The sunflower family (Asteraceae) is characterized by head-like inflorescences (capitula) that resemble single solitary flowers but are typically composed of tens to hundreds of flowers that are often specialized in structure and function. In gerbera, three different flower types are distinguished: the marginal ray flowers, the central disk flowers and the intermediate trans flowers. The presence of different flower types within a single genotype makes gerbera a unique target for developmental and evolutional studies since the traditional model plants for flower development (e.g. Arabidopsis, Petunia and Antirrhinum) bear only single flower forms in their inflorescences.
In each three flower types, four whorls of flower organs can be found:
- Whorl 1 – Flowers of Asteraceae lack conventional sepals, but in most genera, including gerbera, the base of the corolla is surrounded by hairy pappus bristles, specialized organs that are involved in seed dispersal.
- Whorl 2 – Gerbera flowers usually have five petals. Dorsal petals are thin and threadlike in ray and trans florets while lateral and ventral petals are fused together to form a ligule-like structure. This creates the typical bilaterally symmetrical shape of these flowers (zygomorphy). In central disc flowers, petals are shorter and less fused and ultimately, the centermost flower is radially symmetrical (actinomorphic) with separate petals.
- Whorl 3 – Although stamen development starts similarly in all types of flowers, the mature ray and trans flowers have only female sex organs. That is because anthers arrest at early stages of development resulting in the formation of nonfunctional staminodes. In disc flowers, anthers develop fully and form a postgenitally fused structure that covers the carpel (the yellow structure visible in the figure).
- Whorl 4 – Gerbera flowers have one pistil made of two joined carpels. The ovary is inferior located below the floral organs.