DEVELOPMENT AND ANATOMY OF THE ATTACHMENT STRUCTURE OF WOODROSE-PRODUCING MISTLETOES

The formation of woodroses in Sclerocarya birrea (A. Rich.) Hochst, an d Combretum collinum Fresen. in reaction to the parasitic mistletoes E rianthemum dregei (Eckl. & Zeyh.) Tiegh. and Pedistylis galpinii (Schi nz ex Sprague) was investigated. Eight woodroses of varying sizes, and by implication age, were sectioned with the freeze microtome and the sledge microtome. As the mistletoe entered the host tissues it assumed a flask-shape due to mechanical constriction by layers of host cork. There was a clear differentiation between host cells containing tannin and mistletoe cells without tannin throughout the sections. In additi on these were separated by a thin wavy black line of gum. The mistleto e occupied a central position as well as producing finger-like project ions into the host tissue. Older sections indicated an increase in tan nin and sclereids, while the arrangement of axial xylem elements becam e increasingly disrupted. There appear to be two strategies to prevent a functional connection with the mistletoe: 1) the production of woun d periderm at the host surface and 2) internal necrosis and disruption of internal host tissues. The study species showed signs of both but the mistletoes managed to by-pass these strategies and instead lead to the formation of elaborate and intricate woodroses.