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Cleistogamy in Centaurea melitensis L. (Asteraceae): Reproductive Morphological Characters, Analysis, and Ontogeny

In morphological terms, cleistogamy is the most extreme form of autogamy. Among other characteristics, cleistogamy generally entails a reduction in flower size, one that particularly affects the corolla and androecium. In this study, we compare, from a structural point of view, the changes observed in cleistogamous (CL) flower heads of the annual herb Centaurea melitensis to the changes in chasmogamous (CH) heads throughout floral ontogeny. Special attention was paid to capitulum structure, since no other reports of cleistogamic differences similar to those observed here for C. melitensis are to be found in the literature on Asteraceae. Anthesis was absent in both the CL capitulum itself and in the florets within it. In comparison with CH heads, the fertile florets in CL heads were smaller and less numerous, and sterile florets tended to be lacking, as did several floral structures. Floral initiation and development in CH flower heads did not follow the strictly acropetal pattern typically observed in the rest of the Asteraceae family. There is evidence to indicate that the florets of CL capitula evolved from the outermost fertile florets of CH capitula.