Spiranthes: Ladies' Tresses (Lady's Tresses)
The genus Spiranthes reaches its greatest development in North America where most species are found (about 27). Fewer species also occur in Europe, north Africa, northern and eastern Asia and Australasia. The flowers usually grow in a spiral, forming a terminal spike and are usually white or yellow, but sometimes pink. The labellum (lower lip) lacks a spur (an elongated projection for holding nectar).
Two species occur in the British Isles: Spiranthese spiralis (Autum Ladies' Tresses) and Spiranthes romanzoffiana (Hooded or Irish Ladies' Tresses). Spiranthes aestivalis (Summer Ladies' Tresses) is now thought to be extinct in the British Isles (a geographical region that has suffered severe natural habitat loss across the board, as have many parts of the world).
In Spiranthes, the one fertile stamen rests on top of the column (a compound structure formed by fusion of remnants of the sterile stigma and perhaps the two sterile anthers). The rostellum (beak) of the column projects forwards beneath the stamen and bears a long narrow strap-shaped viscidium. The viscidium is a sticky part of the column that can be removed along with the pollinia (a pair of pollen packets released from the stamen) which attach to the rostellum when the anther dehisces (dries and splits open along a predetermined line of weakness). The sticky part of the viscidium faces downwards.
The upper (dorsal) sepal and two petals are fused together to form the upper side of the tube-like flower. The long trough-shaped lower petal forms the lower lip or labellum. The base of the lip wraps closely around the column and is very close to the rostellum, leaving only a narrow gap for a bumble-bee's proboscis to reach nectar secreted by two small nectaries at the base of the lip and which collects in the inner hollow of the lip. The bee rests on the lip whilst feeding in this way. The proboscis can not avoid touching and exposing the sticky viscidium which adheres to it along with the pollinia. The glue sets, firmly fixing the pollinia to the bee. During a day or so, after a flower loses its pollinia, the column and lip move apart to widen the aperture. Now a visiting bee can fit any attached pollinia inside, as well as its proboscis. The pollinia are fragile and clumps of pollen break off and adhere against the sticky stigma, effecting pollination.
Spiranthes spiralis (Common or Autumn Ladies' Tresses)
This species occurs in central and southern Europe and Algeria in North
Africa and northwards as far as Denmark. It is abundant in the
Mediterranean region. It is found over most of England and Wales and
southern Ireland though not common in the British isles it is the most
commonly occurring species of Spiranthes found there. It also
extends across Russia and Asian Minor (Anatolia or Asian Turkey) across
Syria to the Caucasus (the area between the Black Sea and the Caspian
Sea). In the British Isles it may be locally abundant, but the number of
plants flowering each year in a population is very variable (as with
many other orchids).
Above: Spiranthes spiralis (Autumn Ladies' Tresses) (Kingsdown, Kent, British Isles). This specimen was about 5 cm tall and has only opened the lowest four flowers thus far. Spiranthes spiralis grows to 3 to 15 cm in height (up to 28.5 cm) in Britain.
Above: Spiranthes spiralis (Autumn Ladies' Tresses) (Kingsdown, Kent, British Isles). This specimen was about 8 cm tall. The spiral is more-or-less strongly developed and may follow more than one turn as here. the flowers are scented during the daytime and are pollinated by bumble-bees.
Above: Spiranthes spiralis (Autumn Ladies' Tresses) (Kingsdown, Kent, British Isles). Basal rosette. The plant puts out a basal rosette of leaves in the autumn but these die away by late spring. When the plant flowers (in late summer, generally in August and September in the British Isles) it begins to produce the new rosette which persists over the winter. This result is just off to one side from the main stem as shown here.
Above: Spiranthes spiralis (Autumn Ladies' Tresses) (Kingsdown, Kent, British Isles). The basal rosette, visible next to the stem, has up to 10 small leaves (usually 4 or 5, each up to 3 cm or 1 to 2 inches long). Three to seven leaves also sheath the stem. The sheathing leaves have a whitish or translucent fringe. The first leaves appear when the plant is about 8 years old, though in the laboratory they may appear within 6 months and the first flowering spike after about five years. About half of plants live 7 years or less after first appearance above ground.
The underground rhizome does not persist but dies before the first leaves appear. Food storage is taken over by the two or three (up to five) fleshy carrot-shaped tuberous roots up to about an inch in length. After the rosette and flower spike have been produced these roots shrivel and die as their food reserves are exhausted and new roots are produced once the leaves begin photosynthesis.
The roots of the germling are initially free from fungal infection but become infected during the following Autumn. The roots prefer shallow, well-drained soil and the fungus partner is important for mineral uptake, especially during the autumn and spring when the plant prefers high rainfall. It is, however, more tolerant of summer drought. Lateral buds on the stem may give rise to separate root tubers and when the connecting stem dies away this becomes as separate plant. Due to this asexual mode of propagation it is characteristic to see clumps of the plants growing together.
The plant prefers low vegetation (e.g. on short, dry and nutrient poor grassland) as it can not compete well with taller grasses due to its short stature. It is found on sunny slopes, apparently preferring chalky soils, but also occurs in sandy heathy places, dunes, lawns and earthworks and prefers to be near the coast. In the photographs above it was growing at the back of sea defenses formed by a sea wall.