Morphology of Flowering Plants · Lesson 5 of 11
Floral Symmetry, Sex and Ovary Position
“Learn flower symmetry, floral completeness and the relation of floral whorls to the ovary.”
• Identify actinomorphic, zygomorphic and asymmetric flowers. • Distinguish bisexual and unisexual flowers. • Recognise trimerous, tetramerous and pentamerous patterns. • Explain superior and inferior ovary positions using the insertion of other whorls.
How botanists describe a flower
Floral morphology uses a small set of observable traits to describe very diverse structures. The questions are systematic: Is the flower symmetrical? Does it contain both reproductive whorls? How many parts repeat in each whorl? Does a reduced leaf called a bract occur at the base of its pedicel? Most importantly, where do the other floral whorls arise relative to the ovary? Two flowers of similar colour can have very different answers.
Three symmetry patterns
A flower that can be divided into approximately similar halves by more than one vertical plane through its centre; mustard, Datura and chilli provide common examples.
A flower with essentially one vertical plane of bilateral symmetry, as in pea, bean and Cassia.
A flower that lacks a vertical plane dividing it into matching halves, as in canna.
To test symmetry, imagine passing a vertical plane through the floral centre without rotating individual petals. If several such planes can produce corresponding halves, the flower is actinomorphic or radially symmetrical. If only one does, it is zygomorphic or bilaterally symmetrical. If none does, it is asymmetric. This is a description of the whole flower, not simply of one regular petal. A flower with five petals need not be radially symmetrical: their shapes and positioning may produce a single plane of symmetry.
Reproductive whorls and merosity
A bisexual flower has both androecium and gynoecium. A unisexual flower contains only the functional staminate or the functional pistillate reproductive whorl. In either case, the flower may still have a calyx or corolla. Trimerous, tetramerous and pentamerous refer to floral appendages or whorl numbers generally occurring in threes, fours or fives or their multiples. These are useful descriptions, but counts may differ among separate whorls; never infer the complete formula from petal number alone.
A bract is a reduced or modified leaf associated with the base of a flower or flower stalk. A flower with a bract is bracteate; one without is ebracteate. A bract should not be mistaken for a sepal: sepals are floral organs of the outermost whorl, whereas bracts are leaves associated with the floral axis.
| Descriptor | What to look for | Example or clue |
|---|---|---|
| Actinomorphic | Multiple vertical symmetry planes | Mustard, Datura |
| Zygomorphic | One principal vertical symmetry plane | Pea, bean |
| Bisexual | Stamens and carpels both present | Many typical complete flowers |
| Unisexual | Staminate or pistillate whorl only | Functional reproductive whorl count |
| Pentamerous | Parts commonly in fives | Inspect each whorl separately |
| Bracteate | Bract at base of pedicel | Bract is not part of calyx |
Ovary position is about insertion of other organs
Visualise the receptacle as a base, sometimes cup-shaped, that carries the floral whorls. In a hypogynous flower the gynoecium occupies the upper position and the other whorls attach below the ovary. The ovary is therefore termed superior. Common illustrations include mustard, china rose and brinjal. The label superior refers to relative insertion, not to greater size or biological superiority.
In a perigynous flower the other floral whorls arise from the rim of a cup-like thalamus surrounding the centrally placed gynoecium. Plum, rose and peach are familiar examples in introductory descriptions. Some school texts refer to the ovary here as 'half inferior'. However, the careful structural criterion is that the other whorls arise from the surrounding rim; a free ovary in a hypanthium is not necessarily fused to the cup wall. When drawing, show the cup and separate ovary distinctly.
In an epigynous flower the surrounding floral tissue is joined to the ovary region so that the other whorls appear to arise above it; the ovary is called inferior. Guava, cucumber and the ray florets of sunflower illustrate this arrangement. To distinguish it from perigyny, examine whether the ovary is enclosed and fused to the surrounding receptacle tissue rather than merely sitting inside a cup.
The words superior and inferior describe the ovary relative to the attachment of sepals, petals and stamens. They do not indicate whether the ovary is successful at fertilisation or whether a flower is physically upright.
A useful flower-description checklist
Start by observing the flower from above to assess symmetry and count floral parts, then from the side to assess the pedicel, bract and receptacle. When feasible, a carefully opened longitudinal section can reveal whether other whorls arise below, around or above the ovary. Complete the description using separate observations rather than guessing one feature from another. A radially symmetrical bisexual flower can be hypogynous or epigynous; symmetry and ovary position are independent characters.
Quick Quiz
Quiz
A flower has only one plane of bilateral symmetry. Which term fits?
Which condition indicates a superior ovary?
What does bisexual mean for a flower?
In an epigynous flower, where do the other whorls appear attached?
What is a bract?
Practice Problems
- Distinguish actinomorphic, zygomorphic and asymmetric flowers.
- Explain hypogynous, perigynous and epigynous flowers by insertion of whorls.
- A flower is pentamerous and bisexual. What two observations support the description?
- Why is it incorrect to call a perigynous flower always structurally half-fused to the ovary?
Model Answers
1. Actinomorphic flowers can be divided along several vertical planes; zygomorphic ones have only one principal such plane; asymmetric flowers have none.
2. Hypogynous: other whorls below superior ovary. Perigynous: other whorls on rim of cup around gynoecium. Epigynous: other whorls above an inferior ovary associated with enclosing fused tissue.
3. Counts of floral appendages are commonly in fives or their multiples; both stamens and carpels are present in the flower.
4. Perigyny specifies whorl insertion on a surrounding cup, but the ovary may remain free from the cup. Fusion to surrounding tissue is a distinct morphological characteristic.
Key Takeaways
Actinomorphic means many symmetry planes; zygomorphic one; asymmetric none. Bisexual flowers bear both reproductive whorls; unisexual flowers bear one. Hypogynous flowers have superior ovaries; epigynous flowers have inferior ovaries. Perigyny describes other whorls arising from the rim of a cup surrounding the ovary. Bracts are associated leaves; trimerous, tetramerous and pentamerous describe floral part numbers.