Normally, watermelons are "diploid." This means they have two sets of 11 chromosomes, the structures that contain an organism's genetic material. They get one set of chromosomes from each parent, for a total of 22.

Producing a seedless watermelon involves three steps. First, a plant is treated with colchicine, a substance that allows chromosomes to duplicate, but prevents the copies from being distributed properly to dividing cells. As a result, a plant with four sets of chromosomes is created, a "tetraploid."

In the second step, a tetraploid plant is crossed with a diploid to produce offspring that are….? That's right, triploid, with three sets. They get half the number of chromosomes from each parent.

Finally, the triploid seeds are grown into plants. Although they must be germinated under very careful conditions, once the seeds grow into small plantlets, they grow just like normal watermelon plants. They can produce flowers and the female flowers can produce fruit, the watermelons.

However, triploids cannot reproduce sexually. The reason is that the cell divisions that produce pollen and egg cells are very particular; they require precise alignment of chromosome pairs in the middle of the cell, an impossible task with an odd number of copies. Since the triploids have three sets, this crucial process gets mixed up and the eggs inside the watermelon are never formed. Without eggs, the seeds do not grow.

So far so good, except that pollen is still needed to trigger the female flowers to make the watermelons. Since triploid plants cannot produce pollen, farmers grow diploid "pollenizer" plants near the triploids. The diploids produce the necessary pollen, bees carry it to the female triploid flowers, and the seedless watermelons grow. Actually, a few seeds develop partially, so you can find some white, empty seed coats in the red flesh.

When plant breeders developed seedless watermelons, they also selected them for other traits such as sweetness, disease resistance, longer shelf life, and nutritional value.

The people of Knox City, Texas proclaim their city the "Seedless Watermelon Capitol of the World." Perhaps on your next summer vacation you can venture to Knox City for the 17th annual Seedless Watermelon Festival, where you can eat all the free watermelon you please. But don't expect to take part in a seed spittin' contest!