Mistletoe embryos germinate whilst still in the fruit, but are unable to break through the tough exocarp without help. In nature, birds such as blackcap (Sylvia atricapilla) and mistletoe thrush (Turdus viscivorus) remove the seed from the fruit, more or less effectively getting it in contact with the branch of a host (Grazi, 1986). Apart from mucous polysaccharides which in time are washed out, the mesocarp attached to the endocarp contains glutinous substances which firmly attach the seed to the host bark as they dry. After a period of winter rest induced by cold temperatures, the embryos begin to grow in April by elongating the hypocotyl. Mainly negatively phototropic and if necessary also negatively geotropic growth (Tubeuf, 1923) directs the tip of the hypocotyl towards the host bark. The epidermal cells of the hypocotyl's tip secrete a viscous fluid which enables close contact with the bark and helps to affix the embryo directly to the host (Loffler, 1923).
Thoday (1951) describes the way papillae subsequently grow from the epidermis, connecting the base of the hypocotyl even more firmly with the host bark. The tip of the hypocotyl broadens to a flat disk, the so-called holdfast, and the papillae connected to the host are drawn to the periphery, opening up the host periderm
Figure 3 A. Transverse section of slime-enveloped "seed" of hardwood-grown mistletoe (Viscum album ssp. album). Green endosperm enveloped in whitish, translucent mesocarp, with one embryo embedded in it, its hypocotyl pointing to the periphery (x9). B. Transverse section of an apple tree branch with a growing embryo of Visucm album on it. In the autumn, the sinker has become embedded in the wood of the host which responds with hypertrophic growth (x10). C. Part of mistletoe sinker in an apple tree branch, tangential section, stained with astral blue/safranin red. Host wood brownish, with light-coloured vascular strands coming in from the left. On the right the light-coloured sinker parenchyma, with dark xylem structures growing into it from the periphery, where host xylem and sinker tissue meet. These structures join to form a central vascular strand (x33). D. Longitudinal section of a secondary sinker of V. album ssp. album growing from a cortical strand (top, dark green). The sinker tissue consists of stratified xylem strands with green haustorial parenchyma intercalated between (x34). Photos: Raman layer by layer. The slightly oval holdfast shows bilateral symmetry, including a meristematic zone along the major axis and adjacent to the host bark. With cell divisions starting from here, the meristematic tissue is penetrating into the opened-up host periderm. A typical apical meristem develops, driving a wedge through the bark. Apart from mechanical forces, enzymatic processes coming from the mistletoe haustorium probably also help to open up the host tissues (Sallé, 1983). On young apple tree branches the apical meristem, pushing its way in centripetally, will reach the host's cambium about two months after holdfast attachment, which is towards the end of June.
Cell division activity in the mistletoe haustorium is going on then in an intercalary meristem which is established at the level of the host cambium. Like the cambium which produces cells centripetally that differentiate out into woody tissue, the intercalary mistletoe meristem produces new cells towards the central part of the branch. Enclosed by the young wood of the host, this new tissue forms the primary sinker (Figure 3B). Viscum album does not grow actively through the host cambium, but is passively embedded in the host's secondary xylem (Thoday, 1951; Sallé, 1979, 1983).
Host tissues close to the sinker are stimulated into hypertrophic growth. The branch swells, an important signal that the sinker is connected with the host xylem (Figure 3B). Secondary thickening will widen the intercalary mistletoe meristem in the following years, removing it from the centre of the branch in line with the host cambium. The primary sinker gradually assumes the form of a wedge brought in alignment with the host xylem's direction of flow. The oldest tissue of the sinker is resting deep in the wood and dies off only after some years.
For as long as the surrounding host tissue is still young and little differentiated, the sinker consists of pale green, undifferentiated parenchyma. But as soon as the vascular tissues of the host begin to differentiate out and solidify, corresponding differentiation begins also in the adjacent periphery of the sinker where this comes in contact with the host vessels. Parenchyma cells that follow one another are transformed into vessels by dissolving the organelles and hollowing out the cells, with cell walls transverse to the direction of flow reabsorbed, whilst cell walls that lie in the flow direction are reinforced with spiral or reticular thickening. As Melchior (1921) noted, this also involves secondary cell division in the sinker parenchyma. The xylem strands of the mistletoe's sinker sprout at right angles to the original flow direction of the host xylem, from the periphery to the central axis of the haustorium, opening out into the central vessels (Figure 3C).
At the same time as the primary sinker develops, so-called cortical strands begin to grow in longitudinal and also circular direction from the haustorial stem through the host's secondary phloem. Contact between the growing tip of cortical strands and actively dividing host cambium triggers the development of secondary sinkers. As in primary sinker development, their further development is taken over by intercalary meristems embedded in the host cambium. In contrast to the intercalary sinker meristems, which are physiologically adapted to the host cambium's growth rhythms and come to rest in winter, the apical meristems of the cortical strands do not have seasonal growth rhythms. The rate of secondary sinker development, on the other hand, is dependent on the host's cambial growth rhythms and reaches a maximum during the first half of summer (Sallé, 1978).
Cortical strand morphology is fundamentally different from sinker morphology. Growth starts from an apical meristem protected by an anterior zone of elongated cells (Melchior, 1921; Thoday, 1951). Apart from xylem which conducts sap taken up in the secondary sinkers, they also have phloem (Sallé, 1979; Sallé, 1983) which supports the growing tip with organic matter. Primary and secondary sinkers, on the other hand, have no phloem at all. And whilst open connections exist between the host's and the mistletoe's xylem in the sinker region (Melchior 1921; Sallé 1983; Becker 1986), there are no connections between host and mistletoe in the region of the cortical strands. Finally it is worth noting that adventitious shoots may grow from the cortical strands. These break through the cortex at a greater or lesser distance from the primary shoot.
A remarkable feature in the haustorial system of Viscum album is the green colour of the sinker parenchyma (Figure 3D), which persists for several years even deep down in the wood where no light should penetrate. In the sinker of Korthalsella, Fineran (1995) identified green pigments as chlorophyll. He speculated that this mistletoe species might be able to utilise even extremely low radiation potential by photosynthetic activity. Another aspect of interest is the way xylem vessels in the sinker of V. album are piled up (Figure 3D). In studies on Phoradendron, Calvin and Wilson (1995) established that this North American member of the Viscaceae only realises a rather small proportion of theoretically possible connections with adjacent host xylem by differentiating out xylem structures in its sinker. As the relative proportion of xylem structures also shows seasonal variations, they assume that Phoradendron is reacting to different pressures and vascular flow rates in early and late xylem. Research concerning the greening of parenchyma and the xylem organisation in the sinker of Viscum album is still missing.
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