The Occurrence and Nature of Lignotubers in Notelaea longifolia and Elaeocarpus reticulatus

Abstract

A newly discovered form of lignotuber is described for Notelaea longifolia and Elaeocarpus reticulatus. The populations were studied in a Eucalyptus open-forest subject to fires but both species also occur in rainforest communities. The lignotubers start as two swellings at the cotyledonary node and occasionally at several succeeding nodes. Usually only one swelling develops and forms an elongated, unbranched and positively geotropic lignotuber. .Adventitious roots are developed by the seedling lignotuber and these replace the primary root system at an early age. Downward growth of the lignotuber is initiated by expanding buds, one of which becomes apical. The lignotuber exhibits sympodial growth. Elongation of the lignotuber is due to increased cambial activity at the lignotuber apex.

The xylem proximal to the apical bud consists of a core of parenchyma cells forming uniseriate or multiseriate rays and of irregularly arranged vessels. Fibres are lacking. As secondary thickening of the lignotuber proceeds, the cambium produces ray parenchyma and files of vessels alternating with fibres.

The xylem of Notelaea lignotubers does not have an easily discernible sapwood-heartwood zonation. Both old and young parenchyma cells store starch, although their cell walls have bordered pits and are apparently lignified. The relationship between the age of parenchyma cells and the functions of starch storage, mobilization and replacement is unknown.

The lignotubers of Notelaea and Elaeocarpus differ from Eucalyptus lignotubers in that they: (1) usually develop from only one of the paired swellings of a cotyledonary node; (2) do not fuse with the stem or root; (3) produce adventitious roots that replace the primary root at an early age; (4) grow sympodially; and (5) have an elongated form due to the greater activity of the cambium at the lignotuber apex than at the flanks.