The nature of reaction wood. IX. Anomalous cases of reaction anatomy

Abstract

The reaction anatomy of 14 species of Acacia, Lagunaria pattersoni, and the woody monocotyledons Dracaena fragrans and D. marginata was examined with light, polarizing, and ultraviolet microscopes.

Notable features of the reaction wood fibres of Acacia spp. were: the absence of birefringence in the sublayers of the "gelatinous" layer at all stages of their deposition; the absence of radial striations in the gelatinous layer; the frequency of occurrence of fibres lacking protoplasts and with convoluted gelatinous layers at some intermediate stage of development; the relatively wide separation of stages of gelatinous layer formation in passing from cambium towards pith; and the poor adherence of the gelatinous layer to previously formed cell wall layers. It was concluded that the forces bringing about gelatinous layer deposition were either weaker or slower to develop, or both, than in species such as Tristania conferta. Possible reasons for this are discussed. Deposition of starch grains in reaction wood fibres occurred in all Acacia spp, towards the end of the growing season.

A gelatinous layer did not occur in reaction wood fibres of Lagunaria pattersoni. These differed from normal fibres only in the thinness of their walls and their relatively large cross-sectional area. Examination of the wall structure of phloem fibres indicated that changes in microfibril orientation towards a more axial position which occurred in layer S2 of primary phloem fibres were accelerated on the upper sides of bent stems. The significance of this feature, and of possible tensions developed in the outer cortex and epidermis on the upper sides of bent stems because of accelerated phloem and xylem development, are discussed in relation to mechanisms whereby such stems may recover to a vertical position.

The number of vascular bundles and the wall thickness and extent of lignification of the fibres of the bundle sheaths, were increased on the upper sides of horizontally grown stems of Dracaena spp. The radial width of the cortex and the cross-sectional area of cortical cells were less than on the lower sides of the stems.