The ginkgo is a dioecious gymnosperm that produces microstrobili and ovules on separate trees in the spring after maturity is reached. The male reproductive structures are similar in shape to small catkins and the female structures are naked, possess a fleshy integument, and occur in pairs. They both can be found on short spur shoots located at the bases of leaves. A tiny opening in the integument of each ovule enables pollen shed from microstrobili into the air to penetrate the ovule. Following pollination, the ovule expands significantly in size and the integument differentiates into a number of different layers. The outermost fleshy layer can cause nausea if eaten and has a rancid smell, which generally makes male ginkgos more popular as ornamentals. Usually fertilization occurs after ovules have dropped from the tree, a few months after they were pollinated. The embryo that develops is dicotyledonous.