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When an angiosperm, or flowering plant, is in its bud stage, its petals are covered with sepals. These protective, usually green structures are leaf-like, though usually thicker, and keep the delicate bud and its interior parts safe as it blossoms. In botany, a group of these sepals is called a calyx.
The sepals continue to remain in place as the bud blossoms into a full flower, serving to protect its ovary, petals and other parts. After the flower is in full bloom, the calyx is located at the base of the flower, beneath the petals. In some plants, such as tulips, the calyx is not green but a similar color to the petals themselves. In this instance, the term sepal is replaced with tepal. This term can also be used for plants that have colorful sepals and no petals.
In conjunction with the corolla, or the group of petals on the plant, the calyx forms the perianth. In an average flower, this outer envelope takes the form of two whorls, with the petals making up the inner whorl, and the sepals forming the outer whorl. In this instance, petals and sepals are equal in number.
Some flowers, such as day lilies, may contain multiple perianth segments. There are many different varieties of perianths as well. A calcarate perianth has a spurred appearance, while a calceolate perianth is slipper-shaped.
Flower classification is determined by the number of sepals a flower has. This number is referred to as the plant's merosity. Four or five sepaled plants are known as eudicots, while flowers with three, or multiples of three, sepals are called monocots and palaeodicots.
Different kinds of angiosperms feature many different varieties of calyx formations. In many plants, sepals are seen as very small, almost dwarfed or completely hidden by the corolla. Sepals make take the form of teeth, scales, or ridges, depending on the plant, which further offers the petals protection as the plant grows. Like leaves, sepals have veins, and also often resemble leaves in shape and form as well.
Flowers with smaller perianths are usually found in grasses. The sepals may be fused together, closely facing the base. In this case, they may form what is known as a calyx tube. The calyx tube can contain both the corolla and the sepals, as well as the attachment point of the flower's stamens, or male reproductive organs. This tube is commonly formed in flowers of the Lythraceae family.
@Iluviaporos - That's good advice, but you also have to remember which flowers have a visible calyx and which don't. A rose, for example has a really clear and well defined calyx that shows up as green against the petals and is very visible when the rose is still a bud.
A tulip, on the other hand, has tepals, like the article says. If you look at pictures of tulips, they don't have a green outer layer at the bottom of the flower, and if you draw one in, because you think all flowers should have a calyx, it will look wrong.
I personally think you're just better off working from a photo, rather than trying to remember or guess what a flower should look like.
The calyx is something that I find really important to remember when drawing flowers.
I think that forgetting that a flower has sepals can make it look strange, but people don't necessarily realize what the problem is.
If you use careful observation, you'll automatically draw them in, of course, but if you are drawing from memory, it's a good idea to bear in mind all the different parts of a flower, so that you manage to get them all in the right places.