In this article we will discuss about the shoot growth and gross anatomical structure of bamboos.
Bamboo is a fascinating plant for the many thousands of bamboo lovers in temperate climates, both in Western Europe and North America. But for billions of people in Asia, South-America and also Africa, bamboo is an integral part of their daily life requirements and its use ‘from the cradle to the coffin’, is an often cited proverb. Bamboos are used for countless different purposes.
They grow abundantly in many parts of the tropics and sub-tropics. Often only certain species are preferred for distinct uses, whereas, other species are neglected. Restrictions, which exist in processing and utilization of the many bamboo species, are often related to unsuitable properties.
The “bamboo wave” which is presently obvious in many parts of the world with larger plantation schemes requires also considerations about the later utilization, which determines much the economy of any action program. A thorough understanding of the relations between structure, properties, behaviour in and during processing and subsequent product qualities is necessary for promoting the utilization of bamboo.
Shoot Growth:
Bamboo as a giant grass produces a primary shoot without any later secondary growth. The elongation of a culm results from the expansion of its individual internodes and starts at the lower internodes. In the beginning of differentiation, the whole internode consists of an intercalary meristem. In its upper part it differentiates into the various cell types which elongate, while the lower part retains its meristematic activity.
With the elongation of the cells the meristematic activity moves down to the base of the internode, it finally ceases to divide, and the lower cells elongate until the internode has fully developed. Between the internodes a horizontal plate exists which forms the diaphragm and the nodal division.
Whereas, in the internodes all cells are axially oriented, the nodes provide the only transverse interconnection. They also have a lock-like function for which the cell elements of the internodes become modified by branching.
A bamboo culm is developed by extension of the internodes until, its full height of 10-30 m within a period of 3-4 months. This means that bamboos are the fastest growing plants. Taking into account their diameter of 20 cm or more, an enormous biomass must be produced and reported from the stored energy in the rhizomes and by the younger culms within a short period of time. The growing culm hardly possesses leaves for the production of its own energy required for the extension.
Gross Anatomical Structure:
The bamboo culm shows a rather simple anatomical construction and among the about 700 species, differences are rather small, and may be considered as insignificant, compared to the large structural heterogeneity of the about 25000 timber species. Also growth conditions have apparently no significant effect on composition and structure of the culm. But ageing induces some structural changes.
The gross anatomical structure of a transverse section of any culm internode is determined by the shape, size, arrangement and number of vascular bundles. They are clearly contrasted by the darker coloured sclerenchymatous tissue against the parenchymatous ground tissue.
At the peripheral zone of the culm the vascular bundles are smaller and more numerous, in the inner part larger and fewer. Within the culm wall the total number of vascular bundles decreases from bottom towards the top, while their density increases at the same time.
The tissue of a culm consists of parenchyma cells and the vascular bundles, which are composed of vessels, sieve tubes with companion cells and fibres. The total culm comprises of about 50% parenchyma, 40% fibre and 10% conducting tissues (vessels and sieve tubes) with some variation according to the species.
The percentage distribution of cells shows a definite pattern within the culm, both horizontally and vertically. Parenchyma and conducting cells are more frequent in the inner third of the culm wall, whereas in the outer third the percentage of fibres is distinctly higher. In the vertical direction the amount of fibres increases from bottom to top and the amount of parenchyma decreases.
(a) Parenchyma:
The ground tissue consists of parenchyma cells which are mostly vertically elongated (100* 20 µm) with short, cube-like ones interspersed in between. The former are characterized by thicker walls with a polylamellate structure, they become lignified already in the early stages of shoot growth but can still be alive in culms older than 10 years. The shorter cells have a denser cytoplasm and thinner walls. The function of these two different types of parenchyma cells is still unknown.
(b) Vascular Bundles:
The vascular bundle in the bamboo culm consist of the xylem with one or two smaller protoxylem elements and two large metaxylem vessels (40-120 µm) and the phloem with thin walled unlignified sieve tubes connected to companion cells.
The vessels possess large diameters in the inner part of the culm wall and become smaller towards the outside. These water conducting cells have to function without the formation of any new tissue, as in the case of hardwoods and softwoods with cambial activity. Any interruption of the water transport by suction force of the leaves may be critical and has to be stopped efficiently. Special response mechanisms become active, as will be shown later for the alterations induced by ageing.
Both the metaxylem vessels and the phloem tissue are surrounded by sclerenchyma sheaths. They differ considerably in size, shape and location according to their position in the culm and the species. Some genera have additional fibre bundles which are separated from the vessels by parenchymatic tissue.
Four respective types of vascular bundles can be differentiated.
Type I:
Consisting of one central vascular strand supporting, tissue only as sclerenchyma sheaths.
Semi-open. rsp. open type:
Chinese scientists have differentiated Type I into a semi-open type with lateral and inner vascular bundle sheaths linked together, and an open type with the four vascular bundle sheaths of same size and symmetrically located.
Type II:
Consisting of one central vascular strand supporting tissue only as sclerenchyma sheaths at the intercellular space (protoxylem) strikingly larger than other three — tight-waist type
Type III:
Consisting of two parts, the central vascular strand with sclerenchyma sheaths and one isolated fibre bundle — broken-waist type
Type IV:
Consisting of 3 parts, the central vascular strand with small sclerenchyma sheaths and two isolated fibre bundles outside and inside the central strand — double-broken-waist type
Leptomorph genera have only one central vascular strand with supporting fibre sheaths. Pachymorph genera show a greater variation with one or even two isolated fibre bundles outside and inside the central strand.
