Woodland Beauties
Limodorum is a genus of only three terrestrial orchids, all of which are exclusive to the Old World. The most familiar of them is Limodorum abortivum, the Violet Limodore, a species with a wide distribution throughout parts of Europe, the Mediterranean and western Asia.
When it is in flower, between April and June, it produces spectacular stems up to 85 cm in height that bear between 10 and 20 flowers. The entire plant has an intense violet colouration and, when fully open, individual flowers may be up to 6 cm across, with a strongly marked, curved lip and a long downward-pointing spur. The plant is normally encountered singly or in small groups in woodland habitats where flash photography unfortunately leaches away its glorious colour. Seen in the sunlight of a forest glade, however, the Violet Limodore is positively luminescent.
A Subterranean Secret
Even the briefest aquaintance with this unusual orchid will beg the question: where are the leaves? Many herbaceous plants flower before or after developing their foliage, but throughout its entire life cycle, the limodore never produces anything other than a set of greatly-reduced scale leaves on the flowering stem. In fact, this orchid contains insufficient chlorophyll to effectively sustain the process of photosynthesis by which all green plants are able to manufacture their food. Not only this, but for the first eight to ten years of its life, the limodore develops its short thick rhizome and tangle of fleshy roots entirely below ground.
How, then, does this remarkable orchid obtain sufficient energy to survive? The answer lies beneath the soil in a complex interaction with other living organisms that is astonishing even among those most highly evolved of flowering plants, the orchids. The majority of plants, from liverworts to conifers and broadleaved trees, are now known to rely on a mutually-dependant relationship with fungi known as mycorrhiza to obtain at least some of their nutrients.
The roots of such plants are completely surrounded by hyphae, the fine branching threads that make up the greatest part of a fungus (the familiar "toadstools" are only the visible fruiting bodies of some kinds of fungi). This intimate contact between the hyphae and the roots of the plant enables an exchange of nutrients to take place. The fungi are able to use enzymes to obtain otherwise inaccessible minerals in the soil, which they transfer to the plants: in addition, the enormous surface area of the hyphae enables them to absorb extra water and transfer some of this to the partner plants. In turn, the plants transfer compounds produced by photosynthesis, such as carbohydrates, to the fungi.
The Energy Thief
Orchids are especially dependant on mycorrhizal associations. Their tiny seeds contain an embryo but no food reserves, so the germinating orchid must establish a mycorrhizal relationship with a fungus immediately if it is to survive. In the case of the limodores, this usually takes place with members of the Russulaceae, an important group of fungi that contains many familiar parasol mushrooms. Russulas are primarily associated with trees, with whom they exchange nutrients, but if limodores are present the hyphae of the fungus also grow around the roots of the orchids. Because the limodores cannot generate their own food via photosynthesis, they are entirely reliant on the fungi to whom they give nothing in return. The limodore is in fact a parasite on the unfortunate fungus and is able to utilize the russulas ability to obtain minerals from the soil and the food it has been supplied with by the surrounding trees. Plants that are able to parasitise fungi in this way are called myco-heterotrophs.
Since the limodore is not directly dependant on photosynthesis, it is able to survive for years underground without flowering, even after it reaches maturity. Its peculiarity does not stop here. Although the flowers are known to be pollinated by various bee species, they sometimes fail to open but are nethertheless able to self-pollinate. This ability is called cleistogamy and enables viable seeds to be produced even if the pollinating insects have been unable to carry out their function.
An Environmental Indicator
The Violet Limodore has a wide distribution and is found in different kinds of woodland and soil types from sea level to an altitude of 1,300 metres. In southern Spain, for example, a frequent habitat is mixed oak and pine forest which it shares with other emblematic plants such as paeonies and a variety of other orchids. Despite its apparent adaptability, the plant is sensitive to environmental changes and because it requires such exlusive conditions in terms of host fungi and their symbiotic partners, it is vulnerable to disturbance.
It is highly unlikely that the limodores will ever be cultivated under anything other than laboratory conditions. These are plants to enjoy in-situ and then to leave well alone in the knowledge that their presence is a sure indication of a complex and healthy ecosystem.
References
Karl Peter Buttler: Field Guide to Orchids of Britain and Europe The Crowood Press; Swindon: 1991: ISBN 1 85223 591 8
Manuel Becerra Parra, Estrella Robles Domínguez: Guía de campo de las orquídeas silvestres de Andalucía Editorial la Serranía; Ronda: 2009: ISBN 978 84 96607 70 5
Peter Ashby - Peter Ashby lives near Granada in southern Spain, is an expert on natural history, and a dedicated traveller, writer and hispanist.
