I didn't know either, so I did a search and found this.

WHERE DO SEEDLESS WATERMELONS COME FROM?

This article is excerpted from “Seedless Watermelon Production” by Jerry Parsons, Larry Stein, Tom Longbrake, Sam Cotner, and Jerral Johnson, published by the Texas Agricultural Extension Service.

The seedless watermelon is now a reality. Seedless watermelons -- sweet inside but without the numerous seeds found in conventional watermelons -- are the ultimate in convenience food. The obvious question asked about growing seedless watermelons is: ‘How does one obtain seed of a seedless watermelon?’ Obviously, you cannot save seed from a seedless watermelon. So, where do the seeds come from? Simply stated, the chromosome number (threadlike bodies within cells that contain the inheritance units called genes) of a normal watermelon plant is doubled by the used of the chemical colchicine. Doubling a normal (diploid) watermelon results in a tetraploid (have four sets of chromosomes) plant. When the tetraploid plant is bred back or pollinated by a diploid or normal plant, the resulting seed produces a triploid plant that is basically a ‘mule’ of the plant kingdom, and produces seedless watermelons. Seeds of seedless varieties are available from most major seed companies. Seedless watermelons are a warm-season crop, preferring relatively high temperatures for optimum growth. Daytime temperatures of 80 to 95 degrees F, and night temperatures of 60 to 70 degrees F are best. When temperatures are lower, plant growth is slowed considerably. With favorable weather, seeded fields can produce ripe fruit in 85 to 100 days.

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Terry

May the force be with you