Myriopteris aemula, the Texas lip fern or rival lip fern, is a moderately-sized fern of Texas and Mexico, a member of the family Pteridaceae. Unlike many members of its genus, its leaves have a few hairs on upper and lower surfaces, or lack them entirely. One of the cheilanthoid ferns, it was usually classified in the genus Cheilanthes as Cheilanthes aemula until 2013, when the genus Myriopteris was again recognized as separate from Cheilanthes. It typically grows on limestone rock.

Description
Leaf bases are closely spaced along the rhizome, variously described as  or  in diameter. The rhizome bears persistent scales, which are linear to narrowly lanceolate, straight or slightly twisted, and loosely appressed (pressed against the surface of the rhizome). Their margins are entire (untoothed). They may be uniformly brown or tan to orange-brown in color, or be darker at their base, particularly in the center.

The fronds spring up in clusters; they do not unfold as fiddleheads like typical ferns (noncircinate vernation). When mature, they are  long and  wide. The stipe (the stalk of the leaf, below the blade) is  long, representing one-third to one-half of the total length of the leaf. The upper surface of the stipe is rounded, and it is black to dark brown in color, or black to very dark purple. It may be hairless, or bear a few hairs (long ones of 1 mm and short ones less than 0.1 mm) on the upper surface.

The leaf blades range in shape from deltate to ovate. The blade is usually tripinnate (cut into pinnae, pinnules, and pinnulets) to tripinnate-pinnatifid (with deeply lobed pinnulets) at the base, or even quadripinnate. The rachis (leaf axis) is rounded on the upper side. It bears twisted hairs tightly pressed to it on the upper side, and scattered, spreading, straight hairs on the lower side; no scales are present. The pinnae are not jointed at the base, and the dark pigmentation of the rachis enters the edge of the pinnae. The pinnae at the base of the leaf are slightly larger than the pinnae immediately above them and the pinnae are somewhat asymmetric about the costa (pinna axis). The basiscopic pinnules (pointing at the leaf base) are slightly larger and more deeply dissected than the acroscopic pinnules (pointing at the leaf tip). The lowest pair of basiscopic pinnules closest to the stem are noticeably larger than adjacent pinnules and thin in texture. The upper and lower surfaces of the pinnae have a few soft hairs, 0.5–0.8 mm in length or none at all. The costae are black on the upper side for most of their length and lack scales beneath. The pinnulets are narrowly elliptic to elongate-deltate, and not bead-shaped as in some other species of Myriopteris. They are cordate at the base and acute at the tip. The largest pinnulets are  long, and have sparse white hairs on upper and lower surfaces, or lack hairs entirely.

On fertile fronds, the sori are protected by false indusia formed by the edge of the leaf curling back over the underside. The false indusia are slightly differentiated from the rest of the leaf tissue, and are 0.05–0.3 mm wide. The edges of the indusia are not toothed or lobed. Beneath them, the sori are usually not continuous around the edge of the leaf, and are often concentrated on lateral lobes of the fertile pinnulets, particularly at the ends of veins. Each sporangium in a sorus carries 64 tan spores. Individual sporophytes are sexual diploids, with a diploid chromosome number of 2n = 58.

It can be confused with a number of closely related species in the "alabamensis clade" of Myriopteris. M. alabamensis and M. microphylla have slightly less divided fronds which are lanceolate and hence more narrow towards the base, while M. cucullans and M. notholaenoides also have lanceolate fronds and bear abundant golden hairs and narrow, hair-like scales on the rachis.

Taxonomy
Myriopteris aemula was first described by William Ralph Maxon in 1908, as Cheilanthes aemula, based on material collected by Edward Palmer in 1907 from Ciudad Victoria. He distinguished it from Cheilanthes microphylla, found growing with it, by its greater degree of cutting and the triangular shape of the leaf blade. The specific epithet aemula means "rivalling" or "emulating", and is believed to refer to its "emulation" of the C. microphylla found growing with it.

The development of molecular phylogenetic methods showed that the traditional circumscription of Cheilanthes, including that used by Maxon, is polyphyletic. Convergent evolution in arid environments is thought to be responsible for widespread homoplasy in the morphological characters traditionally used to classify it and the segregate genera that have sometimes been recognized. On the basis of molecular evidence, Amanda Grusz and Michael D. Windham revived the genus Myriopteris in 2013 for a group of species formerly placed in Cheilanthes. One of these was C. aemula, which thus became Myriopteris aemula.

In 2018, Maarten J. M. Christenhusz transferred the species to Hemionitis as H. aemula, as part of a program to consolidate the cheilanthoid ferns into that genus.

The common name "lip fern" comes from the position of the sporangia at the edge or lip of the leaf, typical of the genus. This species is referred to as "rival lip fern", a translation of the specific epithet aemula, or "Texas lip fern".

Further molecular studies in Myriopteris demonstrated the existence of three well-supported clades within the genus. M. allosuroides belongs to what Grusz et al. informally named the alabamensis clade, and is sister to a group consisting of M. microphylla, M. moritziana, M. scabra, and M. fimbriata.

Distribution and habitat
Myriopteris aemula is found in scattered locations in southern Texas, including the Trans-Pecos. Its range extends throughout the length of Mexico, particularly in the eastern and central states, as far south as Chiapas.

The species grows on limestone bedrock, on rocky slopes and ledges, and in cracks and openings in the rock. It occurs at an altitude from .

Ecology and conservation
While globally secure (G4), M. aemula is considered by NatureServe to be vulnerable in Texas.

Notes and references

References

Works cited

External links
Type specimen at the US National Herbarium

aemula
Ferns of Mexico
Ferns of the United States
Flora of Texas
Plants described in 1908