During a collecting trip to Thailand, as a member of the Thai-Dutch botanical expedition 1965/1966, I was struck by the excessive amount of mucilage sometimes present on the circinnate fronds of several leptosporangiate ferns. This was especially evident in representatives of the monotypic fern family Plagiogyriaceae and the Thelypteroid ferns. The mucilage is secreted by a multitude of minute glandular hairs. In mature leaves the hairs are shrivelled and easily overlooked, or even have disappeared. Moreover, usually only herbarium specimens are available for investigation.