The number of carpelsA flower may have one or more pistils which consist of a basal portion called the ovary which contains the embryo seeds or ovules and area where the pollen is received called the stigma. Often the stigma may be borne on a stalk-like structure called the style. The pistil consists of ovule-bearing basic units called carpels. Collectively, the carpels form the gynoecium which is sometimes referred to as the female part of the flower because the carpels produce the female gametes, but only the gametes themselves can be male or female. Flowers which bear only carpels and have no stamens are sometimes incorrectly called female, the correct term is carpellate.
There is great variation within the gynoecium of plant species from different plant families. They may differ with regard to:
The number of carpels
| There is considerable variation in the number of carpels found in different plants from different families. Peas and beans (Fabaceae) have only a single carpel which forms a single pistil. Magnolias and Crassulas have several carpels, each forming a separate pistil. |  |
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| The central portion of a magnolia flower showing the central cluster of pistils surrounded by stamens. | ||
Whether the carpels are fused into a single
pistil
or not
| The different parts of the carpels may be fused. In many plant species, the ovaries of the carpels may be fused to form a single ovary and the styles may be fused to form a single style, thus the plant has a single pistil consisting of several fused carpels. Other combinations of fused parts of the carpels are also possible e.g. only the styles may be fused. |
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| A single pistil consisting of several fused carpels (A) and several pistils each consisting of a single carpel (B) | ||
The position of the ovary
in the flower
| The position of the ovary differs in different plant
species. If the ovary is attached above the attachment of the other three whorls,
i.e. the corolla, calyx and
androecium, the ovary is said to be superior
(A). If it is below the level of attachment
of the three whorls it is inferior (C). The third possibility is intermediate
(B) where the ovary is superior but surrounded by a
receptacle from which the
other three whorls sprout.
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The attachment of the ovules in the ovary
| The ovary may consist of one or more chambers or locules which contain one or more undeveloped seeds, the ovules. The area of the ovary wall where the ovules are attached is called the placenta. The arrangement of the placenta varies in different species. In peas and beans (Fabaceae) which have a pistil consisting of a single carpel, the placenta occurs on the ovary wall opposite the main vein. In flowers where the pistil consists of more than one fused carpel the situation is more complex. When the placentas are on the outer wall the ovary it is called parietal placentation (A). If it is central, where the carpels meet in the centre of the ovary, it is known as axile placentation (B). The ovary may have several chambers. In some plants the ovary only has one chamber despite consisting of several fused carpels. In this case, if the placenta is central it is known as free central placentation (C) and (D). |  |
This is confusing - how does one know how many carpels there are ?
When the carpels are fused to form a single pistil, how can one tell how many carpels fused to form the structure? There are several clues which may be used to determine the number of carpels which form a fused gynoecium. In a simple case such as citrus fruit, each chamber in the ovary is formed by a separate carpel. Each chamber later develops into the segments of the fruit. Some plants have a fused ovary and style but a divided stigma, indicating the number of fused carpels.
