One specimen of Dioon is thought to be approximately 1,000 years old. I could not find its height, but it is less than 18m. The anatomy of the trunk is distinctive. The pith and cortex contain large amounts of starch. The vascular strands to the leaf go round the stem for a considerable distance before turning up and entering the leaf.
All cycads are dioecious.
The strobili are large and, in most genera, they are terminal. The female strobili consist of sporophylls that bear seeds where the pinnae of a leaf would be. The male strobili, which are usually smaller than the female strobili, are also produced on the sides of sporophylls, but smaller sporophylls. The sperm are flagellated. It used to be thought that cycads were entirely wind-pollinated, but a few years ago it was shown that seed set is increased if insects are allowed access to the male and female strobili.