The large, bipinnate fronds of Marattia fraxinea reach sizes around 1 m to 3 m in length and 0,5 m to 1,5 m in width. The pinnae or leaflets become 17 cm long and 1,4 cm wide, their margins toothed. The stipe, the petiole from the rootstock to the beginning of the leaf tissue, has a basal swelling, resembling a typical leaf stalk.
Vegetative reproduction takes place by the formation of small plantlets at the stem base. The pale brown sori grow in rows near the pinnae margins on their lower surfaces. A sorus (plural sori) is a cluster of sporangia, the structures that produce and contain the spores from which ferns reproduce (Schmidt, et al, 2002; JSTOR; Wikipedia).