How do leaf scars help estimate the age of a cycad?

Because cycads do not produce annual growth rings, scientists estimate the age of these plants by counting the whorls of leaf scars on their stems.1

These scars are formed by the hardened bases of leaves and cataphylls that remain on the trunk as the plant grows. By relating these whorls to the plant's annual or biennial production of new leaf flushes, researchers can approximate the age of the specimen. Using this method, it has been estimated that some cycads, such as those in the genera Dioon and Macrozamia, may reach up to 1,000 years of age, though such extreme longevity is considered doubtful for most species.1