Camellia sinensis is the plant that produces all true tea. Green tea, black tea, oolong, and white tea all come from the leaves of this single species. The differences between these teas come not from different plants but from how the leaves are processed after harvest. Domesticated over 3,000 years ago in China, it remains one of the most widely consumed plants on Earth.
Origins in Southwestern China
The tea plant traces its roots to the Yunnan and Sichuan provinces of southwestern China. Genetic analysis suggests the two major varieties of the plant first diverged around 22,000 years ago during the last ice age, and deliberate cultivation began at least 3,000 years ago. Excavations at the Han Yangling Mausoleum near Xi’an revealed that tea drinking was already popular among Chinese emperors more than 2,100 years ago.
Two Main Varieties
There are two major varieties of the tea plant, and they look and behave quite differently.
The Chinese variety (var. sinensis) is a small-leafed shrub that thrives in cooler climates and high-altitude regions with significant temperature swings. Its leaves have a thick protective outer layer that shields them from sun and cold damage. This structure also gives the leaves a more subtle, delicate flavor. It’s the hardier of the two, surviving winters as cold as USDA Zone 6 (roughly down to -10°F).
The Assam variety (var. assamica) is a large-leafed plant named after the Assam district in northeastern India. It grows primarily in tropical and subtropical regions, including China’s Yunnan province, Myanmar, Laos, and Vietnam. Its leaves have a thinner outer layer, making them vulnerable to sunburn and frost, but a thicker inner tissue that produces a bolder, more robust flavor. It’s only cold-hardy to about Zone 7.
The plant itself produces small white, scented flowers that appear alone or in clusters of two to four. Its fruits are brownish-green capsules, each containing one to four round or flattened seeds. In cultivation, tea plants are kept pruned to waist height for easy harvesting, though left alone they can grow into small trees. The plant prefers acidic soil with a pH below 6.0.
How One Plant Becomes Every Type of Tea
The key variable that turns a fresh tea leaf into green, black, or oolong tea is oxidation, the same chemical process that turns a sliced apple brown. When tea leaves are bruised or rolled, enzymes inside the leaf react with oxygen and begin transforming the plant’s natural antioxidant compounds into different molecules that change the color, aroma, and flavor of the final product.
Green tea is not oxidized at all. Immediately after harvest, the leaves are heated (steamed in Japan, pan-fired in China) to deactivate those enzymes and lock in the leaf’s original green color and grassy, vegetal character. Black tea sits at the other end of the spectrum: the leaves are fully oxidized, which converts the plant’s natural antioxidants into the darker, more complex compounds that give black tea its deep amber color and malty richness. Oolong falls in between, with partial oxidation that can range anywhere from lightly floral to deeply toasted depending on how long the process runs. White tea undergoes minimal processing of any kind, simply withered and dried with little to no deliberate oxidation.
What’s in the Leaf
Tea leaves contain caffeine, a group of powerful antioxidants called polyphenols, and the amino acid L-theanine, which is largely responsible for tea’s reputation as a calming-yet-alert kind of stimulant. The caffeine content in your cup depends on the type of tea and how it’s prepared:
- Black tea: 40 to 60 mg per cup
- Oolong tea: 30 to 50 mg
- Green and white teas: 20 to 50 mg
- Roasted varieties (hojicha, kukicha): 10 to 30 mg
For comparison, a cup of drip coffee contains 95 to 120 mg of caffeine.
The polyphenols in tea leaves act as potent antioxidants, neutralizing unstable molecules in your body that contribute to cell damage and aging. These compounds can also boost your body’s own antioxidant defenses and help protect DNA from oxidative damage. Green tea retains the highest levels of these antioxidants because it skips oxidation. Research has found that green tea polyphenols support skin health by increasing collagen and elastin production while suppressing the enzymes that break collagen down. There’s also preliminary evidence that these compounds may offer some neuroprotective effects by counteracting processes involved in brain cell damage.
Safety of Tea vs. Concentrated Supplements
Drinking tea brewed in the traditional way is broadly considered safe. The average green tea drinker in Europe consumes roughly 90 to 300 mg of EGCG (the most abundant antioxidant in green tea) per day, and there’s no evidence of harm at those levels.
Concentrated green tea extract supplements are a different story. The European Food Safety Authority reviewed the clinical evidence and found that doses of 800 mg or more of EGCG per day, taken as a supplement, caused measurable signs of liver stress in clinical trials. Some supplements on the market contain up to 1,000 mg of EGCG per dose. France and Italy have both capped the recommended daily intake from supplements at 300 mg of EGCG for adults, and the U.S. FDA has required that human trials of oral EGCG be administered with food. The EFSA ultimately concluded it could not identify a dose of EGCG from concentrated extracts that could be guaranteed safe across all individuals.
The practical distinction: brewing tea leaves naturally limits your intake to a moderate range. Supplements can deliver several times that amount in a single capsule, which is where the risk of liver toxicity appears. If you drink tea, you’re very unlikely to reach problematic levels. If you take green tea extract pills, staying below 300 mg of EGCG daily and taking them with food is the more cautious approach.