Comparative anatomy of bisexual and female florets, embryology in Calendula officinalis (Asteraceae), a naturalized horticultural plant

Calendula officinalis is a perennial gynomonoecious herb, often used for medicinal purpose and as ornamentation. The corolla consists of only one petal for ligulate florets, whereas it consists of four to seven petals for disk florets with four to seven stamens accordingly. The bisexual florets are functionally male and their ovaries are either solid or unilocular with no ovule. In contrast, the ovaries of female florets are always unilocular with an anatropous ovule. For bisexual florets, simultaneous cytokinesis in the microsporocyte meiosis leads to tetrahedral, also decussate tetrads. The mature pollen grain is of the three-cell type. The tapetum is mainly of amoeboid type, yet 6.5% antheral tapetum disintegrates in situ, as called glandular tapetum. The young anther wall is composed of epidermis, endothecium, middle layer and tapetum, but the middle layer degenerates at the microspore tetrad stage. The mature anther wall comprises only endothecium, which develops fibrous thickenings. The ovules are unitegmic, tenuinucellatae and the development of the embryo sac follows the monosporic, polygonum type. Before the differentiation of the micropylar cells into egg cell and two synergids, the two polar nuclei fuse into a secondary nucleus and the antipodal cells start degenerating. Two days after blooming, the secondary nucleus has been fertilized by a single sperm nucleus, generating the triploid primary endosperm nucleus.