I’ve mentioned the bizarre-looking Rheum nobile before in a blog but a reference to a New Scientist article about it by Henry Nicholls piqued my interest again. Most plants growing where Rheum does, at altitudes of 4000 to 4500 m, hug the ground to avoid the cold and wind, especially given the lack of soil to give them a decent foothold. However Rheum nobile has a strategy all its own which allows it to grow up to two metres high in this unpromising environment – it grows it’s own greenhouse!
Rheum nobile. Image: Bhutan tours
A column of overlapping, translucent yellowish leaves or bracts erupts from the normal-looking green leaves at the base of the plant. Within this self-constructed greenhouse the flower spike develops. The tiny green flowers open for pollination, and seeds and fruit form, whilst the spike is still enclosed within the bracts. The bracts only wither and fall away at the end of the season to allow dispersal of the seeds.
It was Joseph Hooker who first came across this plant in Sikkim in the 1840s and recognised it as a member of the dock family, or Polygonaceae, like our more familiar garden rhubarb (Rheum rhabarbarum). The stems are eaten locally, in just the same way, and are believed to have a number of medicinal properties. Though we may associate it with Yorkshire, our rhubarb was also first imported from China along the Silk Road in the 14th Century.
Hooker’s scene showing Rheum nobile in its natural environment. Image licensed under public domain.
Hooker’s botanical sketch of Rheum nobile. Image: Wikimedia commons
The plant’s ‘greenhouse’ is believed to have multiple functions; light passes through the thin, translucent bracts, warming up the air within, which both speeds up flower growth (important where the growing season is short) and creates a microclimate attractive to pollinating insects such as fungus gnats. As there is no such thing as a free lunch in the natural world, the gnats also lay their eggs inside the flowers and the larvae feed on developing seeds. This might seem like an expensive side effect for the plant but it’s actually more like a loose symbiosis; the next generation of plants will need pollinators, after all.
The temperature inside the bracts can be up to 10 °C warmer inside the bracts than outside according to work by Bo Song et al. (2013). Song’s team also carried out a series of experiments where they removed the bracts from flowers at different times to see what effect it would have on the plants’ ability to reproduce. They found that both the pollen grains and the seeds of plants with the bracts kept intact germinated better than those where the bracts had been removed, demonstrating the efficacy of the greenhouse.
From the paper by Bo Song et al., 2013.
What does not pass through the bracts is potentially damaging UV wavelengths of light. The bracts are pigmented with flavonoid molecules which absorb this, and protect the flower within. The high levels of UV light at the altitude where these plants are found are known to reduce germination of pollen grains, which in turn will affect the number of seeds the plant can produce. The bracts also act as a kind of umbrella, sheltering the flowers from the worst of the summer’s heavy rains. A truly multi-purpose evolutionary adaptation!
Bo Song, Zhi-Qiang Zhang, Jűrg Stöcklin, Yang Yang, Yang Niu, Jian-Guo Chen & Hang Sun (2013). Multifunctional bracts enhance plant fitness during flowering and seed development in Rheum nobile (Polygonaceae), a giant herb endemic to the high Himalayas. Oecologia, 172, 359-370.
Nicholls, H. (2013). Extreme rhubarb: The plant that grows a greenhouse. New Scientist, 2939.
Plants for a future plant database (www.pfaf.org)