Cycads: Getting to grips with the reproduction on land
A thallus life cycle stage induces considerable vulnerability, since it is small and possesses little or no protection against herbivory or desiccation. A less vulnerable sexual stage appeared about 350 million years ago with the evolution of plants like the cycads which exist today. Cycads superficially resemble ferns, with some species having spores of the archaic form which are distributed by wind. In other species some spores become large and remain attached to the parent plant where they develop into a conical-shaped structure containing egg cells (that is functionally equivalent to a thallus). When a wind-blown spore, now called a pollen lands on these egg bearing cones, no filmy thallus develops, but a pollen tube which burrows its way into the female cone occurs. The large sperm cell is transported down to the bottom of the pollen tube, where it enters a small drop of fluid secreted by the surrounding tissues of the cone, there it swims to the egg cell and fuses with it and thereby completing the fertilization process.
Cycads are an ancient group of seedplants characterized by a large crown of compound leaves and a stout trunk. They are evergreen, gymnospermous, dioecious plants having large pinnately compound leaves. They are frequently confused with and mistaken for palms or ferns, but are unrelated to either, belonging to the division Cycadophyta.