Watermelons are fruit that develop from the ripened ovary of a flower. Vegetables develop from leaves, stems or roots. A watermelon seed germinates to produce the first cotyledons, or leaf structures, and the first radical, or root, for young watermelon plants. For watermelon fruit to set, the male watermelon flower must be visited by an insect, usually a honey bee or bumble bee, which carries the male's pollen to a female flower on the same plant or a nearby plant.
Watermelon flowers are epigynous, meaning the flower petals, the sepals that cover the petals before the bud opens, and the male flower parts, or the corolla, calyx and androecium, are all attached to the top of the ovary. "Epi" means above and "gynous" means ovary. Another term for the epigynous flower form is "inferior ovary," meaning that the ovary is under the other flower parts. Watermelons and other members of the Cucurbitaceae family, including cucumbers, squash, pumpkins and cantaloupe are classified as "pepo," or false berry, based on the epigynous ovary ripening to a fruit with a hard rind covering a fleshy placenta that carries the seeds.
Flowers in Axils
Watermelon stems are slender vines that creep along the ground to as much as 15 feet in length. The vines bear 6-inch wide by 10-inch long leaves that are deeply lobed. Pale green to yellow flowers are borne in the axils, where the leaves are attached to the stems. In most watermelon, pistillate, or female, flowers occur in every seventh axil, and the staminate, or male, flowers are more abundant. Watermelon flowers are not self-sterile, so the pollen of male flowers from the same vine will fertilise female flowers as successfully as pollen from another plant.
And the Bees
Watermelon flowers open one to two hours after sunrise, exposing nectar to attract pollinating insects. The anthers bear large grains of sticky pollen and separate from the stamen as the flower opens. Pollinators are mostly honeybees, particularly in commercial operations where hives are brought to the fields, however bumblebees pollinate watermelon as well. The stigma on the female pistil is open all day, but most pollination occurs between 9 and 10 a.m. Bees visit each flower for only a few seconds, but better fruit develops from flowers visited by eight or more bees collectively depositing 1,000 or more pollen grains. Longer ovaries tend to be more successfully pollinated than shorter ovaries.
Seedless watermelons are grown from seed produced by cross-pollinating diploid watermelon with tetraploid hybrids. The normal diploids have two sets of chromosomes in their cell nucleus. The hybrid tetraploids have four sets of chromosomes and are usually self-sterile, so that they are pollinated by the diploids. The fruits of this match produce seeds that in the next generation can only develop tiny white seed coats, instead of viable seeds. To the consumer, they appear seedless. Diploid watermelon are planted among seedless varieties, because hormones from healthy pollen produce a hormone that matures the ovary to fruit.