Figure 1. Structure of carpels of the model laboratory species Arabidopsis thaliana. (a) Scanning electron micrograph of a mature flower with several sepals, petals and stamens removed to reveal the central female reproductive organ (medial view). This is made up of two congenitally fused carpels (left and right) that are together called the gynoecium. The three components of the gynoecium are an upper stigma with papillae on which pollen grains germinate, a short style with internal transmitting tract tissues through which the pollen tubes grow and a large, two chambered ovary. Each chamber represents one carpel, and the two chambers are separated by a septum, which also carries transmitting tract tissues. (b) Side (lateral) view of a gynoecium of Arabidopsis with part of the ovary wall dissected away. This reveals the internal ovules within one of the two chambers (corresponding to one of the two carpels). Each ovule has a short stalk (funiculus, f), an internal nucellus that carries the embryo sac (including the female gamete) and two sheaths of tissue (integuments, i) surrounding the entrance to the ovule (micropyle). In this species, the integuments are turned upwards (anatropous). Pollen grains have germinated on the stigma, and pollen tubes (t) have grown down the style and septum, and several can be seen in the chamber (top right). (c) Cleared gynoecium of Arabidopsis in which the pollen tubes have been selectively stained with the fluorochrome aniline blue. Their path of travel from the stigma, down the transmitting tract, and to micropyles of individual ovules, can be observed. (a) and (c) Reproduced from Alvarez and Smyth (2002) with permission of The University of Chicago Press and (b), courtesy of John Alvarez.

Figure 2. Carpel structure in the most primitive angiosperm, Amborella trichopoda. Flowers of this New Caledonian woody shrub have several free urn-shaped carpels, each with a single ovule. In this diagram of a longitudinal section of one carpel, the large cap of stigmatic cells, the narrow canal through which pollen tubes grow (the transmitting tract) and the single, penduluous (orthotropous) ovule within the ovary are visible. Both the canal and the chamber around the ovule are filled with secretion. The floral axis is on the left. For further details see Endress and Igersheim (2000).