Bamboo and Rattan are important resources in China as well as in other countries of the region. Structural research can lead to a better understanding of the properties to improve utilization for producing more value-added products.
Few examples are outlined from recent research — for bamboo- vascular bundle types, nodal influences, the rhizome as the culm’s base, starch as energy reservoir, ageing of culms and wound reactions and for rattan -generic differentiation of rattans, modifications due to ageing, commercial vs. non-commercial rattans and quality improvement by polymer impregnation. Wood anatomists should contribute further to explain and improve the properties behaviour of bamboo and rattan.
Bamboo and Rattan are important forest resources in China as well as in other countries of the region. For the benefit of the billions of people depending on these materials it is necessary to improve the often traditionally processing technologies and to produce value added products.
Structural research can lead to a better understanding of the properties and improve utilization of bamboo and rattan. Our efforts over the years are directed towards these goals. In the following, a few examples are outlined from recent research.
1. Bamboo:
Bamboo is, since ages of great importance for Chinese people. About 400 species belonging to 40 genera exist in China on a total area of 3.66 million ha. Their properties and various uses have recently been summarized; it is an effort to increase further utilization of bamboo in China, as a substitute for timber.
(a) Vascular Bundle Types:
The more than 1000 bamboo species world-wide have different physical and mechanical properties, resulting in differences for processing and product quality. Their anatomical structure in generally, made-up by ground parenchyma (around 50%), fibre (around 40%) and conducting tissue (around 10%, metaxylem vessels and phloem).
Differences are mainly caused by the pattern of the vascular bundles. Leptomorph genera represent the vascular bundle type I, as open type with four sclerenchyma sheaths surrounding the vessels and phloem, like Phyllostachys, or as a semi-open type with the lateral and inner hundle sheaths linked together, like Sasa.
Pachymorph genera, like Bambusa, Dendrocalamus, Melocanna and Schizostachyum represent bundle type II, III and IV with extended fibre sheaths and isolated fibre bundles. The increased fibre content to these types is accompanied by a higher density.
The basic differences in the anatomical make-up must effect a number of properties, such as fibre percentage, density, strength, bending behaviour, shrinkage and splitting. A detailed comparison of all these interrelationships can be of considerable impact for processing qualities as studies on fibre morphology by Ma et al. (1993) on 26 species of 9 genera and by Zhang et al. (1995) on 31 species of Phyllostachys have shown. Pachymorph bamboo species may be less suitable out when sanding the surface. Chopsticks are mainly made from leptomorph species, like Phyllostachys, with only fibre sheaths.
(b) Nodal Influences:
Investigations on bamboo structures consider mainly the internodes, whereas, the nodal part is often neglected. The nodes, however, bear special significance for the intercalary growth and the cross transportation of water and nutrients in the living culm, and influence also seasoning, preservation and technological properties.
Studies revealed the three dimensional arrangement of the vascular system. Whereas some of the axial vascular bundles pass directly from an internode through the nodal region, others bend and intensive vascular anastomoses develop.
The characteristics of leptomorph and pachymorph bundle types vanish and no isolated fibre strands remain. The metaxylem vessel members become branches and intensively pitted, they are surrounded by smaller cells as an “accessory tissue”. Sieve tubes do not branch but develop spindle-like agglomerations of filiform cells for intensive interconnection, similar to the so-called “Phloem Beckenzellen” of Dioscoreacae. At the nodal reason the fibres are considerably shorter than within the internodes and shortest at the diaphragm. Mechanical elasticity is reduced due to shorter, thicker and also forked fibres, the lignin content is higher and thus the density increases.
(c) The Rhizome as the Culm’s Base:
The rhizome is the most important part for expansion and culm growth. Leptomorph species are shown in contrast to their rather uniform culm distinct anatomical differences in the rhizome’s general structure. The percentage of different tissues is changed.
The amount of conducting tissue increases from about 10% in the culm to 20%, likewise the parenchyma cells from 40% to 60%, at the expense of the fibres decreasing from 40% to 20% only. The investigation of 20 leptomorph bamboo species revealed 4 basic anatomical types which are based on the cortex structure and vascular arrangement; they are expressed even by different species within one genus. Remarkable is the presence of large air canals in the cortex of several species. Their relevance for the greater tolerance towards soil moisture or for growth intensity has still to be elaborated.
