Appearance Of Viscum Album

The primary haustorium grows vertically through the host tissue. In addition, cortical strands develop which are orientated parallel to the surface. In contrast to the primary haustorium, the cortical strands retain their mitotic activity throughout the year (Sallé 1978). The cortical strands can give rise to secondary haustoria or to aerial shoots. They have no cell-to-cell connection as seen for the haustoria.

Figure 1 Image of Viscum album growing on poplar.

The shoot forms usually one node every year so that the age of a mistletoe bush can be approximately determined by counting the nodes. During the first three to four years, the shoot grows without branching. Afterwards a dichotomous shoot system or systems of higher order develops. Mistletoe bushes tend to form a globular shape which may reach over 1 m in diameter (Dorka 1996). (Fig. 1)

The coriaceous leaves have a life span of usually two years. Their size varies from about 3.5 to 6 cm in length and 1-2 cm in breadth. The length to breadth index has been used to classify subspecies but the variation is too big to make a clear cut

Figure 2 Male flower of Viscum album (from Becker, 1986; with kind permission of Karger Verlag, Basel) which consists of four perianth members (for better insight, the front segment was cut off). The anthers are implanted on the perianth members; they dehisce by numerous pores, shedding the spherical tricolpate pollen, as shown in Figure 3. Photo: Erbar (Heidelberg)

Figure 2 Male flower of Viscum album (from Becker, 1986; with kind permission of Karger Verlag, Basel) which consists of four perianth members (for better insight, the front segment was cut off). The anthers are implanted on the perianth members; they dehisce by numerous pores, shedding the spherical tricolpate pollen, as shown in Figure 3. Photo: Erbar (Heidelberg)

classification. They are aequifacial as seen by cross section and bear numerous oxalate crystals.

Viscum album is dioecious, i.e. part of the plants is female, the other part is male. The expression of sex is determined by a translocation heterozygoty (Mechelke 1976). Flowers are highly reduced and inconspicuous. According to weather conditions, they open between the end of February and April. Despite their insignificant appearance, they are insect pollinated. Insects are attracted by a sweet fruit-like smell and by floral nectar. Male flowers have no anthers of the usual structure, four tepals bear around 50 pollen compartments that open by pores (see Fig. 2). The pollen grains are oval (35x60 pm), tricolpate with numerous spinulae. Female flowers (see Fig. 3) are

Figure 3 Female flower (from Becker, 1986; with kind permission of Karger Verlag, Basel) which consists of four perianth members with a simple style in the centre. Photo: Erbar (Heidelberg)

even smaller than male ones. True ovula are not formed (Bhandari and Vohra 1983); the embryo develops at the base of the ovary.

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