Growing-Grapes.org

An excellent question!

It turns out that most fruits today do not come from seeds. They come from cuttings instead. This is true of grapes, blueberries, apples, cherries, etc. (pretty much all fruits except citrus, although scientists are working on that, too). A piece of a vine or branch is cut off, dipped in rooting hormone and then placed in moist dirt so that roots and leaves form. Because they come from cuttings, new grapevines are essentially clones of the vine they were cut from.

Seedless grapes actually do contain seeds at some point. But a genetic error prevents the seeds from forming hard outer coats like normal seeds do.

On the other hand, seedless watermelons require a chemical treatment of the parent plant. Simply stated, the number of chromosomes (the threadlike bodies within cells that contain the inheritance units called genes) in a normal watermelon plant is doubled by the use of the chemical colchicine. Doubling a normal (diploid) watermelon results in a tetraploid plant (one having four sets of chromosomes). 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 it produces seedless watermelons. Seed of seedless varieties are available from most major seed companies.