Question

Why are parthenocarpic fruits produced by some plants and apomictic seeds by some others?

Answer 1

Some embryos of seeds can develop apomictically; that is, without development or fusion of gametes (sex cells) but with the normal structures (e.g., ovaries) otherwise being involved. An embryo may develop, for example, from a 2n nutritive cell or other diploid cell of an ovule, instead of from a zygote. After germination, this makes the plant that develops from such a seed the equivalent of a vegetatively propagated plant. Fruits that develop from ovaries having unfertilized eggs are said to be parthenocarpic. Such fruits are seedless and are found in navel oranges, supermarket bananas, and certain varieties of figs and grapes. Apomixis was discussed in Chapter 15.

To further complicate matters, some seedless fruits aren't parthenocarpic. For example, when Thompson seedless grapes are fertilized, the ovules don't develop with the fruit. Also, applying dilute hormone sprays to flowers can bring about artificial parthenocarpy. Seedless tomatoes are often produced in this way. Crossing watermelon varieties with different chromosome

Seed Plants: Angiosperms 449

numbers results in hybrid watermelons that are seedless because the chromosomes can't pair properly during meiosis, and fertilization and seed formation don't occur.

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