IAWA Journal, Vol. 22 (4), 2001: 401–413 REACTION TISSUES IN GNETUM GNEMON A PRELIMINARY REPORT by P. B. Tomlinson Harvard Forest, Harvard University, Petersham, MA 01366, USA, and National Tropical Botanical Garden, 3530 Papalina Road, Kalaheo, HI 96741, USA SUMMARY Gnetum gnemon exhibits Rouxʼs model of tree architecture, with clear differentiation of orthotropic from plagiotropic axes. All axes have similar anatomy and react to displacement in the same way. Secondary xylem of displaced stems shows little eccentricity of development and no reaction anatomy. In contrast, there is considerable eccentricity in extra-xylary tissue involving both primary and secondary production of apparent tension-wood fibres (gelatinous fibres) of three main kinds. Narrow primary fibres occur concentrically in all axes in the outer cortex as a normal developmental feature. In displaced axes gelatinous fibres are developed abundantly and eccentrically on the topographically upper side, from pre-existing and previously undetermined primary cortical cells. They are wide with lamellate cell walls. In addition nar- row secondary phloem fibres are also differentiated abundantly and eccentrically on the upper side of displaced axes. These gelatinous fibres are narrow and without obviously lamellate cell walls. Eccentric gelatinous fibres thus occupy a position that suggests they have the func- tion of tension wood fibres as found in angiosperms. This may be the first report in a gymnosperm of fibres with tension capability. Gnetum gne-mon thus exhibits reaction tissues of unique types, which are neither gymnospermous nor angiospermous. Reaction tissues seem important in maintaining the distinctive architecture of the tree. Key words: Gnetales, Gnetum, reaction fibres, tree architecture, gelati- nous fibres, stem anatomy. INTRODUCTION This article reports a unique reaction anatomy in the Gnetales, where reaction anat- omy may be defined as secondary structural responses to external stimuli, particularly those involving displacement of axes. Reaction tissue (ʻReaktionsgewebeʼ of German authors) is usually defined as the relevant tissue that brings about either secondary re-orientation of mature plant organs or maintains organs in a fixed position relative to gravity by exerting some internal applied force. Reaction tissue is most familiar in the structural changes in secondary xylem of conifers and hardwoods, the tissue be- ing referred to collectively as ʻreaction woodʼ. Reaction wood includes specialized Downloaded from Brill.com09/24/2021 12:45:18PM via free access 402 IAWA Journal, Vol. 22 (4), 2001 Tomlinson — Reaction fibres in Gnetum bark 403 cells derived from cambial fusiform initials after the stimulus is received. Conifers and hardwoods are strongly contrasted in the type of reaction wood they produce and the way in which a mechanical force is generated. Compression wood occurs in coni- fers and Ginkgo (Timell 1980) and consists of modified tracheids (compression wood tracheids) usually developed eccentrically on the lower surface of a leaning stem or branch. The force generated, apparently by extension of cells, is positive, i.e., a ʻpushʼ. In contrast, hardwoods develop tension wood eccentrically, usually on the upper sur- face of leaning axes. The force generated, apparently by contraction of cells, is nega- tive, i.e., a ʻpullʼ and the tissue and cells responsible are termed tension wood and tension wood fibres, respectively. Tension wood fibres, in the absence of certain knowl- edge about their mechanical function, are often termed ʻgelatinous fibresʼ, from their characteristic highly hydrated, refractive and unlignified secondary cell walls, a con- vention followed here. It should be emphasized that reaction tissues, in an ecological context, can play many roles in the dynamics of woody plant organs, although this is often overlooked in the wood anatomical literature. For example, it can occur in leaves (Scurfield 1964; Staff 1974; Sperry 1982) of both dicotyledons and monocotyledons. It is responsible for the contraction of pendulous aerial roots in Ficus when they become rooted dis- tally, an unusual circumstance because the reaction tissue is developed concentrically (Zimmermann et al. 1964). It apparently plays an essential function in the early estab- lishment of the viviparous seedlings of mangrove Rhizophoraceae (Tomlinson & Cox 2000) and has been implicated in the maintenance of overall tree architecture (e.g. Fisher & Stevenson 1981). Reaction wood can be extensively developed in seed- lings (Scurfield & Wardrop 1962). An interesting evolutionary question is to consider the Gnetales in terms of reac- tion tissue. In a typological sense this group of plants stand intermediate between gymnosperms and angiosperms. Despite