Le système endophytique du Viscum album : anatomie et fonctionnement des suçoirs secondaires

Abstract

Sinkers of V. album were studied in infected branchlets of Pyrus malus. Each sinker consists of parenchymatous cells and isodiametric vessels. Moreover, the oldest ones have a radially oriented tissue (radial system) with elongated tracheids. The outermost vessels of the sinkers are in direct contact with those of the host xylem. Serial sections show that vessel members around the periphery of the sinkers have perforations on their longitudinal tangential walls, whereas in the middle of the sinkers, perforations occur on their transversal or longitudinal walls facing the radial system. A scheme is proposed indicating how water and mineral salts travel, through the sinkers, from the host to the cortical strand. An intercalary meristem is observed at the base of the sinkers; it corresponds in position to the cambial zone of the host. Its cyclic activity is demonstrated: Oct.-Jan., this meristematic zone shows no DNA synthesis, no mitosis, and cells ''blocked'' in their G 1 presynthetic phase. During April-June, mitosis and DNA synthesis reach a maximum. A comparison is established between the growth of the host cambium and the one of the intercalary meristem.