Initiation, Development and Anatomy of Lignotubers in Some Species of Eucalyptus

Abstract

An initial survey of the diversity of early lignotuber development in Eucalyptus and an analytical study of the anatomy of young lignotubers and the seedling stem are presented. Studies of the early stages of the morphological development of the lignotuber in 13 species, representative of five taxonomic groups, resulted in the recognition of four modes of lignotuber initiation. The importance to lignotuber formation of the presence of a suite of accessory buds, adaxial to the axillary bud, is emphasized but lignotuber initiation is not in all cases associated with these buds. Lignotuber buds are derived by branching from existing buds, ultimately from the accessory buds of the node. Following its initiation, the possibilities of later morphological development of the lignotuber are discussed. Lignotuber growth may dominate over stem growth and the lignotubers at a node may then fuse laterally to encircle the stem. Stem growth, on the other hand, may dominate over lignotuber growth and the lignotuber then appears to regress. The consequences for the growth habit of the plant of these alternative pathways of development are outlined. The wood of young lignotubers (and that of the swollen hypocotyl) is shown to be different in composition and in the sizes of its elements from that of seedling stem wood; these differences owe their origin to differences in the nature and performance of the cambia of the lignotuber and stem. In lateral fusion of the lignotubers at a node, and their upward and downwards extension over the stem, e.g. over the hypocotyl, stem cambial initials are either progressively lost or, more likely, converted to lignotuber-type initials. The possibility of the reverse process occumng in stem dominance is discussed.