Zoid - Evolution

Evolution

In plants, the zoid, or swimming sperm, is considered to be a trait of the "lower" land plants. In aqueous environments, the necessity for motile reproduction is obvious, but on land this adaptation losses its relevance. The zoid is most common among the non-vascular plants and the "lower" vascular plants. It is hypothesized that as the land plants evolved enclosed ovules, the necessity for a film of water and therefore motile sperm became unnecessary. The occurrence of motile, flagellated sperm or zoids in angiosperms is incredibly rare.

Along the same lines, the Ginkgo is a species that has no close living relative. It is believed to be most closely related to the giant seed ferns which date back to the Jurassic era. This represents what would also be considered a "lower" land plant. Ginkgo were originally classified in the Taxaceae, or yew, family. When it was discovered that ginkgo had motile sperm, they were moved to their own family, Ginkgoaceae.