synthesis and functional roles remain understudied. The young leaves of the tropical ferns Amauropelta cheilanthoides , Amauropelta pachyrhachis and Steiropteris decussata are covered by a hyaline layer of highly hydrophilic mucilage that is produced by glandular trichomes (GTs). In order to better understand the morphology and ultrastructure associated with mucilage secretion in these plants, we employed a combination of light and electron microscopy along with histochemical studies. The distribution and abundance of GTs varies during leaf growth. The origin and differentiation of GTs occurs early in leaf development and their secretory activity is restricted to the first stage of leaf differentiation. Glandular trichomes can be sessile or pedunculate and the secretory head commonly shows a single cell in the distal portion. In general, at the beginning of development, the distal secretory cell has a large nucleus, thin wall and dense cytoplasm with large vacuoles, endoplasmatic reticulum and active dictyosomes. In mature secretory cells the large vacuole is filled with secretion, and represents almost the entire protoplast. Observations of the early onset and duration of functional mucilaginous trichomes of Thelypteridaceae, the physicochemical nature of the secretion and the location of these trichomes indicate that the product of these structures provides protection for the apical meristem and young leaves from excessive sunlight and high levels of transpiration, thereby reducing water loss. This functional concept is the common link connecting most reports on the structure of colleters and, therefore, this work represents the first report of colleters for ferns.