Development Of Fruit And Embryo

Successful pollination is followed by the development of mistletoe fruits. The pollen germinates on the stigma, with a pollen tube growing into the style and the tissues of the central body. As soon as germination starts, the generative nucleus divides, and the two new generative nuclei as well as the vegetative nucleus move into the pollen

Figure 5 A.Axillary bud with primordia of V. album twig. By mid-July the main elements of next year's shoot generation have already developed in the axils of the current year's mistletoe leaves. They are the two bud scales (top and bottom), and decussate to these the leaf primordia (left and right), with the spherical bud of the inflorescence between them (x48). B. Typical shoot of V. album with female flower buds in November. Photos: Raman.

tube. When the tip of the pollen tube penetrates one of the embryo sacs, the vegetative pollen nucleus dissolves and further development is triggered by the two generative pollen nuclei. One fuses with the egg cell, with the resulting zygote resting for about two months. The other generative pollen nucleus unites with the endosperm nucleus; further cell divisions will start immediately and will lead to the formation of the endosperm. When the first division of the zygote comes in mid-May, it is already completely enclosed by the multiplying endosperm. The bicellular embryo stage goes through another rest period of about two weeks, and then development becomes more rapid (Pisek, 1923; Steindl, 1935).

The primordia of the cotyledons become visible as light-colored zones in the pale green endosperm in the beginning of July (figure 13), and the hypocotyl grows towards the periphery of the endosperm in August. By the end of September, a viable embryo has differentiated out in the mistletoe fruit, which is still green. Viscum album shows polyembryonie, i.e. one seed may contain one or two, and, rarely, even three or four embryos. Depending on the climate, and evidently also on other yet unknown factors relating to individual mistletoe bushes, the fruits begin to ripen in mid-October. The pericarp loses its green colour and appears white (figure 14). Sugars are produced in the colourless, translucent mesocarp, giving it a sweetish taste and making the outer parts in particular a favourite food for mistletoe-distributing birds.

Mesocarp development bases on the tissues of the flask-shaped central body, the cell walls of which turn mucilaginous and dissolve towards the end of June. Morphologically the exocarp arises from the walls of the calyx and the carpel tissues fused with it. With axial tissue also involved in this mode of fruit development, the correct botanical classification of mistletoe fruit is as a pseudo berry. And with the embryos developing from embryo sacs that lie without integuments naked in the central body tissue, mistletoe also does not have true seeds in the strictly botanical sense (Steindl, 1935).

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