Field of Science

What is a seed?

Seeds are perhaps the most misunderstood and misinterpreted of botanical subjects. You plant a radish seed, a radish grows, flowers, gets pollinated, fruits, and the seeds within contain an embryonic radish plant. This seems simple enough, rather like chickens. In fact the traditional name for an immature seed at the time of pollination is an ovule, literally an egg. So of course the floral structure that houses ovules must be an ovary. Oh so very like an animal, NOT!

An ovule is actually a sporangium, a jacketed, unopening sporangium that produces 1 spore via meiosis thus reducing the chromosome number from diploid to haploid. The spore develops into a female organism, a gametophyte, that ultimately produces a real egg. Pollen is also a spore at its inception, and it too develops into a haploid male organism, albeit a very tiny and reduced fellow. Having a small dispersible male solves the swimming-sperm problem that land plants with free-living gametophytes have, e.g., ferns.

Pollination is the completion of the male’s dispersal trip. A pollen tube that grows from him to her delivers the sperm, fertilizes the egg, and results in a new embryo. When the entire kit and caboodle is mature, the jacketed sporangium, what’s left of the female, and the resulting embryo of a new diploid plant (a sporophyte) gets dispersed as a seed.

The seed plant life cycle only makes sense as an evolutionary solution to a reproductive problem that limited the distribution of land plants.

Here are some great false color images of seeds and pollen complements of the Royal Botanical Garden at Kew. HT to DD at Neuron Culture.

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