Cycads

Cycads, like all seed-producing plants, have a dominant diploid sporophyte phase in their life cycle—this is the large, familiar, green plant seen in nature. Cycads and other gymnosperms do not have true flowers and their seeds are borne naked. In the more evolutionarily recent angiosperms (flowering plants), the seed is enveloped by a coat or fruit which originates from the ovary.

All species of cycads are dioecious, meaning the male and female reproductive structures are borne on separate plants. The male reproductive structure, known as an androstrobilus, superficially looks like a large pine cone, though it is much simpler in structure. It consists of many densely packed, modified leaves, known as microsporophylls. Each microsporophyll produces a large quantity of pollen grains on its dorsal surface. The pollen grain is the small, multicellular, male haploid gametophyte phase of the cycad life cycle. The pollen is dispersed by wind or by insects to the gynostrobilus, or the female reproductive structure.

The gynostrobilus of cycads also looks like a large pine cone, but it has a morphology different from the androstrobilus. When a pollen grain lands on the gynostrobilus, it germinates and grows a pollen tube, a long tubular cell that extends to deep within the multicellular, female haploid gametophyte. Then a sperm cell of the pollen grain swims through the pollen tube using its whip-like tail, or flagella, and fertilizes the egg to form a zygote. The zygote eventually develops into an embryo, and then a seed. Cycad seeds are rich in starch and have a pigmented, fleshy outer layer known as the sarcotesta. The seeds are often dispersed by birds or mammals, which eat them for the nutritious sarcotesta, and later defecate the still-viable seed.

It is significant that the cycads have flagellated sperm cells, which is considered a primitive (i.e., ancient) characteristic. Other evolutionarily ancient plants, such as mosses, liverworts, and ferns, also have flagellated sperm cells. More evolutionarily recent plants, such as the flowering plants, do not have flagellated sperm cells. In fact, other than the cycads, only one species of A cycad in Hawaii. JLM Visuals. Reproduced with permission. gymnosperm, the gingko, or maidenhair tree (Ginkgo biloba), has flagellated sperm cells. In other gymnosperms and angiosperms, the sperm is transported directly to the female ovule by a sperm tube.