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.