Gymnosperms : Ovules

Each ovule is actually the female spore-producing organ (megasporangium, called nucellus) surrounded by a protective envelope called integument. Inside the ovule, a cell undergoes meiosis to produce four megaspores (female spores). Three of them degenerate; the remaining megaspore is retained inside the megasporangium and gives rise to a female gametophyte (megagametophyte) inside the spore. The gametophyte produces female sex organs (archegonia), each of which contains an egg.

ovule
  strobili
a gymnosperm ovule   young pine cone

Because the ovules are naked, pollen grains can enter the ovules through an opening (micropyle) on the integument. Each pollen grain germinates inside the ovule into a pollen tube, which delivers and releases two male nuclei or two flagellate sperms. One of the male nuclei or sperms fertilizes an egg in a female sex organ, resulting in the formation of a zygote. The zygote develops into an embryo, whereas the ovule matures into a seed and the integument serves as the seed coat. A seed in gymnosperms, therefore, is a package composed of the diploid sporangium tissue from the mother plant, the haploid female gametophyte tissues serving as food reserves in the seed, and a diploid embryo representing the next sporophyte generation.

Because the seeds are not enclosed in an ovary or fruit, gymnosperms are generally regarded as bearing 'naked' seeds. The 'naked' ovules do not mean that they do not have any protection. In fact, the ovules are generally borne on stalks or flattened structures (megasporophylls) that cluster together to form cones. The cones do offer some protection but not exclusion of pollen grains from direct contact with the ovules.


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