Botany 101: Flowers

We probably all learned some basic botany back in school … and have almost certainly forgotten some of it. In a blog about gardening, therefore, it can’t hurt to do a bit of revision from time to time. So here is a bit of basic botany about flowers.

What Is a Flower?

The structure of a typical flower. 1. Receptacle. 2. Sepal. 3. Petal. 4. Stamens. 5. Pistil. Source: Petr Dlouhý, Wikimedia Commons

The flower is the reproductive structure of higher plants, the place where seeds, which will produce the next generation of the plant, are produced.

The flower is carried on a small stalk called a peduncle that joins a receptacle: the base of the flower.

The outer envelope of the flower, especially visible when the flower is in bud, is called the calyx. It generally consists of green, leaflike sepals, although they are sometimes colored. The flower sometimes has one or more colored leaves called bracts arising from its base. The poinsettia (Euphorbia pulcherrima), for example, is well known for its colorful bracts.

The inner envelope of the flower is called the corolla. It is often colored and is made up of petals.

Sometimes petals and sepals look much alike. This is the case with lilies, tulips and daylilies, for example. They’ll then be called tepals.

The Sex of Flowers

Most flowers are bisexual (hermaphrodites) and bear both male reproductive organs (stamens) and female reproductive organs (pistils).

 

Stamens consist of a filament bearing an anther at its tip. The latter contains pollen sacs which are, as the name suggests, filled with pollen (male gametes). Pollen is usually carried from one flower to another by insects (less often by birds or mammals) or by the wind, more rarely by water (a few aquatic plants). There can be from one to over one thousand stamens per flower.

The pistil (the female reproductive organ) is made up or one or more carpels. When it carries only one carpel, it’s said to a simple pistil, but a pistil can also be compound and have several carpels. The carpel is made up of three parts: a swollen base called the ovary, a narrow, tubelike style and, at the tip of the style, a stigma. The ovary can be simple or compound and contains the ovules that will become the seeds once pollinated. The stigma is knoblike and has a sticky surface. It serves to receive pollen grains.

Fertilization

Once the pollen grain is deposited on the stigma, it emits a pollen tube that extends to the ovary and the sperm cells descends the tube to fertilize the egg. An embryo develops that turns into a seed. When the seed is mature and falls to the ground (or any other suitable surface), it sprouts and produces a new plant.

Sex Can Be Complicated

Most flowers (about 90% of them) are hermaphroditic: they house both sexes and are said to be perfect flowers. Other flowers are imperfect: they are entirely female or entirely male. Sometimes female flowers—those of the kiwi (Actinidia spp.) for example—have visible, but non-functional, stamens. The opposite is also possible: male flowers can have visible but non-functional carpels.

Some plants bear separate male and female flowers on the same plant. They’re called monoecious, from mono, for one. Squashes and cucumbers as well as begonias are monoecious.

If a plant is said to be dioecious, that means is it carries male and female flowers on two different plants. (Note the “di,” for two, in the name.) The plant that bears only female flowers is called gynoecious; the one that bears only male flowers is androecious. Remember when the nursery guy told you to plant at least one male holly to ensure your female hollies produce berries? That’s because hollies are dioecious. Willows, poplars and yews are other examples of dioecious plants.

The More the Prettier

Most flowers are single flowers. They bear the “usual” number of petals for the species: five is most common, but three, four and six petals are also frequent and there can be many more.

Double flowers though have a far greater number of petals than normal. Double flowers are rare in the wild, as they tend to be sterile or nearly so, but gardeners like them because they are more colorful than regular flowers and often longer lasting. Usually double flowers show up as mutations, but doubleness can also be an inheritable trait.

Double flowers can originate from different plant parts. Sometimes they come from petals that multiply abnormally and other times, from extra sepals, but in most cases, double flowers result from the stamens that turn into petals, in which case they are called petaloid stamens or just petaloids.

Semi-double flowers have more than the usual number of petals single flowers bear, but less than double flowers. Notably, they usually have non-mutated stamens clearly visible in the center of the flower and are therefore quite fertile.

Flower Heads

An inflorescence is the arrangement of flowers on a plant and includes stems, stalks, bracts and flowers … and the possibilities of how flowers are placed—singly, in umbels, in cymes, in spikes, etc.—are almost limitless. This is a subject for a future Botany 101 blog, but for now, I simply want to clarify the situation of flower heads, because they cause much confusion among gardeners.

A flower head, also called capitulum, is the compound inflorescence seen in the huge Asteraceae family (daisy family), containing more than 22,000 species. It’s confusing, because while it looks like a single flower—apparently a circle of petals surrounding carpels and stamens—it’s actually something quite different.

Instead the flower head is made up many tiny flowers (sometimes called florets) closely packed together. Some of the best-known examples are sunflowers, dandelions, daisies and thistles. Often, there are two types of florets: tubular florets in the center that form a disc (they’re called disc flowers) and that are generally hermaphroditic and fertile and ray flowers with one large lobe that form a circle around the disc, giving that typical daisy appearance and thus mimicking the petals of simple flowers. Often, but not always, ray flowers are sterile and serve only to attract pollinators to the fertile disc flowers in the center.

Theoretically, you shouldn’t call an Asteraceae “bloom” a flower, but always use the terms flower head, inflorescence or capitulum, but I must confess that I break this rule regularly. Indeed, often voluntarily. It’s too easy to call something that looks like a flower a flower. I do it when I have no specific reason to point out the difference, although I sometimes do slip in the word inflorescence just so any botanists reading the text know I really do know the difference.

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And there you go. Hopefully this article helped demystify the complex subject of the flower!