Spiranthes (Ladies tresses)

Iowa's Spiranthes species

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), is rated as infrequent, but is the most common of the Iowa species and of the North America species as well.

Spiranthes cernua 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 new 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 polyploid species which has been named Spiranthes 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 S. cernua. 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 acquires (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 it a single name which is Spiranthes cernua. The polyploid and each of the diploid species that have contributed to its genetic makeup are called a Spiranthes cernua complex. Because S. cernua doesn’t 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.

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 and S. romanzoffiana. A recent 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 base (haploid) chromosome number n=22. C. cernua and the remainder of the Iowa species have base number n=15. Therefore, the diploid number for S. lucida is 2n=44 and for all of the other Iowa diploids it is 2n=30. The polyploid species are either some multiple of n=22 in the case of S. romanzoffiana or some multiple of n=15 in the case of S. cernua. Counting chromosomes is an important taxonomic technique, but 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 ; but, so far as I know, they have not been found in any of the other Iowa Spiranthes species (there have been reports in other places, (see FNA)). All of Iowa's other Spiranthes species 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 most frequent Spiranthes ID problem, in Iowa, is the distinction between S. magnicamporum and S. cernua. 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 has Polyembryonic seeds (see species page) compared to S. magnicamporum's monoembryonic seeds, and (2) S. magnicamporum has a strong odor (variously described as coumarin—like 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.

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 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..."


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.