The Mistletoe Shoot

The hypocotyl of young mistletoe embryos that have achieved connection with the water conduits of the host during summer may come upright already in autumn. The shoot apex is drawn forth from the endocarp, grown empty and dry in the meantime, so that the rudiments of the opposite cotyledons become visible. In most cases, visible growth only begins when the next spring is coming. Two elongated leaves will arise on a more or less extended internode, decussate to the rudimentary cotyledons. Stem and leaf tissues grow firmer in texture as the vegetative period progresses, but no further organs develop at this time. A new twig finally develops in the following spring, its only pair of leaves decussate to those from the previous year (Figure 4A). With this simple growth gesture, Viscum album makes decussation as a typical characteristic held in common by most mistletoes (Kuijt, 1969). Because of the analogous appearance between mistletoe twigs and dicotyledonous seedlings, the mistletoe bush is occasionally referred to as an "aggregate of seedlings" (Grohmann, 1941).

Figure 4 A. Viscum album growing on apple tree at the beginning of the third vegetative period. B. Old and young twigs of V. album in May. A typical feature at this developmental stage is the negatively geotropic orientation of the pale green shoots of the current year. Photos: Raman.

Mistletoe leaves do not develop the typical bipolar differentiation into palisade and spongy parenchyma. Their tissue remains at a meristematic stage, with further growth in thickness and elongation in the following spring (Gobel, 1994). The normal case is that mistletoe leaves drop in the late summer of the second vegetative period, being still green and turgescent. Leaves that are three or four years old and correspondingly larger can, however, frequently be found on mistletoe growing on elms and fir trees. Goedings (1997) drew attention in this respect to the host dependence of some morphological and in conjunction with this also physiological properties of V. album.

From the second or third year of growth, new mistletoe twigs arise not only from the apical meristem but also from basal buds. Apical shoot meristems are no longer available for vegetative development from the fifth, sixth or seventh year of growth. From this time on they provide the basis for generative development and differentiate as highly compressed generative short shoots with flower buds. The result is a typical pseudo-dichotomy with two dominant forked shoots which grow from the axillae of the opposite leaves from the year before. Each of these can be accompanied by two lateral complementary shoots (Figure 4B) which grow from the axillae of the paired bud scales that initially protect every shoot (Figure 5A) but later drop off.

The transition from vegetative to generative organ development occurs during June when the primordia of the next year's twig generation are developed (Gobel, 1994; Ramm, 1995; Dorka 1996). When these arise in the leaf axils of developing young shoots, the intervention of the flowering impulse shows itself as soon as the primary pair of leaves has differentiated out, with the spherical structure of the inflorescence bud developing from the apical meristem (Figure 5A). At its base, the primordial meristem will rest for about nine months, and only begin to differentiate out flowering organs in the following spring.

Starting in the second half of May the currently developed twigs of Viscum album show characteristic growth movements (Gobel and Dorka, 1986; Dorka, 1996). These nutations, synchronised in time but largely independent in spatial terms, release the young shoots from their original, negatively geotropic orientation (Figure 4B). When growth movements come to an end in July, all the young mistletoe twigs have assumed their appropriate orientation in the spherical form of the bush. Mistletoe bushes may reach up to 1 meter in diameter during a life time of 20 or more years.

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