(d) Starch as Energy Reservoir:
Starch is a cell constituent of special significance for the living culm and its later utilization. It is stored abundantly in the culm and rhizome, and has to be mobilized and transported to the emerging shoots. Only few investigations have dealt so far with the occurrence of starch in bamboo and were mostly concerned with its significance to beetle borer infestation. Some species, such as Bambusa, are said to be less liable towards beetle attach due to a lower starch content, others with a higher content, such as B. vulgaris to be more susceptible facts or opinion?
The starch grains represent only the solid part of the energy stored within a parenchyma cell, so that for such relations also the sugar content has to be considered.
Distribution and frequency of starch have been investigated in a number of bamboo species according to culm age and season. Since, the presence of starch indicates a living cell, it is noteworthy that even in culm of 12 years (Phyllostachys viridi-glaucescens); the parenchyma was filled up. Starch mobilization occurs with the growing season for the new shorts, but also as a reaction towards wounding with a special accumulation within the diaphragm.
(e) Ageing of Culms:
Properties and utilization of bamboos are influenced by structural changes due to ageing. This concerns mainly the fully elongated culm during its “maturation”. Especially the fibres show a wall thickening during these first two years. But even older culms of ages between 9 and 12 years reveal an additional increase in wall thickness. Associated is wall thickening also of parenchyma cells. This may explain sporadic observations about an increase in density even of older culms.
These structural modifications in older culms can have an effect on the stability of the culm wall. It is sometimes mentioned that culms of higher age are preferred for bamboo furniture since; they show less shrinkage and splitting, the worst danger for a furniture producer besides beetles.
The practice in well managed forests of China to cut only culms of at least 5-6 years old may find thus a structural explanation, because the culms become apparently stronger. Detailed investigations are warranted.
(f) Wound Reactions:
Bamboo culms can occasionally be injured by mechanical damage or by beetle attack, but they are wounded frequently during the planting process by vegetative propagation. In China the top part of Phyllostachys edulis culms is regularly cut off to prevent snow damage and to be used as an additional materials resource.
In order to protect the conducting tissue in the living culm against inactivation by invading air, efficient response mechanisms must exist to seal-off the damaging influences. Extensive investigations on artificially wounded culms of many species have revealed a time dependant sequence of distinct cellular reactions. The metaxylem vessels are closed by slime formation and in several species also by tyloses. Sieve tubes show callose plugs, slime and lignification.
Parenchyma exhibits additional cell-wall-layers, and in the short cells a cell-wall-lignification and the formation of phenolic compounds. Fibres form also additional wall-layers and sometimes septa. Starch is deposited around the wound edge in parenchyma cells and fibres. Thus, definite wound reactions develop to protect the functions of the living tissue.
Similar response mechanisms occur in the rhizome, which is even more endangered in any planting operation due to the divided root masses. As a special feature, the intercellular canals along the short parenchyma cells may be mentioned, which become filled with brown-black substances.
(g) Structural Influences on the Treatability of Culms:
Since, bamboo culms as monocots with no secondary growth do not develop protective secondary metabolites, a chemical protection can be necessary for certain fields of use, such as constructions. The special culm structure restricts very much any lateral penetration, but offers easy axial pathways through the large and long metaxylem vessels.
Thus, the sap replacement method appears particularly suitable for the treatment of fresh bamboo culms. However, the metaxylem vessels amount to only less than 10% of the total tissue to be protected. Therefore, as a second treatment-phase a diffusion of a suitable preservative is required from the vessels into adjacent fibres and parenchyma.
The vessel area and their size vary considerably within a culm, but show also distinct differences between species. Detailed measurements of the vessel area will provide reliable information of the species to be treated for determining parameters of the treatment process.
In addition, the wound reactions developed by the tissue at the cross ends will have a considerable influences on the treatability. Slime and in numerous species also tyloses close the vessels and restrict the inflow of the preservative as well as its outflow. For an easy exchange these structural influences on the permeability have to be considered and avoided.
2. Rattan:
Rattans, the climbing palms in the tropical rain forest of Southeast Asia and also West-Africa, provide the most important “minor forest” products. About 90% of the world’s rattan produce is still harvested from the wild, the remainder from the few plantations.
Of the about 650 species belonging to 13 genera, China has 40 species of 3 genera. The area of natural forest with distribution of rattan in China is about 0.3 million ha, with an annual output of raw rattan of about 4000-5000 tons.
The history of rattan usage in China is about 150 years old. With, the developed rattan industry with almost ten thousand varieties of furniture and handicraft is creates several ten thousand employments and exceeds several hundred million RMB Yuan of annual output value.
