The exception seems to be the rule: Nectaries in Serpocaulon and an update of their presence in Polypodiaceae

Abstract

Although nectaries are widely known among angiosperms, few records exist for ferns, and there are even fewer studies focusing on their anatomy. We noticed nectaries in Serpocaulon (Polypodiaceae) and confirmed their presence in six taxa of the genus. We conducted a structural analysis using light and scanning electron microscopy and revised occurrence records of nectaries in polypodiaceous ferns. Nectar secretion was observed in young fronds and was restricted to the basal pinnae/segments. We did not detect any differentiated glandular tissue in the portions where the nectar was released, which implies that the nectar secreting glands in the studied taxa should be regarded as non-structural nectaries. Nonetheless, we found the mesophyll in these portions to usually be compact (i. e. with inconspicuous and relatively fewer intercellular spaces), sometimes with cells with distinct dense protoplast. In addition, prominent stomata with elevated guard cells were observed in nectary portions, probably constituting pathways through which nectar exudates. An update record for nectaries in Polypodiaceae was presented; these structures were recorded in 8% of Serpocaulon and ca. 1% of the species of the family, most being epiphytic with pinnatisect laminae dissection. While extrafloral nectaries are mostly related to protection against herbivores, nectary ecology in ferns is poorly understood, and a protective role of these structures cannot be discarded. However, further investigation is needed and encouraged to develop a comprehensive understanding of fern nectaries in light of their evolutionary history and ecological roles.