Rheum nobile, the Sikkim rhubarb or noble rhubarb or पदमचाल, is a giant herbaceous plant native to the Himalaya, from northeastern Afghanistan, east through northern Pakistan and India, Nepal, Sikkim (in India), Bhutan, and Tibet to Myanmar, occurring in the alpine zone at 4000–4800 m altitude. It is an extraordinary species of rhubarb (genus Rheum). At 1–2 m tall, the monocarpic inflorescences of R. nobile tower above the other shrubs and low herbs in its habitat, and it is visible across valleys a mile away. R. nobile is often called a glasshouse plant because of its outer curtain of translucent bracts which pass visible light, creating a greenhouse effect, while blocking ultraviolet radiation. These are likely defenses against the increased UV-B exposure and extreme cold in its high altitude range. Description An individual R. nobile is a conical tower of delicate, straw-coloured, shining, translucent, regularly overlapping bracts; the higher ones have pink edges. Large, glossy, green radicle leaves, with red petioles and nerves, form a broad base to the plant. Turning up the bracts reveals membranous, fragile, pink stipules. Within these are short branched panicles of diminutive green flowers. The root is often long and as thick as an arm, and bright yellow inside. After flowering, the stem lengthens and the bracts separate one from another, turning a coarse red-brown. As the fruit ripens, the bracts fall away, leaving a ragged-looking stem covered with panicles of deep brown pendulous fruits. As Hooker put their appearance: "In the winter, these naked black stems, projecting from the beetling cliffs, or towering above the snow, are in dismal keeping with the surrounding desolation of that season." Karyotypy R. nobile has a chromosome count of 2n=22. Bracts The bracts of R. nobile are 110-170 µm thick and do not differentiate into palisade and spongy layers. They selectively block ultraviolet radiation while letting almost all visible light through; thus the developing flowers and the apical meristem are protected from the intense radiation found in high altitudes. The major UV blockers found in the bracts are all quercetin flavonoids: Rutin, quercetin 3-O-rutinoside: widespread in higher plants and previously reported in leaves and petioles of other Rheum species Guaijaverin, quercetin 3-O-arabinoside: first report in Rheum Hyperin, quercetin 3-O-galactoside: widespread in plants and previously reported in leaves and petioles of R. rhaponticum Isoquercitrin, quercetin 3-O-glucoside: widespread in plants and previously reported in leaves and petioles of R. rhaponticum