Viscum album has some remarkable features that do not fit into the categories of classical botany. The haustorial system is comparable to the roots of higher plants only in so far as it serves to conduct water and minerals into the shoot. V. album does not have a true radicle, its sinker is green and has neither a protective root cap nor phloem structures. The cortical strands, on the other hand, do not only spread the haustorial system through the formation of secondary sinkers but also give rise to
Figure 6 A. Male inflorescence of V. album (x5). B. Female inflorescence of V. album (x9). C. Fruit of V. album in early July. The mesocarp is slimy, and the primordia of the embryo become evident in the spindle-shaped endosperm (x11). D. Ripe mistletoe fruit in November. The green colour of endosperm and embryo is contrasting with the whitish pericarp (x8). Photos: Raman.
shoot development. Because of this, the mistletoe's haustorial system is seen as an "organ sui generis", a term introduced by Tubeuf (1923) and repeatedly confirmed (Thoday, 1951; Sallé, 1983).
The shoot development of Viscum album is much reduced. Each twig of one year only consists of a single internode with one decussate leaf pair and a generative short shoot with three flowers. It takes three years to form the primordia of these few organs from an axillary meristem, to unfold stem and leaves, to blossom and finally to develop a vital embryo in the ripening fruit (Ramm, 1995). Urech and Ramm (1997) considered the greatly reduced shoot development seen in the mistletoe twig, and the absence of typical root-like growth in the sinker to be in relation with viscotoxin and mistletoe lectin formation and the distribution of these typical substances in the mistletoe bush.
Another characteristic feature is the green colour of the mistletoe's embryo and endosperm which persists during the winter time. Their photophysiological potential might be crucial for the survival of the embryo. Tubeuf (1923) could show that the viability of mistletoe embryos is greatly reduced if the bushes are kept in the dark. He called Viscum album a plant that is "green through and through". Indeed, the green colour both at the centre of the fruit and in the sinker indicates that Viscum album has a special relationship to light.
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