A bit of basic terminology will help us to understand such mysteries. Complete flowers are composed of four parts, sepals, petals, stamens, and pistils. These four parts are arranged in whorls from outside to the center of the flower.
A sepal is a leaflike flower part that is the outermost whorl of a flower. Usually it is green and can be found underneath the petals. The sepal is the part that encases and protects the flower when it is in the bud stage. All of the sepals together make up the calyx of a flower.
A petal is a leaflike part that is often colorful. In complete flowers, the petals are the second whorl and are positioned inside the sepals. All petals make up the corolla of the flower. The sepals and petals together are the perianth.
The sepal (calyx), however, is not always green. Sometimes it is colored like the petals, and sometimes it is a contrasting or different color than the petals. In many flowers, such as amaryllises, lilies, and orchids, the sepals and petals look very similar, so they are called tepals. The daylily, the amaryllis, and the 'Stargazer' lily shown in this article have three sepals and three petals. Since they look very similar, they are collectively called tepals. A close look will reveal two different whorls with the sepals on the outside and the petals on the inside nearest the viewer.
The third and fourth flower components are the reproductive parts of the flower. The male part is the stamen, consisting of the pollen-bearing anther that sits atop a stalklike filament. The female part is the pistil, consisting of the stigma, the style, and the ovary.
An incomplete flower lacks one or more of the four major parts. It may not have petals (apetalous), it may not have sepals (asepalous), or it may have neither petals nor sepals. Likewise, it may be missing the stamen/s and be a pistilate or female flower, or it may be missing the pistil/s and be a staminate or male flower. Male and female flowers are said to be imperfect. Perfect flowers have both male and female parts but may not have sepals and/or petals, so they are not necessarily complete flowers.
It is interesting to note that the flower parts are basically different expressions of the same structure. It helps to distinguish among them when you realize that the four main parts of a flower are generally defined by their positions and not by their function, and that the functions frequently change from flower to flower.
Some incomplete flowers such as hydrangeas and clematis shown here have colorful sepals that look like petals. A look at the flower reveals only one whorl of petal-like parts. In cases where only one whorl exists within the perianth, the whorl is made up of sepals and the petals are missing.
In some double flowers, such as roses, camellias, and others, the stamens and pistils may be transformed into petals, correctly called petaloid stamens or petaloid pistils. While the word "petaloid" is an adjective, the word is sometimes used as a noun and instead of saying petaloid pistil or petaloid stamen, it is shortened to "petaloid."
So, when trying to decide if a flower part is a petal or a sepal, simply look at its position in the whorl. If there are two definite whorls in the flower perianth, you know that it has both petals and sepals. They may look alike or very similar, as in the case of tepals. However, if only one whorl is present, it is generally understood that the structure is a sepal, and not a petal. In double flowers where no stamens or pistils are evident, one can guess that they are expressed as petaloid stamens, petaloid pistils, or both.
Some inquiring minds may question such things as whether a flower part is a petal or a sepal. Others don't care, and simply choose to enjoy the beauty of the flowers without concern as to whether they are male or female, complete or incomplete, apetalous or asepalous, or any other of the hundreds of variations that occur. The variations, however, make the world of flowers infinitely interesting and keep some of us captivated for a lifetime.Double Camellia (petaloid stamens)
Double rose (petaloid stamens)