Li Ling remembered a passage in Materia Medica Explained, a Song-dynasty pharmacopoeia from his previous life's world: "Agarwood trees grow throughout the prefectures of Lingnan, especially those beside the sea. Their trunks intertwine and their branches join, covering ridge after ridge without a break for a thousand miles."
The Incense Compendium likewise recorded that fewer than one or two trees in a hundred contained incense, for agarwood could only form fragrant resin after exposure to water.
Mountain folk entered the forests and hacked wounds into crooked trunks and slanting branches with knives, allowing rainwater to soak them year after year. Such trees generally needed to be more than ten years old before their resin glands were developed enough to form fragrant deposits, which then took a long time to become agarwood.
There were many varieties. Among those of superior quality, only those produced in Qiongzhou and Yazhou were called horn agarwood.
What came from branches was yellow agarwood, suitable for medicine. What formed along the bark was called blue cassia, with an exceptionally fresh fragrance.
Thin flakes that formed after years in the soil without requiring wounds or scraping were called dragon scale. Those that curled of their own accord after being shaved and were supple when chewed were called yellow-wax agarwood, an especially rare variety.
Broken branches and withered trunks might yield agarwood, plank incense, or yellow-ripened incense. What came from trees that withered and died naturally was called water-plate incense.
Once it was sawn off and the whitewood scraped away, pieces with fragrant deposits forming speckles were called partridge spots. When burned, their fragrance was exceptionally pure and potent.
Beyond these varieties, there were also hemp-leaf, bamboo-leaf, and more than a score of other kinds.
It was never easy for agarwood to form fragrant resin. Even within large