Structural research on bamboo can contribute to a better understanding of how its properties will behave under given conditions. Such knowledge is, essential for optimizing the current uses as well as for innovating new uses of bamboo. The research needs concern broad areas such as culm properties, durability and preservation, processing technologies and product development, to all of which anatomists can contribute considerably.
Vascular Bundle Types:
The more than 1000 bamboo species worldwide have different physical and mechanical properties, resulting in differences for processing and in product quality. Their anatomical structure is generally made up by ground parenchyma (around 50%), fibres (about 40%) and conducting tissue (metaxylem vessels and phloem, around 10%).
Differences are mainly caused by the pattern-of the vascular bundles – Leptomorph genera represent the vascular bundle type I- open type with four- sclerenchyma sheaths surrounding the vessels and phloem, like Phyllostachys, or semi-open type with the lateral and inner bundle sheaths linked together, like Sasa. Pachymorph genera like Bombusa, Dendrocalamus, Melocanno and Schizostachyum – represent bundle type II, III and IV, with extended fibre sheaths and isolated fibre bundles. The increased fibre content of these three types is accompanied by a higher density.
The basic differences in the anatomical make-up must reflect in a number of properties, such as fibre percentage, density, strength, bending behaviour, shrinkage and splitting. A detailed comparison of all these interrelationship can be of considerable importance for processing, as fibre morphology studies by Ma et al. (1993) on 26 bamboo species of 9 genera and by Zhang et al. (1995) on 34 species of Phyllostachys have shown. For instance Pachymorph bamboo species may be less suitable for bamboo parquet because the isolated fibre bundle can stick out when sanding the surface. Chopsticks are mostly made from Leptomorph species, such as Phyllostachys, with only fibre sheaths.
Influence of Nodes:
Usually, research on bamboo structure tends to focus on internodes, often neglecting the nodal part. The node, however bears special significance for the intercalary growth and the cross transportation of water and nutrients in the living culm, and nutrients in the living culm. It also influences seasoning, preservation and technical properties.
Studies have revealed the arrangement of the vascular system to be three-dimensional. While 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 branched, intensively pitted, and surrounded by smaller cells as an accessory tissue. Sieve tubes do not branch but develop spindle-like agglomerations of filliform cells for intensive interconnection, similar to the so-called “Phloem Beckenzellen” of Discoreaceae. At the nodal region, the fibres are considerably shorter than within the internodes and shortest at the diaphragm. Mechanical elasticity is reduced owing to shorter, thicker and forked fibres. The lignin content is higher and thus the density is increased.
The Rhizome as the Culm’s Base:
The rhizome is the most important part for expansion and culm growth. Leptomorph species show, 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 in the culm increases from approximately 10% to 20% and the parenchyma cells from 40% to 60% at the expenses of the fibres which, however, decreases from 40% to 20% only.
The investigation of 20 Leptomorph bamboo species revealed four basic anatomical types, 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 to a greater tolerance for soil moisture or to growth intensity is yet to be established.
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 the rhizome, and has to be mobilized and transported to the emerging shoots. The few investigations that have so far dealt with the occurrence of starch in bamboo are concerned mostly with its significance to beetle borer infestation.
Some species, such as Bombusa textills, are said to be less susceptible to borer attack owing to their lower starch content, and some other species, such as B. vulgaris, more susceptible because of their higher starch content, are these facts or opinions? The starch grains represent only the solid part of the energy stored within parenchyma cells, to establish such relations, the sugar content also has to be considered.
Mobilization and distribution of starch have been studied in a number of bamboo species at different ages and in different seasons. Since the presence of starch indicates a living cell, it is noteworthy that even in culms 12 years old (Phyllostachys viridiglaucescens) the parenchyma has starch. Starch mobilization occurs with the growing season of the new shoots, but also as a reaction to wounds, with a special accumulation within the diaphragm.
Ageing of Culms:
Properties and utilization of bamboos are influenced by structural changes brought about by ageing. This mainly concerns the fully elongated culm during its ‘maturation’. The fibres in particular show a wall thickening in these first two years. But even older culms of ages between 9 and 12 years reveal an additional increase in wall thickness. An associated thickening of parenchyma cells is also observed. This may explain the sporadic observations about an increase in density even in 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 as they show less shrinkage and splitting, the worst fear for a furniture manufacturer besides beetles. A detailed investigation on this aspect is warranted.
Wound Reactions:
Bamboo culms can occasionally be injured by mechanical damage or by beetle attack, but they are wounded more frequently during the planting process by vegetative propagation. In China, the top part of Phyllostachys edulis culm is regularly cut-off to prevent snow damage and for use as an additional material resource. Such operations result in a big wound for the remaining culm.
Efficient response mechanisms must exist to protect the conducting tissue in the living culm by sealing it off against the damaging influences of the invading air. Extensive studies on artificially wounded culms of many species have revealed a time dependent sequence of distinct cellular reactions.
The metaxylem vessels are closed by slime formation and, in several species, by tyloses also. Sieve tubes show callose plugs, slime and lignification. Parenchyma exhibits additional cell wall layer and in the short cells, a cell wall lignification and the formation of phenolic compounds occur. Fibres form 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 owing to the divided root masses. It may be mentioned as a special feature that the intercellular canals along the short parenchyma cells become filled with brown- black substances.
Structural Influences on the Treatability of Culms:
Since bamboo culms, as monocots with no secondary growth, do not develop protective secondary metabolises, a chemical protection may be necessary when the culms are used in certain fields, such as construction. The culm structure restricts any lateral penetration of the chemicals, 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, diffusion of a suitable preservative is required from the vessels into the adjacent fibres and parenchyma.
The area and size of vessels vary considerably within a culm, and show distinct differences between species. Detailed measurements of the vessel area will provide reliable information of the species to be treated for determining treatment parameters.
In addition, the wound reactions developed by the tissue at the cross-ends will have a considerable influence on the treatability. Slime and, in several cases, Moses closes the vessels and restricts the in-flow and out-flows of the preservative. For an easy exchange, these structural influences on the permeability will have to be considered and avoided.