Lesser twayblade

This beautiful orchid is often overlooked because of its small size. Further study is needed to elucidate its lifecycle and role in the ecosystem.

Global distribution

Lesser twayblade has a circumboreal distribution, occurring throughout the world’s northern latitudes. Its range includes north Asia and Scandinavia, and it grows further south in Europe at higher elevations in the Apennines, at up to 2,000 metres in the Alps and in mountainous parts of Greece, the former Yugoslavia and Turkey. It also occurs in Greenland and much of North America, including Alaska and all of Canada, and in the mountains of the USA as far south as California and West Virginia. It grows in several habitats, including conifer forest, but is most common in areas with acid soil conditions such as wet heaths and peat bogs.

Distribution in Scotland

Lesser twayblade has been recorded in all parts of Scotland, including Orkney, Shetland and the Western Isles, but is most common in the north and west of the mainland, and in the Cairngorms and Grampian Mountains. Its range extends into northern England, north Wales and parts of Somerset and Devon, as well as much of Ireland, but Scotland is the stronghold for the species in the UK.

The principal habitat for lesser twayblade in Scotland is heather moorland, where it grows amongst moss in the shade of the heather, but it also occurs in wet woodlands such as willow and alder carr, and in native pinewoods.

Lesser twayblade is a small member of the orchid family, Orchidaceae , and is similar to the closely-related larger species, common twayblade ( Listera ovata ). It usually grows up to 15 cm. tall, although it can range from 4 to 25 cm. in height. It is distinctive in having only two leaves (which gives rise to its common name), and these are heart-shaped (hence the specific, scientific name ‘cordata’). They occur about one third of the way up the stem, opposite each other, and are a dark glossy green colour and from 1 – 3 cm. long. Very occasionally there can be a third leaf, growing further up the stem, and below the flowers. The leaves have a distinct central rib, and are similar to those of blaeberry (Vaccinium myrtillus ), which can make identification of non-flowering specimens difficult, as the two plants occur in the same habitat.

Lesser twayblade has a single, upright flower stem, which is often reddish-purple in colour, and is hairless below the leaves, with some fine glandular hairs occurring above them. There can be up to 20 tiny flowers on an individual plant. They are reddish-green or reddish-bronze in colour, and are clustered at the top of the stem.

The flowers of all orchids, though highly variable between species, are easily recognisable by the lay person, because of the features common to them. They all consist of three inner petals that are framed by three outer sepals. The two side petals are mirror images of one another while the bottom petal is quite different from the other two, and is known as a lip. The sepals and petals of lesser twayblade’s flowers are about 2-3 mm long, while the lip is from 3-6 mm long and is forked or bifurcated in the middle, forming two long, narrow spear-like lobes in its lower part. The ovary is relatively large and prominent, and is green with reddish ribs.

There is a lack of data on the lifespan of individual lesser twayblade plants, although they are perennial, and it is estimated that they live for ‘a few years’.

In Scotland, lesser twayblade flowers from May to July, and pollination is by insects. These include small flies, beetles, ichneumon wasps and female gnats ( Sciara thomae ). In North America, fungus gnats have often been observed acting as pollinators. After pollination, the flowers persist for some time while the ovary ripens to form the seed capsule. The seeds are minute and dust-like, and are dispersed by the wind. Lesser twayblade also reproduces vegetatively, with new shoots growing off the roots, or rhizomes.

Little information is available about the germination of seeds in natural conditions, and it is not known if there’s a period of dormancy first. In a cultivated situation, 15 months after germination, the new plant was still a protocorm (the initial stage of development of all orchids), and consisted of a small ball, or embryo, with a few hundred cells. Another report indicated that the first green leaf appeared after 2-3 years of development of a plant’s roots underground. The closely-related common twayblade does not produce a green leaf until its fourth year, and apparently does not flower until it is 12-15 years old.

Like all orchids, lesser twayblade has a crucial mycorrhizal relationship with fungi. Because of the lack of food resources in their minute seeds, and the absence of seedling leaves (which enable other plants to gain energy from the sun through photosynthesis at an early stage in their growth), orchids are dependent, when they are young, on an external food source for their nutrients.

The relationship is an endomycorrhizal one, in which the fungus lives inside the cells in the orchid’s roots, although a part of it grows outside the plant and accesses nutrients that are passed to the host as carbohydrates. In the case of lesser twayblade, the mycorrhizal partners are fungi in the genus Rhizoctonia , and the fungal hyphae occur in the long, hairy roots, but not in the rhizomes, which are storage organs. In most orchids, it is thought that the nutrient flow is one way only, from fungus to orchid, thereby making the mycorrhizal relationship a parasitic one, rather than a mutually-beneficial symbiotic one. It is unknown whether the orchid continues to draw nutrients from the fungus once it has green leaves and can therefore produce its own food through photosynthesis.

Like many plants in the heather moorlands and woodland areas where it grows, lesser twayblade suffers from overgrazing by red deer (Cervus elaphus ) and sheep. However, some grazing at a reduced level is beneficial, as it limits the height of the surrounding vegetation, thereby making it easier for the twayblade to grow successfully. Often overlooked because of its small size, the lesser twayblade is a beautiful plant that requires further study for a fuller understanding of its lifecycle and role within the ecosystem.

The other main relationship that lesser twayblade has with other organisms is the provision of nectar to the insects that act as its pollination agents. Nectar is secreted in a groove that runs centrally down the lip of each flower, and provides the ‘reward’ that attracts the insects that transfer pollen from plant to plant, thereby facilitating sexual reproduction of the twayblade.

Alan Watson Featherstone