In the filmy ferns (ferns with membranous translucent fronds) the sorus is surrounded on all sides by a structure called an involucre, which may be two-lipped (bilabiate) at the tip or funnel-shaped. This character is only applicable to filmy ferns with this type of indusium. Decide whether the fern has:
Australian Tropical Ferns and Lycophytes - Online edition
5.3.3 Involucre opening
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