A Carpel consists of an ovary, a style and a stigma. A flower may have 1 or more carpels that collectively make up the Gynoecium, the female part of the flower, which is  called the Pistil. The carpels may be arranged in three different ways – Monocarpous, Apocarpous or Syncarpous – as explained  below, and this is a diagnostic feature used in plant identification. 


Single carpel of Prunus avium Wild Cherry

Monocarpous – It has  one carpel. Examples are Wild Cherry, shown here,  Common Hawthorn and most Legumes.

Apocarpous – It has more than one carpel but the carpels are ‘free’ as in the Hellebore shown here with 5 free carpels. The Buttercup and Stonecrop families have free carpels as has the Rosa and Fragaria (strawberry) genera. 

5 fused carpels of Hypericum x ‘Hidcote’ a hybrid St John’s Wort

Syncarpous – It has more than one carpel but they are ‘united’ (sometimes called ‘fused’). The Hypericum shown here has 5 ‘united’ carpels with 5 free styles.  80% of flowering species have syncarpous ovaries.

A  complete flower has flower parts arranged in four concentric circles around a floral axis.  Each circle (known as a whorl) has a set number of parts for a species. The outer two, non-reproductive whorls are called the perianth. The two inner whorls are the reproductive parts. From the outside towards the centre the flower parts are:-

1. The sepals, collectively known as the calyx (Greek for husk or pod) protect the flower parts as the flower bud develops. Most sepals are green but in some flowers the sepals and petals are identical and both are called tepals (as in most Monocots) 

2. The petals, collectively known as the corolla, attract pollinators and provide landing sites for them. 

3. The stamens (male Androecium) – each one consists of a filament supporting an anther which produces and releases pollen grains.

4. The carpels (female Gynoecium) – each one consists of an ovary with ovules, a style and a stigma on which pollen grains are received. The ovary becomes a fruit and the ovules become seeds. A flower may have one or more carpels.