Nymphaea thermarum

Nymphaea thermarum is a species of water lily that is endemic to Rwanda. Once thought to be extinct in the wild, all wild plants were believed to be lost due to destruction of its native habitat, but it was thought to be saved from extinction when it was grown from seed at the Royal Botanic Gardens, Kew in 2009. A previously-unknown wild population was discovered in 2023.

Taxonomy
Nymphaea thermarum was discovered in 1987 by German botanist Eberhard Fischer. The specific epithet, thermarum, refers to the hot spring and temperature that provided its native habitat. There are no common names for the plant, though Kew Gardens is informally calling it "pygmy Rwandan water lily".

Within the genus Nymphaea it is placed within Nymphaea subgen. Brachyceras.

Cytology
The diploid chromosome count is 2n = 28. The genome size is 498.78 Mb.

Description
This is the world's smallest water lily species; the pads (leaves) of N. thermarum can measure only 1 cm across, less than 10% the width of the next smallest species in the genus Nymphaea (though they are more usually about 2 cm or 3 cm). By comparison, the largest water lily, Victoria amazonica, has pads that can reach 3 m. Nymphaea thermarum forms rosettes 20 to 30 cm wide, with bright green lily pads growing on short petioles. The very small flowers are white with yellow stamens, with the flowers held upright a few centimeters above the plant.

Breeding
They can self-pollinate, and after blooming the flower stalk bends so the fruit contacts the mud. The fruit contains 300 to 400 seeds. The sepals are slightly hairy, and as large as the flower's petals. The plant is a tropical day bloomer displaying protogynous flowering patterns, opening early in the morning on the first day with female floral functioning, closing in early afternoon, and opening on the second day with male functionality. It is in the Nymphaea subgenus Brachyceras, though the leaves are more typical of the subgenus Nymphaea. It apparently does not form tubers. Seeds are large for plants in subgenus Brachyceras. The lifespan of Nymphaea thermarum can be greater than 10 years.

Conservation
The plant's native habitat was damp mud formed by the overflow of a freshwater hot spring in Mashyuza, southwest Rwanda. It was thought to have become extinct in the wild around 2008, when local farmers began using the spring for agriculture. The farmers cut off the flow of the spring, which dried up the tiny area—just a few square metres—that was believed to be the entire habitat. Before the extinction of the first known population, Fischer sent some specimens to Bonn Botanic Gardens in Germany when he saw that their habitat was fragile. The plants were kept alive at the gardens, but botanists could not solve the problem of propagating them from seed.

The first published occurrence of N. thermarum germination was by Carlos Magdalena, at the Royal Botanical Gardens, Kew. By placing the seeds and seedlings into pots of loam surrounded by water of the same level in a 25 C environment, eight began to flourish and mature within weeks and in November 2009, the waterlilies flowered for the first time. During this time, a rat had eaten one of the last two cultivated plants in Germany. With the germination problem solved, Magdalena says that the tiny plants are easy to grow, giving it potential to be grown as a houseplant. In January 2014, a surviving water lily was stolen from the Royal Botanic Gardens.

Uses
It has been proposed to be used as a model species for basal angiosperms, due to its small size, rapid lifecycle, and small genome.