Spiranthes (Ladies tresses orchids)

Iowa's Spiranthes species

This painting, formerly titled Spiranthes cerna, has been retitled Spiranthes incurva. Based on the non-drooping flowers and the pointed labium (lower petal), the new title is more consistant with the information provided in the Pace and Cameron (2017) paper in the references below.

BONAP (The Biota of North America Program) shows 31 species in the U.S. with 7 species in Iowa-none of which are considered noxious.

In The Vascular Plants of Iowa (1994) Eilers and Roosa report the following seven species in Iowa—all of which are native to Iowa.

The first six are rated rare. The last (S. cernua now recognized in Iowa as Spiranthes incurva), is rated as infrequent, but is probably the most common of the Iowa species .

Spiranthes incurva is shown in the image at right (click on the image to go to the species page).

Telling them apart

Perplexing, is how taxonomists describe Spiranthes taxonomy. This is because taxonomists like to identify plants by morphological features which are generally (allowing for slight variations) consistent within a species. Spiranthes species show enough variation from place to place, that some taxonomists are tempted to regard the variants as separate species.

However; what appears to be the case, is that most diploid species are fairly consistent in their morphology from place to place. The variation seems to come from a collection of similar polyploid organisms which have been named the Spiranthes cernua complex. It has been suggested that Spiranthes incurva and Spiranthes cernua can be regarded as two separate but "cryptic" (confusing) species within that complex. A few visual clues have been offered to distinguish between S. incurva and S. cernua:

1. The flower heads of S. incurva tend to be parallel to the ground or slightly ascending. The heads of S. cernua tend to nod or droop.
2. Lateral sepals are longer more noticeable and appear attached to the upper petal in S. incurva , they are shorter and less prominent in S. cernua .
3. The labellum is longer and more pointed in S. incurva than in S. cernua .
4. Basel leaves usually wither before flowering in S. incurva . They may persist during flowering in S. cernua .
5. S. incurva generally flowers earlier in the season then does S. cernua .

Occasionally, a polyploid species arises when a diploid species fails to reduce its chromosome numbers during meiosis. The new polyploid may share the features of its precursor or it may have new features of its own. If the ability to produce polyploids is shared by several closely related species, and the polyploids can cross with each other and perhaps even other related species, It's easy to imagine the genetic compilation of genes which would provide the sort of variation seen in the S. cernua complex. The observation that S. cernua also exhibits the ability to produce viable seed both sexually and asexually can explain how the polyploid, once established, can become the dominant member in the group. The polyploids produce other polyploids and these are capable of increased variety by sexual reproduction as well as increasing their numbers by forming clones asexually. Over several generations the polyploid species acquire (compile) a genetic storehouse reflecting the genetic makeup of the related plants in the area. Different areas will produce polyploids with different genes and therefore morphologically different individuals. So, the diversity that troubles taxonomists seems to be the result of these regionally different polyploids. A decision has been made to call these diverse Spiranthes polyploids a single species and to give them a single name which is Spiranthes cernua. The polyploids and each of the diploid species that have contributed to their genetic makeup are called a Spiranthes cernua complex. Because S. cernua does not fit the usual notion of a single species; that is, it can have the appearance of any of its donar species and because it is a compilation of the genes from those several related species—It is called a compilospecies. It could be said the S. cernua was a compilospecies that developed in the NE U.S. and that S. incurva was a compilospecies that developed in a more westerly region that would include the state of Iowa. The differences seen between the two species can be less significant than the differences seen within each species.

Fortunally, when you can't rely on morphological distinctions to separate species, there are other options available.

Ploidy level and chromosome number are basic tools for species identification. In Iowa, Spiranthes contains only two polyploids, S. cernua/incurva and S. romanzoffiana. The work by L. Dueck, D. Aygoren, and K. M. Cameron (AJB 2014)1 provides chromosome numbers as well as a cladistic analysis of species relationships both inside and outside of North America (S. romanzoffiana is the only Iowa species to have crossed the Atlantic ocean). The chromosome numbers show that S. romanzoffiana and one other Iowa species (S. lucida) have the haploid chromosome number n=22. S. cernua and the remainder of the Iowa species have haploid number n=15. Therefore, the polyploid species are either some multiple of n=22 as in the case of S. romanzoffiana or some multiple of n=15 as in the case of S. cernua. Taxonomy of the Spiranthes cernua complex has been as tedious as untying the "Gordian knot" and the recent work of M.C. Pace, and K.M. Cameron (2017) has done much to unravel it. Their paper The Systematics of the Spiranthes cernua Species Complex (Orchidaceae): Untangling the Gordian Knot is, at this writing, an open source pdf. It nicely extends and refines the work of Dueck et al. (2014). It introduces the new cryptic species name Spiranthes incurva, it also includes photographs of the plants under consideration and is well worth reading.

While counting chromosomes is an important taxonomic technique, it is impractical in the field and there are a couple of other equally useful identity clues when working with Spiranthes.

