6. Asplenium rhizophyllum L. (walking fern) Pl. 2e,f; Map 5

Camptosorus rhizophyllus (L.) Link

C. rhizophyllus f. auriculatum Hoffm.

Leaves 5–40 cm long, monomorphic. Petioles brown at the base, green above, not shiny. Leaf blades simple, rarely forking at the tip, narrowly deltoid in outline, the tip long-attenuate, often rooting and forming plantlets vegetatively, the base with a pair of auricles, these rounded to long-attenuate. Veins anastomosing. Spores 64 per sporangium. 2n=72. May–October.

Common nearly throughout Missouri, although less common north of the Missouri River (eastern U.S. and adjacent Canada west to Kansas). Ledges and crevices of shaded dolomite and limestone bluffs and boulders; rarely found on noncalcareous substrates.

This distinctive species is treated in the segregate genus Camptosorus by some authors based upon the netted venation and simple leaves. However, it forms hybrids with several other species of Asplenium and the characters that distinguish it are unremarkable in the context of overall morphological variation within the genus. Thus most modern botanists treat the walking fern as a species of Asplenium.

Where the walking fern grows on boulders of wooded slopes it frequently occurs in close proximity to the ebony spleenwort, which is primarily a plant of the forest floor. Scott's spleenwort, the sterile, diploid hybrid between A. platyneuron and A. rhizophyllum (Asplenium ¥ebenoides R.R. Scott, Pl. 2c,d), occurs on dolomite and limestone boulders sporadically throughout the Ozark and Ozark Border regions. It is intermediate in morphology between the parental species, with leaf blades that are pinnately compound in the lower half and with the pinnae and lobes irregular in size. Some plants of this hybrid resemble those of A. pinnatifidum. However, the petioles are dull and green except at the base in A. pinnatifidum, whereas they are shiny and reddish brown to dark brown in A. ¥ebenoides.