Adventitious embryony in this species commonly gives rise to polyembryonic seeds and seeds exhibiting irregularities such as embryos erupting through or developing outside the testa and naked embryos lacking testae. Often a capsule contains a mixture of polyembryonic and monoembryonic seeds. Rarely all or most seed is monoembryonic, yet irregularities can still be seen.
Polyembryony is also characteristic of Spiranthes casei and S. parksii, species that are probably derivatives of S. cernua. For the most part, at least, related diploids and all other species are strictly sexual and characterized by wholly monoembryonic seed. Some indication of apomixis and polyembryony in S. magnicamporum, S. ochroleuca, and S. odorata in the northeastern portion of the range of the complex has been reported (P. M. Catling 1981, 1982), but the difficulty in distinguishing between diploid donors and tetraploid chromosomal segregates makes interpretation difficult (C. J. Sheviak 1982, 1991). In particular, the range of S. odorata northward along the Atlantic Coast is uncertain. Plants that appear typical of S. odorata in all respects but with polyembryonic seed occur from Virginia northward and quickly replace monoembryonic plants toward the north. Such plants from as far south as Virginia, however, have proven to be tetraploids and furthermore to assume morphology typical of S. cernua when grown away from the Coast.
Gene flow from Spiranthes magnicamporum generates cytogenetically based peloria, a common element in portions of the prairie region. Most commonly differentiation of the lip is suppressed, but in some forms the petals are elaborated and approach the lip in form. Additionally, buds may fail to open and a functional column may be lacking, yet seed is produced apomictically. See the discussion under 16. S. parksii.
Spiranthes cernua, commonly called nodding lady's tresses,[2] or nodding ladies' tresses,[3] is a species of orchid occurring from Maritime Canada to the eastern and southern United States. As the common name suggests cernua means "nodding", or "bowed" in Latin.
Spiranthes cernua plants grow to 50 centimetres (20 in) tall. They have 1 to 5 narrow, basal, upright leaves, 2.6 centimetres (1.0 in) long and 2 centimetres (0.8 in) wide.[4] The leaves are present during flowering but wilt afterwards. The white flowers are arranged in a spiral around the stem. Each flower is 6–12 millimetres (0.2–0.5 in) long and consists of 3 sepals and 3 petals, all curved forward to give the flower a long bell shape. Flowers are slightly to strongly nodding (hence the name), with older flowers usually nodding more than new ones. The dorsal sepal (the one at the top) is convex and recurved upwards towards the tip. The lip (bottom petal) curves strongly downwards towards its tip.[5][6]
The genus name, Spiranthes, originated from the Greek words speira (coil) and anthos (flower), describing the spiral flowers common to the orchids in this group.[7] Cernua is from the Greek word cernuus and refers to the drooping or bowing flowers.[8]
The species is native in central and eastern North America, from Ontario and Quebec to the north, Nebraska to the west, Texas to the south, and Novia Scotia to the east.[2] Spiranthes cernua grows in wet meadows, mossy seeps, maritime dune swallows, Sphagnum areas around pond and lake edges and along roadsides.[6]
The orchid blooms in autumn, from August to November.[4] Native bees, including bumblebees, feed on the nectar and pollinate the plant.[9]
Before more recent identification of several separate species Spiranthes cernua had long been described as a species complex, exhibiting different morphologies throughout its range. Charles Sheviak, now retired curator of the New York State Museum Herbarium, explored this diversity in great detail,[10] eventually describing Spiranthes magnicamporum to represent large, later-flowering individuals with tuberous roots from the Midwest.[11] More recently, molecular and morphological work has recognized additional cryptic species in need of description; these newly recognized species are sometimes not closely related to S. cernua.[6]
Broadly, the Spiranthes cernua species complex includes:[6]
Spiranthes odorata was also formerly included in the cernua species complex but newer research shows it belonging to a different clade containing most of the remaining Eastern North American Spiranthes species.[6]
A commonly cultivated variety is Spiranthes cernua 'Chadds Ford', grown because of its larger flowers, ease of cultivation, and other merits.[12] This cultivar is also often labelled as Spiranthes odorata however botanically it is neither but a third species called Spiranthes bightensis.[13]
Spiranthes cernua, commonly called nodding lady's tresses, or nodding ladies' tresses, is a species of orchid occurring from Maritime Canada to the eastern and southern United States. As the common name suggests cernua means "nodding", or "bowed" in Latin.
