1. Dioon occurs in abundance at Chavarrillo, Mexico. 2. It is probable that plants often reach an age of more than one thousand years. 3. The ovulate strobilus is more like the loose ovulate strobilus of Cycas than the compact ovulate strobili of the other genera. 4. The megasporophylls are more leaf-like than those of any other genus of cycads except Cycas. 5. The integument consists of three layers: an outer and an inner fleshy layer, with a stony layer between them. The integument is probably double in nature, the outer fleshy layer representing the outer integument. 6. Only a small portion of the nucellus is free from the integument. 7. One vascular bundle passes from the sporophyll toward each ovule. Before entering the ovule the bundle forks, one branch forming the continuously branching system of the inner fleshy layer of the integument, and the other forming the slightly branched system of the outer fleshy layer. 8. The megaspore membrane varies in thickness from 3-4. 5 μ in young ovules to 9-10 μ...