The Mistletoe Flower

In August, a short shoot becomes visible between the two mistletoe leaves, which in the typical case is bearing a terminal and two lateral generative buds (Figure 5B). Male flowers are enclosed in a single bract, whilst in a female inflorescence the two lateral flowers have one bract and the terminal flower another (Kuijt, 1969; Rispens, 1993). Viscum album is a dioecious plant, and bushes bearing female flowers are about four times more frequent in nature than those bearing male flowers. The genetic basis of dioecism, connected with differences in the distribution of sex-specific chromosomes, was established by Mechelke (1976).

Mistletoe flowers are inconspicuous and simple in form. Male flowers are distinctly larger than female, and consist of four simple perigon leaves (Figure 6A). The stamens are fused into a kind of cushion, with the pollen grains produced in chambers embedded in these cushions. The female flower consists of a spherical green fruit primordium, with small yellowish perigon leaves, usually four in number, surrounding the papillous stigma (Figure 6B).

Male and female flowering organs differentiate out in spring and summer as the unfolding mistletoe twig finds its orientation in the spherical bush. Initially, a floral calyx grows from the meristem of the female flower, with the perigon leaves in its outer margin. Two carpels develop from the primordial meristem that has remained at the base, fusing with the surrounding axial tissue of the calyx and also with one another. A "central flask" will remain between, in which 7-9 embryo sac mother cells evolve, finally producing 1-5 embryo sacs. When the distal part of the flask grows into the papillous stigma by the end of September, all the organs of the female flower have differentiated out. The male flowers whose pollen grains have gone through the requisite maturation divisions will also be fully developed in autumn, now being in the binucleate stage with a vegetative and a generative nucleus (Pisek, 1923; Steindl, 1935; Zeller, 1976; Dorka, 1996). Both types of flowers rest until temperatures reach a level where flowering can begin.

Depending on geographical position and actual weather conditions, the main flowering period of Viscum album is in February or March. In warmer years and regions, however, the first flowers may open as early as January or even at the end of December. Pollination is by insects that are active also in winter. In field observations we have seen ants (Formica), different types of fly (Muscidae, Drosophilidae) and hoverflies (Syrphidae) on both male and female flowers. Honey bees (Apis mellifica) were only seen on male mistletoe flowers and are unlikely to count among mistletoe pollinators, which is also the view taken by Wallden (1961). The insects are clearly attracted by the scent arising from the nectaries at the base of the funnel-shaped male flowers, and by the nectar which is secreted all around the stigmata of female flowers.

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