Flowers lacking petals are described as apetalous and may defy our notion of what a flower is for. Petals, especially colorful and showy petals, attract pollinators such as birds and bees. Some flowers are pollinated by the wind, though, and these don't need petals or any other showy elements. Still other plants produce colored leaves or showy sepals--sepals are a kind of modified leaf--and use these petal posers to attract attention instead.
Not only is the calla lily apetalous, it is asepalous, lacking the sepals that would normally be located on the outermost part of a complete flower. (Complete flowers have all four of the basic parts that flowers can possess.) With neither petals nor sepals, calla lilies are considered "naked." In botany, naked flowers are "achlamydeous."
What looks like a single furled petal protecting a yellow spike on the inside of a calla lily is really just a pretty leaf called a bract, specifically a spathe bract.
Calla lilies like the sun and can grow in a boggy section of the garden. Besides white, hybrid callas may have bracts of yellow, pink, purple and cream.
The anemone belongs to the buttercup family, most of which have no petals. Indeed, the true buttercup is the only member that has petals.
Instead of a ring of petals, the anemone sports a ring of sepals, collectively called the calyx. (Petals are collectively called the corolla.) Since there are many species of anemones, sepals are found in a variety of colors, from yellows to reds to purples to white.
Clematis is a popular flower among gardeners and another member of the buttercup family, or, botanically, the Ranunculaceae family. Many species of clematis exist and because gardeners like the flowers, a lot of hybrids are available as well. Most of the plants tend to be vining and come in many colors. These colored parts of the clematis are not petals, but sepals.
Hepatica is a pretty flower that belies its name: hepatica refers to liver. The flower is so-named because its leaves look like a human liver. Three green bracts beneath the flower might look to some like sepals instead of leaves, fooling some into thinking the blue sepals are petals.
Hepatica can take shade, making it welcome to many gardeners who have wooded areas.