Flowers make plant identification much easier, especially for non-woody plants. To identify something, you need to group it with its relatives, and separate it from unrelated groups, gradually narrowing your search. Taxonomy, the science of classifying living things, goes hand-in-hand with identification.

It’s hard to determine genealogy with leaves: Because they’re around all season, evolution changes them very greatly. Unrelated plants in similar environments, with matching survival strategies, often evolve similar leaves. Flowers don’t change as greatly because, as reproductive structures, they’re around for a shorter time. Natural selection has less opportunity to work, so flowers retain more of their group's common ancestor's features. Wild roses, for example, evolved different colors and sizes, but they all look like roses. Shakespeare got it right: Smelling them confirms their identity with 100 percent certainty. (It’s amazing how many beginners don’t think of smelling plants.)

Linnaeus, the eighteenth century Swedish biologist who invented modern scientific nomenclature, scandalized plants by classifying them according to their sexual parts. Long before Darwin, he didn’t know that related plants had evolved from common ancestors, but he saw how anatomical similarities between living things formed natural groups and subgroups: species, genus, family, class, phylum, and kingdom. This was a great stride from medieval botany, where anthropocentric systems competed to classify things according to their usefulness to man. Thus rodents and insect pests were grouped together as vermin.

Common names are still confusing today: The same plants have different names in different regions. Unrelated plants have similar names for no reason. Different edible and poisonous plants may even have the same name!

Universal scientific names clear up this confusion. They’re in Latin or ancient Greek—dead, unchanging languages. This makes them hard to remember, so I suggest you use them as references: If you compare information about plants from different sources, check the scientific names to make sure the plants are identical. Eventually, you’ll be able to impress people because you’ll automatically remember the scientific names of the plants you use the most.

Scientific names are often descriptive, and we've included translations where they’re meaningful. Sometimes, they’re whimsical: Linnaeus sent scientists around the world to gather specimens. One became severely seasick, sailing across a stormy Mediterranean sea, to collect African specimens. He vowed never to set foot on a ship again. When it was time to return, he spent years walking around the entire Mediterranean Sea. So Linnaeus named a tree after him—a species of arid areas, that never grows near water.

Flowers evolved as modified sets of leaves specializing in reproduction. Resting on a base called the receptacle, they’re arranged on a short stem in concentric whorls.

What floral parts do we look at to identify plants? The petals, modified leaves that are collectively called the corolla, are the most obvious features. Simply noting their color and counting them differentiates your specimen from many other plants.

Surrounding the corolla is another set of modified leaves—the calyx, composed of sepals—usually green. They’re often smaller and less conspicuous than the petals—most noticeable when they enclose the flower bud. If the petals are fused into a tube, you can’t count them. But you can enumerate a flower’s basic divisions by counting the sepals. Sepals also vary: Some flowers have large, colored, sepals, often replacing absent petals. Sometimes sepals fuse, forming a calyx tube.

Surrounding the calyx may be another set of modified leaves, the bracts—collectively known as the involucre.

Enclosed by the petals are the reproductive parts of the flower: The male stamens consist of threadlike filaments, each supporting a tiny sac-like, pollen-producing anther.

The female part or pistil consists of three parts: The stigma, which catches the pollen, is the expanded tip of the style, a tube for conducting pollen to the ovary. The ovary is the bulging container of the ovules. It may be simple or compound. The ovary and ovules eventually develop into fruit and seeds.

Under selective pressure (the gradual force of natural selection), these basic structures may change radically. Some flowers lack male or female parts. Some reproduce asexually, opening only when the seeds are mature. Some don?t set seed at all, relinquishing reproduction to other parts of the plant—asexual vegetative reproduction. Such nonfunctioning flowers, like the daylily's, are vestigial: They're present only because the genes that create them are still active.

A major distinction between flowers is whether they’re small, numerous, and inconspicuous, or large and showy. The former produce way more pollen, depending on wind and pollen volume for pollination. Their parts are often so greatly reduced, we hardly recognize them as flowers. They often cluster as catkins—long stems bearing stalkless male or female flowers. Catkins fall from the plant as a unit. Most wind-pollinated trees, such as oaks and walnuts, bear catkins. Wind-borne pollen often lands in people's noses, causing hay-fever.

Showy flowers are the most highly adapted and efficient. They entice insects, birds, or bats to pollinate them. The most advanced are the orchids: Some are so specialized, one wasp species pollinates one orchid species: The pollen-bearing part may resemble the female wasp’s genitalia. The male wasp futilely tries to mate with it, gets covered with pollen, and flies off in frustration. Endowed with more libido than brain, he gets fooled over and over, repeatedly pollinating orchids without gratification.

A major distinction among animal-pollinated flowers is whether they’re radially-symmetrical (regular), like an apple blossom, or bilaterally symmetrical (irregular) like a violet. Flower shape and color are adaptations for selecting efficient pollinators. Insects land on a flower for a meal of nectar or pollen, but not all insects transfer pollen.

Some flowers, like wood sorrel’s, are bowl-shaped. Effective and ineffective pollinators alike are admitted. Others, such as jewelweed’s, have nectar at the end of a long tube that forms a conspicuous spur. Only hummingbirds and insects with long tongues, such as butterflies, gain access (although you sometimes see a tiny hole at the base of the spur, evidence that a non-pollinating invertebrate cheated by making a hole to get to the nectar). Flower and animal head shapes evolved in unison, like locks and keys, so pollination and feeding is assured.

Pea-like flowers—members of the legume family, such as wisteria and black locust—are bilaterally symmetrical, with a winged keel similar to a boat’s, positioned under a prominent flag-like standard, that attracts the pollinators. You must be a robust, sturdy insect, such as a bee, to shoulder your way through the keel to reach the nectar within, picking up pollen in the process.

Flower arrangement provide further distinctions to help you identify plants. When the flower is out of season, the fruit usually retains the configuration. The pattern may even remain on dead, out-of-season plants.

Stalked flowers often grow on a long central axis, with the lower flowers maturing first. This is a raceme—typical of the mustard family. If the flowers are stalkless, as in the mints, it’s a spike. If the central axis branches, like a grape’s, it’s a panicle.

An umbel resembles an umbrella. Stemmed flowers of equal length originate from one point, as with wild onions. If the lower flowers of the central axis have longer stalks, and the cluster is flat-topped, you have a corymb, like the sweet cherry tree's. When the flowers at the end of a flat-topped, branched cluster mature first, you have a cyme, like the viburnums and elderberries.

It’s also important to note where the flowers originate. The dandelion arises from the middle of its rosette. Some flowers grow in terminal clusters, at the tips of the plants. Others come from the leaf axils, the crotch between the branch and the leafstalk.

A clover looks as if it has one flower, but closer examination reveals many small flowers grouped together into a flowerhead. Here the recurrent themes of repetition and grouping reap a great advantage. One insect can pollinate all the flowerhead’s flowers at once.

The composite family, which includes burdock, yarrow, and sunflowers, takes this strategy one step further: The radially-symmetrical flowerhead has two kinds of flowers: Sterile ray flowers, resembling petals, radiate from the circumference. They attract insects to the many tiny, inner, fertile disk flowers. This arrangement is so efficient, composites outnumber all other North America families. As always, there are variations: The dandelion, for example, is a composite lacking ray flowers.