Botany 101: Flowers

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This hibiscus is a typical single flower: five petals and clearly defined pistil and stamens in the center. Source: Petr Kratochvil,

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?

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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).


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Structure of a stamen. Source:

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.

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Structure of a pistil. Source:

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.


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The pollen grain forms a pollen tube through which the sperm cells descend to fertilize the ovule. Source:

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.

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This double tulip is sterile: all its reproductive organs have been turned into petals, a common situation with double flowers. Source: Max Pixel

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.

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A typical Asteraceae inflorescence. Source: and

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.

And there you go. Hopefully this article helped demystify the complex subject of the flower!20171205B Petr Dlouhý, Wikimedia Commons


Garden Myth: When a Plant Doesn’t Bloom, It’s Because It’s a Male


20170407B.jpgIt’s always a bit mysterious when a plant doesn’t bloom. After all, most bloom annually. How then to explain why a plant doesn’t flower at all?

Often one of your friends suggests what seems to be a logical answer. “If your plant doesn’t flower, it’s because it’s a male.” Well, that does sound logical… but it’s totally false. You see, male plants also bloom. In fact, they have to. If they didn’t and therefore didn’t produce pollen, how would female flowers receive the pollen they need to produce seeds?

Perfect and Imperfect Flowers

20170407CENIn fact, the vast majority of flowering plants, about 90%, are said to be “perfect”: that is, they are hermaphrodites (bisexual) and carry both stamens (male) and at least one pistil (female) in the same flower.

When a flower does not have both sexes, it’s said to be imperfect. Some plants, for example, are monoecious: there bear separate male and female flowers on the same plant. Squashes (Cucurbita spp.), begonias (Begonia spp.) and most conifers belong to this group.

About 6% of plants are dioecious, that is, they produce male flowers (with stamens) and female flowers (with a pistil) on separate plants. This is notably the case with hollies (Ilex spp.) and kiwis (Actinidia spp.). Do note though that both plants, although one is male and one is female, do bloom.

So Why Do Some Plants Not Bloom?

Let’s return to the initial problem: how to explain why a plant doesn’t flower. Here are some of the (many) possibilities:


It can take years before a tree first blooms.

  • It’s not mature enough. True enough, some plants bloom within months of sowing, but others wait several years before they start. In the case of large trees, it’s not at all unusual for it to take 40 years before the first flowers appear.
  • It may not bloom annually. There are plants that bloom only every two years, and others that flower even less frequently. If you take notice of it in an off year, no, it won’t bloom.
  • It has been incorrectly pruned. This is a common occurrence in home gardens. If you prune a plant while it bears flower buds, even if they’re still nearly microscopic, that will keep it from blooming. To avoid this situation, and if you have a good reason to prune (few plants need to be pruned on a regular basis), always prune it after the plant blooms.
  • Maybe it did bloom, but you didn’t notice. Not only are some flowers pretty insignificant, but they may also bloom for such a short period or at such odd hours that it’s unlikely you’ll notice them.
  • It’s not growing under the right conditions. This is the number one reason a plant doesn’t bloom. The plant may lack light or receive too much light, the soil or air may be too humid or too dry, it may be too cold or not cold enough in the winter or too cool or too hot in the summer, the soil may be too poor (or too rich)… and the list goes on and on. In fact, three cheers for the capacity of plants to adapt to new conditions, because it’s almost surprising that a plant removed from its natural environment and cultivated in a very different climate still manages to bloom, yet so many do.

As a laidback gardener, it’s up to you to grow the plant under conditions the most closely meet its needs. Then all you have to do is wait patiently for it to start to flower!20170407B