What is the difference between Single and Double

• On single flowers there is a layer of petals forming the flower
• Double flowerheads have multiple petals forming around and covering or replacing the stigma and stamen.
• Semi-double flowers have two or more layers of petals but the centre of the flower is still open.
• Roses with 5 petals are one of several species known in both single and double varieties as are Poppies (below) and Peonies as shown above.
• An inflorescence is a group or cluster of generally single flowers arranged on a stem.

Design of a Flower

• A bloom has four components which are clearly visible, the outermost parts are the  green leaf-like structures called sepals that enclose and protect the bud prior to opening.

• Second are the petals, the decorative layers that attract insects to pollinate.
• Third are the sexual structures, stamens the male reproductive organs and finally in the centre the carpels or female reproductive organs.

About Doubles

• Historically there were few if any double flowers until gardeners sought out the plants with extra petals.
• A genetic misfire results in the sexual structures mutating to petals resulting in the so-called double flower.
• Replacement of the sexual organs with petals renders the blooms sterile, so the double-flowering plants will not pollinate the next year.
• Doubles  frustrate pollinating insects, which are enticed to the flowers only to find an absence of stamens,   no pollen to harvest and less nectar.
• Doubles often have less of the chemicals responsible for the speed at which a flower ages. Generally the stronger the scent, the shorter the life.
• Any gardener seeking a long floral display should opt for doubles whilst singles make the fragrant garden.
• Doubles lack  viable seed so gardeners must rely on vegetative propagation.

So Doubles are harder to obtain from seed as less viable seed is produced in favour of more petals. The Begonia has got around this with ‘unisexual male and female flowers occurring separately on the same plant, the male containing numerous stamens, the female having a large inferior ovary and two to four branched or twisted stigmas. In most species the fruit is a winged capsule containing numerous minute seeds’ wikipedia