Examine the lamina against the light with a hand lens. This character is for ferns with membranous and translucent laminas only - see 4.7.2.

Are there dark lines of cells that appear to be veins? They may not branch from the veins in the normal way, and never contain actual conducting cells? These often lie between the real veins (but are usually not continuous with them) or along the margin of the lamina.