Although resembling palms and tree-ferns in appearance, cycads differ in reproductive behaviour. All are dioecious, with separate male and female plants (2) (5) (6), and produce large seeds with a hard, stony layer (sclerotesta) surrounded by a fleshy outer coat (sarcotesta), which attracts animals that serve as dispersal agents (2) (6). Cycads do not produce flowers, the reproductive organs instead being borne on cones, which are formed from modified leaves. Although previously thought to be wind-pollinated, cycads are now known to be mostly pollinated by insects such as beetles (2) (5) (6). The female cone of Dioon spinulosum is the largest of any gymnosperm, at up to 80 centimetres in length and 18 kilograms in weight, and containing up to 300 seeds, each of which measures around 4 to 5 centimetres in length (2) (3) (5) (6). The male cones are slightly smaller, at up to 55 centimetres in length (6). The female plant only produces a single cone at each reproductive cycle, generally every three to four years (2).
All cycads are slow-growing and long-lived (5) (6), with wild specimens of Dioon spinulosum estimated to reach an impressive 500 to 1,000 years old (5). In addition to normal roots, cycads also possess specialised roots containing cyanobacteria that form a symbiotic relationship with the plant, providing it with extra nutrients by converting (fixing) atmospheric nitrogen into a usable form. Cycads also possess roots which can be retracted for protection against fire and drought (2) (6).