Crouch et all.: " In southern Africa the naturalised colonies of N. cordifolia have long been mistaken for N. exaltata, the supposed source of the 'Boston sword fern' (which is probably a hybrid) and its cultivars. Owing to various misconceptions and the apparent difficulty to identify members of the genus, the name N. exaltata has been widely and mistakenly applied worldwide. N. exaltata is native to Central America and the Caribbean, there is no indication that it invades and naturalises readily on any significant scale."
Crouch et all: " The other Nephrolepis that has apparently escaped from cultivation in some parts of the world, N. exaltata, never produces water-storing tubers along its stolons, as N. cordifolia does. The cordate acroscopic pinnae base and lunate or broad indusium further distinguishes N. cordifolia from N. exaltata in which the acroscopic base of the pinna is truncate and the indusium reniform.
Burrows, J.E. (1990) Southern African Ferns and Fern Allies. Frandsen, Sandton. Pages 206 - 207. (Includes a picture).
Crouch, N.R., Klopper, R.R., Burrows, J.E. & Burrows, S.M. (2011) Ferns of Southern Africa, A comprehensive guide. Struik Nature. Pages 518 - 519. As Nephrolepis cordifolia (L.) C.Presl subsp. cordifolia (Includes a picture).
Roux, J.P. (2009) Synopsis of the Lycopodiophyta and Pteridophyta of Africa, Madagascar and neighbouring islands. Strelitzia 23, South African National Biodiversity Institute, Pretoria. Page 148.
Roux, J.P. (2001) Conspectus of Southern African Pteridophyta.Southern African Botanical Diversity Network Report, 13 Pages 111 - 112. (Includes a picture).