recent vicissitudes about the phylogenetic status of the Gnetales (e.g., Donoghue & Doyle 2000) recent studies show them to be most closely related to the conifers. However, as is well known, their secondary xylem includes well-developed vessels, with mainly simple but sometimes forami- nate perforation plates (Fisher & Ewers 1995). This raises the question addressed here: “Are the Gnetales gymnosperm- or angiosperm-like in their reaction anatomy?” The preliminary observations reported here show, at least in Gnetum gnemon, that they exhibit reaction tissues of unique types, neither gymnospermous nor angiosper- mous. MATERIALS AND METHODS Gnetum gnemon L. is a small dioecious tree widely cultivated in Southeast Asia as a kampong (village) plant for its edible leaves, seeds and for fibre (Burkill 1966), with a natural distribution in the Malay Archipelago from Malaysia to Papua New Guinea. Material was derived from a small population of male and female trees, appropriately cultivated at ʻThe Kampongʼ of the National Tropical Botanical Garden, Coconut Grove, Miami, Florida, USA. Trees reiterate trunk axes abundantly to provide an am- ple supply of stems for destructive sampling. For experimental procedures orthotropic Downloaded from Brill.com09/24/2021 12:45:18PM via free access 402 IAWA Journal, Vol. 22 (4), 2001 Tomlinson — Reaction fibres in Gnetum bark 403 axes 3–5 cm diameter were tied down by nylon string, the bark protected by bands of bicycle tire inner tube. A series of plagiotropic axes were similarly bent below the horizontal (i.e., below their natural orientation). In both experiments this introduced abnormal eccentric mechanical stress. After 6 months (June 2000 to January 2001) the axes were cut and an approximate map made of their shape, dimensions and inter- node number with lengths. To preserve the manipulated orientation of the stems a shallow groove was scored along their upper surface at the time they were collected. The groove appears as a shallow notch in subsequent transverse sections, indicating the upper sector of the axis (e.g. Fig. 10 n). The results section deals almost exclu- sively with orthotropic axes, the greater complexity of plagiotropic axes is only com- mented upon briefly. Transverse sections were cut without embedding from the middle of each inter- node, starting with the youngest, using a Reichert sliding microtome. Section thick- ness varied from 40 μm to 100 μm since it was necessary to produce an overall view without regard to section quality, as appears clear from the illustrations of thicker axes (Fig. 11–16). Longitudinal sections of a few axes were prepared in a similar way. Most sections were stained in 0.1% aqueous toluidine blue, washed in tap water and mounted in glycerine:water, 1 : 1 by volume, making semi-permanent prepara- tions. For histological purposes sections were stained in 95% alcoholic phlorogluci- nol and concentrated (38%) hydrochloric acid, 1 : 1 by volume as a test for lignin. Starch distribution was determined by mounting sections in aqueous iodine/potas- sium iodide (Lugolʼs iodine). For a study of cell types material was macerated by boiling slivers of tissue for three minutes in 10% aqueous potassium hydroxide followed, after repeated wash- ing, by 20% aqueous chromic acid (chromium trioxide). After 15–20 minutes the material was sufficiently soft to be rinsed and teased apart on slides in the glycerine mountant. Material was examined on an Olympus S2H compound microscope, photographed on Ektachrome and the colour transparencies converted to illustrative plates in the computer graphic laboratory at Harvard Forest. Cell wall features were enhanced, where necessary, by use of polarizing optics with a supplementary colour plate. Some material was embedded in ʻParaplastʼ and sectioned on a rotary microtome in the usual way, but gave poor initial results because of the high crystal content of young tissues. RESULTS Architecture Gnetum gnemon provides a precise example of Rouxʼs model in the Hallé-Oldeman system of tree architectural models (Hallé & Oldeman 1970; Hallé et al. 1978). Phyl- lotaxis is decussate throughout. Orthotropic (trunk) axes (O in Fig. 1, 3 & 4), with radial symmetry, have continuous branching with a branch pair produced by syllepsis at each node so that four orthostichies remain conspicuous (Fig. 1). Plagiotropic axes (P in Fig. 1, 3 & 4), initially close to the vertical, become horizontal with age. Dorsi- ventrality of each branch complex is established as each leaf pair rotates by twisting Downloaded from Brill.com09/24/2021 12:45:18PM via free access 404 IAWA Journal, Vol. 22 (4), 2001 Tomlinson — Reaction fibres in Gnetum bark 405 Fig. 1– 4. Gnetum gnemon. Habit. – 1: View from the ground up the trunk (orthotropic axis,