Gymnosperm cones, pollen and ovules

Gymnosperms are heterosporous and make haploid microspores and megaspores, inside sporangia borne on sporophylls. The sporophylls spiral along an axis to form loose or compact strobili (cones).

Cones with microsporophylls and microsporangia are male (microsporangiate) strobili. Each microspore becomes a male gametophyte reduced to a few cells, the pollen grain, which develops inside the microsporangium.

Cones with megasporophylls carrying ovules (megasporangia) are female (macrosporangiate) strobili. Pinus bears male and female cones on the same tree; in Cycas, male cones and megasporophylls are on separate trees.

One cell of the nucellus becomes the megaspore mother cell. The nucellus wrapped in its protective envelopes is the ovule.

The megaspore mother cell divides by meiosis into four megaspores. One of them, still inside the megasporangium, grows into a multicellular female gametophyte bearing two or more archegonia, and this gametophyte too stays inside the megasporangium.

Unlike bryophytes and pteridophytes, neither gametophyte of a gymnosperm lives independently; both stay within sporangia on the sporophyte.

Pollen grains released from the microsporangium are carried by air currents to the opening of the ovules. A pollen tube carrying male gametes grows towards the archegonia and releases them near the archegonium's mouth.

After fertilisation the zygote becomes an embryo and the ovule becomes a seed, which stays uncovered.