Let's look at the basic flower structure of several flowers found around my place, relating the parts of those blossoms to the parts of the "Standard Blossom" at the right:
Our Standard Blossom's calyx consists of a cup-like part with five sepals of equal size and shape. In the drawing above, only a tip of a single sepal is visible. Five sepals arising behind the white petals are much more visible in the much-magnified Chickweed flower (Stellaria media) at the right. By the way, chickweed flowers try to trick you. Instead of bearing ten white petals, which it seems to, it only has five, but each petal is deeply divided down the middle so that it looks like two. You can easily see this if you pull a petal off. It looks like a little white Y. Much in contrast to the calyxes of both the Standard Blossom and the Chickweed, the calyx of the St. Andrew's Cross, Hypericum hypericoides, shown at the left, has only two broad sepals arising below its flowers' four yellow petals..
At the right you see a Blackberry flower (genus Rubus) with 5 separate petals. (and five green sepals). The inset at the bottom left in the picture shows the blossom from above, and the larger picture shows the same flower from below. In the larger picture, notice how the sepals are "reflexed," or bent downward. Also above the blossom you can see dozens of stamens arising from the flower's center.
Much in contrast to the blackberry's separate petals, at the left you see a Periwinkle's purple flower (Vinca major), where the corolla consists of a cylindrical tube atop which the corolla lobes suddenly spread out horizontally. Usually the word "petal" is just used when the corolla parts are separate from one another, as in the blackberry flower. A blossom with a slender tube bearing horizontally-spreading lobes, such as the Periwinkle's, is said to be salverform. Also notice that in the Periwinkle's flower, in contrast to the blackberry's calyx, the sepals stand straight up.
Sometimes a calyx's sepals are so similar in color and texture to the corolla's lobes or petals that the calyx and corolla together are thought of as one thing, the perianth, and the individual parts are called tepals. This is often the case in the showy Lily, Iris and Amaryllis Families. The sepals and petals of the False Garlic flower (Nothoscordum bivalve, of the Lily Family) at the right are so similar that they can be called tepals.
Stamens are structures composed of anthers and filaments. Our Standard Blossom has five stamens, but other flowers may have none, or perhaps dozens, or even hundreds. Stamen number can provide critical cues as to an unknown plant's identity.
For instance, members of the Iris Family typically possess three stamens, while members of the closely related Amaryllis Family usually have six. Members of the Mustard Family generally have six stamens, of which two are shorter than the other four.
Some plant groups shed their pollen through anther pores, others through vertical slits, and others through horizontal slits. Also there's endless variation in anther size, color, and form.
Pistils are the blossom's female parts, and they are composed of stigmas, styles and ovaries.
One of the most fundamental ways pistils can be different from the Standard Blossom's is for the flower to have more than one. The picture at the right shows such a flower, one belonging to the common wildflower called Avens, Geum canadense. At the left in the picture you see the flower with five white petals, like the Standard Blossom's, but above the petals are many stamens instead of the Standard Blossom's five, plus each of those green, spiny-looking things is a separate pistil with its own slender style. At the right in the picture a more mature Avens flower has dropped its petals and the pistils and styles have enlarged enormously. Eventually each of the matured pistils will fall separately as a distinct fruit -- a special kind of simple fruit known as an achene.
There are many other kinds of pistils, too. Pistils in members of the Mint Family have ovaries deeply divided into four bulging lobes, with the style strangely arising from amidst them. Members of the huge Composite Family, which includes everything from sunflowers, asters, and ragweeds, to daisies, are structured so that every flower has one pistil, but the flowers themselves are packed together on a "receptacle" so that a whole bunch of flowers looks like just one... You can learn more about that on our Composite Flower Page.
If you were to cut across the middle of our Standard Blossom, from top to bottom and passing through the ovary's center, no matter from which petal or corolla lobe you began cutting, each resulting half of the blossom would be a mirror image of the other half. A huge percentage of blossoms are like this -- they are radially symmetrical.
Many other blossoms, however, are bilaterally symmetrical in the same way that humans are -- there is only one way to cut through them so that mirror images result. All the blossoms illustrated above are radially symmetrical, but the one at the left is bilaterally symmetrical. There you see a head-on view of a member of the Bean Family. It's Vetch, Viccia angustifolia.
When identifying plants by their flowers, one of the first things to notice is whether the blossom is radially or bilaterally symmetrical.
To start enjoying flower diversity, as soon as you can you should find yourself a blossom and, with your hand lens, locate in your flower the above-mentioned parts. Do this with a number of different kinds of blossoms, and don't become discouraged if you can't match things up.
For example, at the right you see the flowers of two Black Willow trees, Salix nigra. They are very different, right? That's because the whitish, wormlike things (technically referred to as aments) on the left are bunches of male flowers, while on the right you see bunches of female flowers. That's right, Black Willows come in boy trees and girl trees, so, if you have a boy tree and you're trying to find the pistils in the aments, you're just out of luck. And female aments have no stamens. Moreover, neither male nor female flowers possess any sign of a calyx or a corolla.
Therefore, when you begin trying to locate the various parts of your backyard blossoms, try to begin with large, simple-looking blossoms. Once you learn the basics, then you can begin discovering the big world of flowers out there which differ from our Standard Blossom in really mind-boggling ways.