The leaves range from six to fourteen inches in length. One of its subvarieties is the dark-leafed Assam which grows at high altitudes at Darjeeling, India. The large-leafed trees that grow in China's Yunnan province are of the Assam type. The Cambodian type, growing to about fifteen feet, is also considered a tree, and is used mainly to cross with other varieties. Scientists currently believe that all of the different types of tea descended from one center, probably located near the source of the Irrawady River in Burma, and gradually spread throughout Southeast Asia in a fanlike movement.
Tea is grown commercially in a belt that circles the earth on either side of the equator.
The altitude and mountain mists help protect against excessive sunlight and create the right temperature and humidity to enable the leaves and buds to develop slowly and remain tender. Thus they produce a higher content of caffeine, amino acids, and essential oils. The frost, heat, and dampness of the lowlands are not conducive to good growth. Tea plants grow best in an acid soil of pH4.5 to 6.5 with a moisture content of 70 to 80 percent, and air humidity above 70 percent. Zhejiang, Hunan, and Sichuan rank first in order of importance, and Auhui and Fujian are also big producers.