The citrus family (Rutaceae) contains some of the world's most delicious fruits, including numerous hybrid crosses between species. The popular tangelo grown in San Diego County is a hybrid produced by crossing a grapefruit (C. x paradisi) with a tangerine (C. reticulata). The grapefruit is sometimes called a pomelo, and this explains the blending (portmanteau word) of tangerine and pomelo. Actually, the grapefruit is a hybrid produced by crossing the shaddock or pummelo (Citrus maxima) with a sweet orange (C. sinensis). The shaddock is a large, thick-skinned, tropical citrus fruit up to six inches (15 cm) in diameter that is occasionally sold in supermarkets.

According to Apples: A Catalog of International Varieties by Tom Burford, there are 17,000 varieties of apples! Most of the apples grown commercially are probably diploid (2n), although there are many triploid varieties. For example, 'Gravenstein' apples are triploid with a chromosome number of 51 (3n=51). They are produced by the union of a diploid egg (2n=34) and a haploid sperm (n=17). This is accomplished by crossing a tetraploid plant (4n=68) with an ordinary diploid plant (2n=34). Because the triploid (3n) varieties are sterile, they must be propagated by grafting, where the scions of choice cultivars are grafted to hardy, pest-resistant root stalks.

Apples are mentioned throughout most of recorded human history. The generic name Malus is derived from the Latin word malus or bad, referring to Eve picking an apple in the Garden of Eden; however, some biblical scholars think the fig, and not the apple, was the forbidden fruit picked by Eve. One of the earliest records of any fruit eaten by people of the Middle East is the common fig (Ficus carica). Remnants of figs have been found in archeological excavations dating back to the Neolithic era, about 1000 years before Moses. The fig is also the first tree mentioned in the Bible in the story of Adam and Eve. There are some scholars who think the apricot is a more likely candidate because it was an abundant fruit (along with figs) in the ancient Palestine area. Other interesting tales about apples include Johnny Appleseed, William Tell, Sir Isaac Newton, and Apple Computers.

The rose family (Rosaceae) includes many economically-important fruit trees known as stone fruits in the genus Prunus. Each species has many vamed cultivated varieties (cultivars). Botanists have moved some of these species into separate genera, including Amygdalus (peach) and Armeniaca (apricot). Some examples of stone fruits are fuzzy-skinned peaches (P. persica syn. Amygdalus persica), smooth-skinned peaches called nectarines (another variety of P. persica), plums (P. domestica), apricots (P. armeniaca syn. Armeniaca vulgaris), and cherries (P. avium and P. cerasus). Like apples and pears, there are hundreds of cultivated varieties. These fruits are technically referred to as drupes because they consist of an outer skin or exocarp, a thick, fleshy middle layer or mesocarp, and a hard, woody layer (endocarp) surrounding the seed. The part of these fruits that is eaten by people is the mesocarp layer and also the exocarp if you don't bother to peel them. The woody endocarp layer protects the seed and probably aids in the dispersal of drupaceous fruits by hungry herbivores. In wild plants with drupes, the seeds can pass through the entire digestive system of grazing animals and be planted in new locations. The almond (Prunus amygdalus syn. Amygdalus communis) is also a drupe with a green exocarp and thin mesocarp surrounding the pit. When you crack open an almond to get the seed, you are actually cracking open the endocarp layer.

Some species of Prunus have been artificially crossed to produce some unusual hybrids. The peachcot (Prunus persica x P. armeniaca) is a hybrid between the peach and apricot; the cherrycot (P. besseyi x P. armeniaca) is a hybrid between the cherry and apricot; the plumcot (P. domestica x P. armeniaca) is a hybrid between the plum and apricot. Some of these hybrids have many different named cultivars, depending on which varieties of stone fruits have been crossed together. In addition, hybrids often retain more characteristics of one parent and are given special names. For example, some cultivars of plumcots are called "pluots" because these resemble plums more than apricots. Plumcots called "apriums" resemble apricots more than plums.

Modern varieties of the watermelon are derived from the native African vine Citrullus lanatus (syn. C. vulgaris). Cultivated for thousands of years in the Nile Valley, this species still grows wild in the arid interior where it supplies native people with water during drought seasons.

According to R.W. Robinson and D.S. Decker-Walters (Cucurbits 1997), wild populations of C. lanatus var. citroides, which are common in central Africa, probably gave rise to domesticated watermelons (var. lanatus). Wild, ancestral watermelons (var. citroides) have a spherical, striped fruit, and white, slightly bitter or bland flesh. The pale flesh tastes like the rind of a typical watermelon. They are commonly known as the citron or citron melon, not to be confused with the "citron" Citrus medica of the Citrus Family (Rutaceae). The citron is also called "preserving melon" because the fruit rind is used in preserves, jellies and to make pickles or conserves. Because of its high pectin content, it is added to fruit juices to make them jell more rapidly. One plant may produce up to 100 fruits, which are commonly fed to livestock. Citron melons become weedy vines in cultivated melon fields of North America, and are unmistakable among other cucurbits because of their pinnatifid (pinnately dissected) leaves. The citron is naturalized in the Cape Region of Baja California, along with the curious teasel gourd.

Modern triploid watermelons (with three haploid sets of chromosomes) are unable to produce viable gametes during meiosis, and much to the delight of growers, their ripened melons are seedless. [Note: The word "set" is defined here as one haploid set of chromosomes.] They are produced by crossing a tetraploid (4n) seed parent bearing 2n eggs with a diploid (2n) pollen parent bearing haploid (n) sperm. Tetraploid plants are produced by treating the terminal buds of diploid plants with colchicine, causing the chromosome number of the meristematic cells inside to double. The haploid (n) sperm from a pollen grain from the male flower of the 2n parent fertilizes the diploid (2n) egg inside the ovule of a female flower on the 4n parent. The resulting 3n zygote develops into a 3n embryo inside a seed. Planting this seed will yield a 3n watermelon plant bearing 3n seedless watermelons. The following illustration shows this cross resulting in a triploid watermelon plant:

The triploid seed will germinate and grow into a triploid plant bearing triploid male and female flowers, but the flowers will not produce viable sperm-bearing pollen or eggs because of the odd number of chromosome sets (3). With three sets of chromosomes, one set will not have a matching (homologous) set to pair up with during synapsis of prophase 1 of meiosis. This synaptic failure results in gametes that are not viable, therefore double fertilization inside the ovule does not occur and an embryo-bearing seed is not typically formed. When you buy seedless watermelon seeds, you get two kinds of seeds, one for the fertile diploid plant and one for the sterile triploid. The triploid seeds are larger, and both types of seeds are planted in the same vicinity. Male flowers of the diploid plant provide the pollen which pollinates (but does not fertilize) the sterile triploid plant. The act of pollination induces fruit development without fertilization, thus the triploid watermelons are seedless.