An anatomical study of floral variation in Thymelaea hirsuta (L.) Endl. related to sexual dimorphism

Thymelaea hirsuta is a subdioecious plant, bearing hermaphroditic (perfect) or monosexual (male or female) flowers from September to May. Flowers are grouped in a condensed raceme (capitulum) in which the flower sequence may occur within the same flowering season or from year to year; nevertheless floral variation is constant within sexually stable individuals (subgynoecious, subandroecious, protandrous and protogynous) or may vary within labile sexual forms. Sexual dimorphism in flowers is linked to the developmental arrest of male or female reproductive organs within bipotential flower buds. In the present study, we provide a morphological characterization of the flower and a detailed description of the developmental timing in hermaphroditic and unisexual flowers by using light and scanning electron microscopy. Particularly, we analyse the early development during which sexual dimorphism is established, by comparing the differentiation stages during pistil and stamen development/regression. The point of transition from hermaphroditic to unisexual flowers is also clarified: in female flowers the stamens arrest their development after a few mitotic divisions of the sporogenous tissue, while in male flowers ovule development is stopped after the first meiotic division.