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Wednesday, November 7, 2018

Whorled Wonders

Great Plains Ladies Tresses

The spiral, which is a fundamental form in nature, is most splendidly illustrated in a genus of wild, native orchids called Spiranthes.  Commonly known as ladies tresses, the genus name comes from the Greek speira meaning ‘coil’ and anthos meaning ‘flower’, and refers to each species spirally arranged inflorescence.  The most predominant species of orchid found in Texas prairies, several members of this genus are colonizers of sparsely vegetated areas, appearing on newly disturbed sites such as roadsides and cleared fields, increasing in number until outcompeted by other vegetation.

Of the 15 native Spiranthes species in Texas, several are so similar in appearance that either a hand lens or microscope is often needed to distinguish one from another. To add to the confusion, many closely related species are also known to hybridize. However, Central Texas, the most common include the Great Plains Ladies Tresses (S. magnicamporum) and the Nodding Ladies Tresses (S. cernua).  

Great Plains Ladies Tresses has 2 to 4 narrow, grass-like basal leaves, up to 6 inches long, that are usually absent or withering during the flowering period.  The flower spike can range from 4 to 24 inches tall, and is made up of 12 to 54 small white tubular fragrant flowers, tightly or loosely spiraled, that nod abruptly from the base.  Blooming from September to November, it prefers calcareous grassland habitat, often growing in association with our native Seep Muhly.  In wet years, this orchid may appear in robust spikes numbering in the hundreds, and in dry years it may not flower at all.

Nodding Ladies Tresses has 3 to 5 narrow, grass-like, basal leaves, 8 to 10 inches long, and are typically present at flowering.  It has a flower spike that can grow from 4 to 19 inches tall, and consists of 10 to 50 small white tubular flowers, tightly or loosely whorled in 2 to 4 rows along the upper portion of the stem.  Blooming from late September through November (and sometimes even into December), it can grow on wet or dry sites, but prefers more acidic, sandy soils.

Flowers of Spiranthes orchids begin opening at the bottom of the inflorescence.

Like most orchids, the flowers of these Spiranthes species are resupinate, or twisting during development into an upside-down position.  In fact, the tendency of the flowers to droop slightly gives the Nodding Ladies Tresses both its common and species name, for cernua comes from the Latin and means ‘drooping.’  Unlike other closely related species, the flowers of the Nodding Ladies Tresses have little or no fragrance, but like other closely related species, the flowers are pollinated by bumblebees.  As with most Spiranthes, bumblebees start at the bottom and move upward on the inflorescence in search of nectar. Older flowers at the base of the flower stalk have more nectar, which makes them an efficient first stop for the foraging bumblebees.

As mentioned above, many Spiranthes are difficult to identify to species, and both the Great Plains Ladies Tresses and the Nodding Ladies Tresses are no exceptions.  In fact, Nodding Ladies Tresses is known as a compilospecies, which is defined as a genetically aggressive species that incorporates the heredities of a closely related species by hybridization through unidirectional gene flow, and may even completely subsume that species over time.  Now that’s a whorled wonder!