are stimulants, and xanthines similar to caffeine. The astringency in tea can be attributed to the presence of polyphenols. These are the most abundant compounds in tea leaves, making up 30–40% of their composition. Polyphenols include flavonoids, epigallocatechin gallate (EGCG), and other catechins. Although there has been preliminary clinical research on whether green or black teas may protect against various human diseases, there is no evidence that tea polyphenols have any effect on health or lowering disease risk. Health effects Although health benefits have been assumed throughout the history of Camellia sinensis's consumption, there is no high-quality evidence showing that tea consumption gives significant benefits other than possibly increasing alertness, an effect caused by caffeine in the tea leaves. In clinical research conducted in the early 21st century, it was found there is no scientific evidence to indicate that consuming tea affects any disease or improves health. Black and green teas contain no essential nutrients in significant amounts, with the exception of the dietary mineral manganese, at 0.5 mg per cup or 26% of the Reference Daily Intake (RDI). Fluoride is sometimes present in tea; certain types of "brick tea", made from old leaves and stems, have the highest levels, enough to pose a health risk if much tea is drunk, which has been attributed to high levels of fluoride in soils, acidic soils, and long brewing. Cultivation and harvesting Camellia sinensis is an evergreen plant that grows mainly in tropical and subtropical climates. Some varieties can also tolerate marine climates and are cultivated as far north as Cornwall in England, Perthshire in Scotland, Washington in the United States, and Vancouver Island in Canada. In the Southern Hemisphere, tea is grown as far south as Hobart in Tasmania and Waikato in New Zealand. Tea plants are propagated from seed and cuttings; about 4 to 12 years are needed for a plant to bear seed and about three years before a new plant is ready for harvesting. In addition to a zone 8 climate or warmer, tea plants require at least 127 cm (50 in) of rainfall per year and prefer acidic soils. Many high-quality tea plants are cultivated at elevations of up to above sea level. Though at these heights the plants grow more slowly, they acquire a better flavour. Two principal varieties are used: Camellia sinensis var. sinensis, which is used for most Chinese, Formosan and Japanese teas, and