The groove between areole and axil on the upper surface of the tubercle is an organ typical of the genus. The point of time and the way of arrival of the grooves are diagnostic characteristics for certain groups of Cory-phantha spp. (see Chap. 5). Young grooves often produce white wool which often disappears with age. Another function of the groove is the production of offsets or stolons. Coryphantha spp. usually sprout from old tubercles near the ground from the groove directly behind a spine-bearing areole and never from the axil itself. Tubercles which have already reached into the ground may also form stolons in certain species from which young offsets originate some distance from the mother plant (e.g. in C. tripu-gionacantha, C. glassii).

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