A first flowering for Kew
With the appearance of a tiny greenish-white flower, Kew's horticulturists have recorded the first flowering in cultivation of Metastelma anegadense, a plant found only on the low-lying island of Anegada in the Caribbean.
For over ten years, members of Kew’s UKOTs team have paid regular visits to Anegada, one of the British Virgin Islands in the Caribbean. Despite its small size, Anegada is home to a variety of plants and animals, notably the Anegada rock iguana, a lizard found nowhere else in the world. Another of the island’s unique species is Metastelma anegadense, a scrambling thin-stemmed plant belonging to the milkweed family, Apocynaceae. It is commonly known as wire wist, reflecting the use of the stems as a natural string.
In its natural habitat Metastelma anegadense twists around both itself and supporting bushes
Collecting the seedlings
During fieldwork on the island in July 2011, Martin Hamilton and Michele Sanchez from Kew collected some tiny wire wist seedlings which had established themselves alongside a path. Rather than see these rarities face death by trampling, the Kew team carefully lifted them and took them back to the J R O’Neal Botanic Garden on Tortola, managed by the BVI National Parks Trust. Several seedlings were potted up at the Garden for cultivation there whilst five were cleaned and placed in pots of sterile perlite for the journey back to the UK to ensure that they did not represent a plant-health risk.
Metastelma anegadense seedlings on arrival at Kew's Quarantine House
Arriving at Kew
As soon as they arrived in Kew’s new Plant Quarantine Facility, the seedlings were transferred from the sterile growing medium used for transport and grown on in compost. Within six months of their arrival, the first of the wire wist seedlings had grown large enough to begin flowering - the first time that flowers have been recorded for this species outside its native environment.
The first Metastelma anegadense flower at Kew
Not only is this a major achievement for Kew’s horticulturists, but also enables Kew scientists to study this plant in more detial. Until now, it has never been possible to collect seeds from this plant as their mature fruits split open without warning and the seeds, which are covered with silky hairs, rapidly disperse in the wind. By hand-pollinating the flowers, researchers at Kew will be able to monitor the development of the fruits and discover how long it takes for them to reach maturity. This knowledge will help seed collectors harvesting wire wist fruits from their natural habitat for seed storage at the Millennium Seed Bank.
Metastelma anegadense fruits (photographed on Anegada) split open to release the wind-dispersed seeds
Challenges in the pollination process
But pollinating the flowers is not as easy as it sounds. Firstly they are very tiny, and Michele Sanchez and Noelia Alvarez from Kew’s Great Glasshouses team needed to use a magnifying glass as they began the pollination process. David Goyder, a botanist in Kew's Herbarium who specialises in the family Apocynaceae, explained that the unusual structure of the flowers would make them tricky to pollinate. He pointed out that the receptive part of the stigma (pollen-receptor) is on its lower surface, hidden from view behind the pollen-bearing stamens. And as in orchids, pollen is distributed en masse, not as individual pollen grains. All the pollen within two adjacent pollen sacs is joined together to form a removable pollinarium, which would normally become attached to nectar-feeding insects and be transported from one flower to another.
As with many other plants in this family, this species is probably self-incompatible. This means that for successful pollination, flowers from different plants need to be open at the same time so pollen can be transferred from one plant to the other. So to produce the next generation of wire wist plants in cultivation, Kew’s horticulturists will have to wait for more plants to flower and then cross pollinate them with ‘surgical’ precision. If successful, the team also wants to collect seed at different stages of ripeness to test the effect of age on germination.
The good news is that both cultivation and seed storage will ensure that this Critically Endangered species can be conserved away from the threat of habitat loss that it faces in some parts of Anegada.
- Marcella -