Phenology can be some help with identification—but, notice that some blooming periods overlap. The best time to identify these plants is when they are in flower and shortly thereafter. Among Iowa plants, S. vernalis and S. lucida typically flower in the early summer (May–July). S. lacera and S. romanzoffiana flower in late summer (June–Sept.). S. ovalis, S. magnicamporum, and S. cernua flower in the fall (Aug.–Nov.)1.

Polyembryonic seeds are found in numerous orchid taxa. They are evident in S.cernua and S.incurva ; but, so far as I know, they have not been reported in any of the other Iowa Spiranthes species (They have been reported in S. casei and S. parksii which are not Iowa species. (see FNA)). All of Iowa's other Spiranthes species appear to have only monoembryonic seeds. The seeds (often called dust seeds) are numerous and tiny (less than 1mm long). But, with a compound microscope it is easy to see the number of embryos in the seeds. No staining or special preparation is necessary. The seeds should be examined when the fruits have turned brown and the seeds have matured.

The most commonSpiranthes ID problem, in Iowa, is the distinction between S. magnicamporum and S. cernua (or S. incurva). Morphologically they can be very similar, and they also exhibit the same phenology and habitat preferences. But, there are two handy distinctions; (1) S. cernua/incurva has Polyembryonic seeds (see species page) compared to S. magnicamporum's monoembryonic seeds, and (2) At the time of flowering, S. magnicamporum has a frangrance (variously described as like; coumarin, new mown hay, vanilla, or even chewing tobacco). One botanist claims to have detected the odor from 30 feet away. S. cernua, on the other hand, has lessor or even no odor. In fact, the strength of the odor produced by S. cernua may be indicative of the extent of its compilation of genes originating from S. magnicamporum. Of course, evaluating odor strength is subjective and there is little doubt that botanists differ in their olfactory skills. Distinguishing between these two species by odor is often done by workers in the field. However; until more is known, the taxonomic value of this technique seems tenuous.

Facultatively agamospermic polyploid compilospecies is the description used by orchid expert Charles J. Sheviak to explain the nature of S. cernua. Two of his often quoted studies2,3 offer clues to how the term came about. You may need the help of a good librarian at a university library to access these publications. A quicker option would be to read the FNA page for S. cernua.

Interest in the Spiranthes orchids goes way back; In 1877 Charles Darwin wrote a book about orchid fertilisation that is available on line as a pdf. Scroll to Spiranthes (P. 106 Chap. IV). I found his description of Spiranthes autumnalis, it's structure and interaction with "humble-bees", fascinating. His correspondence with Asa Gray (p. 111) confirmed that most of Darwin's observations about Spiranthes pollination also applied to S. cernua in North America.

Reproduction by S. cernua can be sexual, asexual or both. It is believed capable of producing viable seeds either way. Pollination by bees provide a means by which genetic material is sexually transfered between closely related species. Sexual reproduction, through genetic recombinations, provide a means for morphological variation. Asexual reproduction, which doesn't require pollinators, provides clones with little or no variation and is thought to occur in one of two ways; either by seed development inside (cleistogamous) flowers that never open and are self pollinated, or by means of agamospermy (apomixis). Agamospermy refers to embryo development that occurs from multiploid cells rather than the haploid cells involved in sexual embryogenisis. Facultative agamospermy indicates that it doesn't have to occur all of the time. Polyembryonic seeds are thought to have one sexually produced embryo and one or more (rarely more than two) embryos produced asexually—probably, from the integument or nucellus during development of the seed. For S. cernua integument embryony is most likely since S.cernua seeds have little or no nucellus (i.e. they are tenuinucellate). But, whatever the source, polyembryony is an important identifying characteristic for S. cernua.

You may come across individual Spiranthes plants, but it is not unusual to find them in clusters. When the plants are not as close together as would be expected if they reproduced by rhizomes or stolons; the distance between individuals suggest propagation by seed. Clusters may include nearly identical clones suggesting asexual reproduction. Others may include individuals intermediate in form between two nearby species which suggests hybridization and some may be polyploids (mostly tetraploids) that look very much like nearby diploids. Some clusters may include all of these. Such a group could be a hybrid swarm. Hybrid swarms develop when a group of diploid plants and their hybrids develop polyploid individuals which can cross with each other and backcross with progenitors. In such a group gene flow could be in two directions. In the S. cernua complex it is postulated that gene flow is in only one direction, from diploid to polyploid.

Sheviak2 (p17) argues that: "much of the variation of S.cernua ...may be due to introgression from S. magnicamporum..." and "the incorporation of genes of related species is a fundamental characteristic of S. cernua; it is, consequently, a compilospecies..."


1. DUECK, L. A., Aygoren,D. AND K. M. CAMERON. 2014. A molecular framework for understanding the phylogeny of Spiranthes (Orchidaceae), a cosmopolitan genus with a North American center of diversityS. incurva American Journal of Botany 101 (9): 1551-1571.

2. Sheviak, C. J. 1982. Biosystematic study of the Spiranthes cernua complex. New York State Museum Bulletin 448:1-73.

3. Sheviak, C. J. 1991. Morphological variation in the compilospecies Spiranthes cernua (L.) L. C. Rich.: ecologically limited effects of gene flow. Lindleyana 6:228-234.