Some Examples:
Type I alone- Arundinaria, Phyllostachys, Fargesia, Sasa and Pseudosasa
Type II alone- Cephalostachyum, Pleioblastus
Type II and Type III- Melocanna, Schizostachyum
Type III alone- Oxytenanthera, Gigantochloa
Type III and IV- Bambusa, Dendrocalamus, Guadua
These basic differences in the anatomical make-up affect also a number of properties like density, strength, bending behaviour, splitting and shrinkage.
The phloem of the vascular bundles consists of large thin-walled sieve tubes, among which smaller companion cells are distributed. These cells have to remain alive and active throughout the lifetime of the culm.
(c) Fibres:
The fibres contribute 60-70% of the weight of the total culm tissue. They are long and tapered at their ends. The ratio of length to width varies between 150: 1 and 250: 1. The length show considerable variation between and within species. Mean values are – Bambusa tulda 3 mm, B. vulgaris 2.3 mm, Dendrocalamus giganteus 3.2 mm, Guadua angustifolia 1.6 mm, Phyllostachys edulis 1.5 mm. Generally, the fibres are much longer than those from hardwoods (1-1.5 mm). Different values have been reported for one and the same species. The reason is mainly due to considerable variation of fibre length within one culm.
Across the culm wall the fibre length often increases from the periphery towards the middle and decreases towards the inner part. Along the culm from base to top no remarkable pattern for the fibre length exists except a slight reduction, whereas a great variation is evident within one internode of up-to 100% or more. The shortest fibres are always near the nodes, the largest are in the middle.
Thus the nodal part has a reduced strength due to its shorter fibres and marks the breaking point for the standing culm. In service however bamboo breaks hardly at the nodes because of a higher fibre portion due to reduced parenchyma and increased Lignification.
Furthermore, the irregularity of the grain with an interwoven structure provides more shearing strength than the internodes with their axial arrangement of fibre sheaths and bundles embedded in homogeneous parenchyma. Test samples with nodes however, exhibit lower bending, compression and shear strength, although a higher specific gravity.
The ultrastructure of some of the fibres is characterized by thick polylamellate secondary walls. The lamellation consists of alternating broad and narrow layers with differing fibrillar orientation. This polylamellate wall structure is present especially in fibres at the periphery of the culm, and their significance for bending properties appears obvious.
Thin walled fibres do not have such a lamellation. The occurrence and distribution of such fibres will influence certain properties and processing qualities, so that the detailed studies with technologically superior and inferior bamboo species on the fibre type appear useful.
(d) Outer and Inner Skin:
The culm is protected at the outer and the inner side by a special tissue against loss of water and mechanical damage. The outer part consists of two epidermal cell layers with high silica content and a cutinization of the cell walls.
The inner layer consists of numerous thick walled parenchyma cells, is highly lignified and shows sometimes a suberized membrane. Any loss of water or penetration by liquids is thus much restricted with consequences for drying the culm and their treatment with preservatives. Pathways are only the cross ends of a culm and to a rather small extent the sheath scars around the nodes.
Structural Changes due to Wounding and Ageing:
Structural changes occur due to wounding and ageing. If a culm is damaged mechanically, the transport functions of the vessels are endangered. Response reactions of the surrounding parenchyma cells occur, which lead to a blocking of the vessel lumina, either by formation of tyloses or by the production of slime-like substances. A browning of the parenchyma indicates the production of phenolic substances. During ageing of culms definite changes occur from its immature stage towards maturation upto dying/death of the culm.
They appear in the first and second year in the form of an additional cell wall thickening of fibres and parenchyma cells as well as by progressing lignification. In older’ culms, vessels and sieve tubes can partly become blocked by tyloses and depositions in vessels and sieve tubes, thus loosing their conductivity, which may cause death of the aged culms.
The numerous investigations of strength properties in relation to culm age have mostly shown an increase during the maturation period, but also for consecutive years. Dying culms become brittle and are often bent down and break. The structural/ chemical basis for such drastic changes in properties is still much concealed.
Natural Resistance and Preservation:
During the ageing of culms no toxic substances are formed, which constitute in many trees the heartwood with high natural resistance against microorganisms and beetle attack. Bamboo is generally liable toward degradation, but fungi can become active only in humid environments. The attack by the common bamboo borers depends, mainly on the starch content as nutritional sources. The amount of starch present changes with the growing season, and the ageing of the culm.
Preservation of bamboo respectively its penetrability is another area where anatomical structure is decisive. The culm is covered at its outer and inner side by an impermeable skin. Also the absence of ray cells as radial pathways determine the need for axial penetration through the long and wide vessels. In green condition the natural easy flow of water can be used by so-called sap replacement treatment.
It is only necessary to fill the vessels with a suitable. Tyloses formation into a metaxylem vessel as wound or preservative so that during the following resting ageing response in Phyllostachys sp. period the salt components can diffuse into the surrounding tissue. Treatment time depends much on the vessel area of the bamboo. Generally the metaxylem comprises 6-8% but differences exist between the lower and upper part of a culm and also among species.
Bamboo is an essential material in many countries still for many purposes, like constructions, supports, fences and furniture. It has to be processed and used properly, which means, according to its biological properties.
We have to be aware that the fascination of bamboo as a material has encouraged competitors for bamboo imitations from plastic. They look real, are durable and often cheaper than the original. And the old, much unjustified saying “Bamboo is the poor man’s timber”, is in certain quarters already rephrased in- “Only the rich ones can afford bamboo”.