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The Story of Flowers

How do plants know when to flower?

Dr. Adam Dimech takes a look at the science of flowering.

Floral Structure

Flowers are composed of whorls.

Starting from the outside of the flower, these are whorls of sepals, petals, stamens and carpels.

Typical structure of a flower
Typical basic floral structure.

The sepals are green and protect the other floral organs. The green 'sheaths' on a flower bud are the sepals. The sepals are arranged around the flower to form the corolla.

The petals are often (but not always) coloured, and are usually present to attract pollinators or to assist in the pollination somehow. Sometimes petals are fused together to form a single structure. In monocots, the sepals and petals are combined to form tepals.

The stamens are the male part of the flower. Stamens are composed of a filament which supports an anther. The stamen releases pollen. The filament positions the anther to maximise pollination. The entire male part of the flower is called the androecium.

The carpels are the female part of the flower. Carpels are each composed of a stigma, style, and an ovary which contains ovules. Many plants have individual flowers with multiple carpels. These are collectively termed the gynoecium. Pollen lands on the stigma and fertilises the flower. Seeds can then develop. (Note: American scientists tend to refer to carpels as pistils).

Sometimes parts of flowers are fully or partially fused together. It is also common in more advanced plants to have fused carpels with a number of divisions, each representing a single carpel.

The number of whorls a flower has depends on the species (and sometimes the cultivar). For instance, in some 'old world' roses, there is a single whorl of petals, while in some modern hybrids there are many whorls of petals.

The arrangement, structure and number of floral organs gives rise to the classifications of flowering plants. The more similar a plants flower is to another, the more likely that plant is to be related to the other. However this is not always the case, and other features such as leaves, seeds, fruits and genetic compositions are also used to determine plant relatedness.

Flowering plants are split into two groups; monocotyledons and dicotyledons. 'Monocots' are more modern in evolutionary terms than 'dicots'. Here are some key features of each group:



Whilst many plants have flowers with both 'male' and 'female' sexual organs (i.e. an androecium and gynoecium), there are exceptions to this. Plants which have separate 'male' and 'female' flowers on the same plants are described as monoecious (eg: zucchini). Plants which have either 'male' or 'female' flowers on one plant (but never the two together, thus giving rise to 'male' and 'female' plants) are called dioecious (eg: kiwifruit).

Sometimes, due to mutations, some floral organs are omitted from the flower. For instance, the green rose (Rosa viridiflora) does not have any petals, stamens or carpels. Instead it has multiple whorls of sepals, hence the flower is green. (You can read more about this in Floral Identity).

This page is only designed to serve as a brief introduction to floral structure and is by no means complete.