(a) Generic Identification of Rattans:
Unlike bamboo, the rattan genera present distinct anatomical differences. Of special significance is the number of phloem fields and vessels per vascular bundle, the type of ground parenchyma, the occurrence of fibre-rows in the cortex, of the “yellow cap” and rapid sacs, further the arrangement of sieve tubes and size of the fibre sheath of the outer vascular bundles.
On the basis of these features a dichotomous key for the identification of the rattan genera has been developed, including also the three African endemic genera. Since the technological properties of the many species differ considerably, identification at least at the generic level is of practical value, also for disputes in trade. For the rattans from South India, even a differentiation up-to the species level was possible ‘Further identification efforts are worthwhile for the increased utilization of this vast resource.
(b) Modification due to Ageing:
A considerable anatomical variation characterizes the rattan stem. Besides horizontal differences from the cortex towards the middle, a pattern exists also along the stem length due to ageing. Age related changes occur with respect to wall thickness of fibres and parenchyma cells, although with distinct differences between species.
Thus, the decrease with height is strong for Calamus rotang, C. hookerianus and C. thwaitesii, but lesser for C. tranvancorius and C. metzianus. Detailed studies on a 5 years old stem of C. axillaries with 55 internodes showed, that the fibres thicken by addition of alternating broad and narrow lamellae.
Thus, the average wall thickness increased from the top internodes towards the base from about 1.5 pm to about 4 pm. Since the base of the only 5 years old plant contained living fibres, a further increase of wall thickness could be assumed.
The structural immaturity of the upper internodes, evident by their thin fibre walls, will lead to an easy breakage, e.g. if, these parts are used for furniture making. Fibre wall thickness can be criterion for strength properties.
(c) Commercial Vs. Non-Commercial Rattans:
Of the around 650 species only about 25 are traditionally used for furniture making. Factors for selection are- availability, diameter, and length of internodes, colour, but also processing qualities. These are based on properties, mainly reflecting their anatomical structure. A comparative investigation of the tissue make-up of the best and the neglected species has revealed certain parameters as typical for traditionally preferred species.
A Commercial Species Is Characterized by the Following Criteria:
(a) Distribution of vascular bundles over the cross-section, approximately 20%-25% fibres, 45% conducting elements, 307f-55% ground parenchyma;
(b) Fibres in caps of equal size, fibre walls with polylamellate structure and an equal cell length over the inner part of the central cylinder,
(c) Ground parenchyma cells with thick, polylamellate walls.
These characters should be tested on other species of adequate size which were neglected so far. This applies also to regions, where rattans are not yet much appreciated and commercially used. An analysis of 17 samples from 7 species (four genera) from West Africa revealed that Calamus deeratus shows similarities with the Asian species used, whereas the species from the three endemic genera resemble the non-commercial rattans.
(d) Quality Improvement by Polymer Impregnation:
To overcome certain inferiorities of rattans like- the low natural resistance, swelling, cracking or breaking, rattan-polymer composites have been considered, as also for bamboo and cocos-wood. Such trails are based on the good experiences for wood to improve technological properties and also the natural resistance without toxic chemicals.
The efficiency of monomer penetration, however, depends on the anatomical structure of the material to be treated. Monomers do penetrate through the lumina of the cells, but infiltration of the cell wall is limited to only few substances with low molecular weight.
To overcome structural barriers and the hindrance by compressed air due to the invading monomer and evaporation of the woody materials is necessary, followed by a pressure forced penetration. Good results as for softwood and hardwoods, however, cannot be expected for rattan and bamboo.
Their permeability is restricted to an axial penetration, e.g. around 20% respectively below 10% for bamboo in comparison with around 70% for softwoods and 30% for diffused porous hardwoods. Since, the monomers hardly diffuse further into the surrounding tissue; only a small portion of the whole tissue can be penetrated. Therefore, a rather limited application appears possible like for a stabilization of furniture parts or for an improvement of certain technological properties of inferior species.
Outlook:
These few examples should outline some areas, where structural investigations could contribute to a better understanding of the properties behaviour of bamboo and rattan. More has to be done, since; the needs are great to utilize bamboo and rattan more efficient for the benefit of the valuable natural resources and the billions of people depending on them.
The research needs concern broad areas such as: culm/stem properties, durability and preservation, processing technologies and product development, to all of which anatomists can contribute considerably.