Merism

Abstract: The numbers of flower parts are considered among the most constant angiosperm characters However, nearly 33% of the plants in 13 populations of Ipomopsis aggregata in Colorado had at least one anomalous flower, and almost 10% of the flowers displayed abnormal merism (abnormal floral formulas) Floral inconstancy varied significantly among populations The number of abnormal flowers per plant increased with the proportion of plants displaying abnormal merism at a given site Abnormal flowers were nonrandomly distributed; more plants had multiple anomalous flowers than expected by chance The gynoecium was the most variable flower part and varied independently of the other parts, which tended to vary in a coordinated fashion These observations are compared with other examples of abnormal merism in the Polemoniaceae

Abstract: Stephania is the sole genus in the basal eudicot family Menispermaceae that possesses both actinomorphic and zygomorphic flowers. Variation in perianth merism can have an important impact on flower symmetry and thus eminent biological significance in evolution of Menispermaceae. Using SEM, we studied the floral development of four representative species, which present the two predominant floral patterns of the genus, namely, homomorphy of both male and female flowers (actinomorphy) or heteromorphy (actinomorphy/zygomorphy). The sepals of the male flowers are arranged mostly in two alternate whorls of three or four each, whereas in female flowers they are in a single whorl of three or four or there is only a single sepal. Petals of male flowers are in a whorl of three or four organs, whereas female flowers of some species have only two petals. Trimerous and tetramerous perianths can coexist in the same umbellets of some species. Variation in perianth merism and loss of perianth parts of the female flowers ...