bamboo structures in colour 133 Fig. 133: Vietnamese ladies’ hats made from Neohouzeaua dulloa 128 the anatomy of bamboo culms structuralChapter Five modifications +107 129 CHAPTER FIVE structural modifications Growth Effects DEVELOPMENT AND AGEING The previous chapters have outlined the general structure of mature culms and its variations. Few investigations have dealt with structural development until maturation (Grosser 1971; Fujii 1985; Alvin and Murphy 1988). Recent observations have shown that some structural modifications occur after maturation also (Liese and Weiner 1996, 1997). The ageing of a bamboo culm influences certain properties and, consequently, its processing and utilization. This section describes the changes during culm elongation and maturation, and certain modifications that occur in mature culms. The elongation of a culm results from the expansion of individual internodes, already present in the bud. Differentiation starts at the upper part of an internode by elongation of different cell types, and proceeds down to its base. The differentiation of an individual internode is completed in just a few days (Hsiung et al. 1980). Longitudinally, there is no major difference in the composition and structure of the tissue within an internode, except for the expansion of the cells, especially the fibres. The fibres undergo a significant elongation from only a few microns to about 2 mm. Within the internode, the fibres near the nodes are always shorter than in the middle portion (see page 61 & Fig. 82). Elongation, however, causes only a few anatomical changes along culm length. The narrowing of the culm wall in the upper part results in a reduction of its inner portion with less parenchyma, and the vascular bundles decrease in size but increase in number (see page 27). The upper culm part has a higher fibre content with a higher specific gravity. The younger stage of a culm is marked by certain external characteristics that are useful indicators to avoid premature harvesting. These include the presence of culm sheaths, bud break, branching pattern, number of leaf scars, and colour changes of the stem ranging from fresh green to often yellow-grey (Banik 1993). A young hairy culm becomes glabrous at maturity. Structural modifications during the maturation phase and the years following relate to fibres and parenchyma cells. The results obtained by Alvin and Murphy (1988), Majima 130 the anatomy of bamboo culms et al. (1991), Liese and Weiner (1996, 1997) and Murphy and Alvin (1997 a,b) have demonstrated that the maturation process of fibres proceeds quite differently over the transverse section of a culm wall. It is influenced by the position of vascular bundles and by the position of the fibre within the vascular bundle. Murphy and Alvin (1997b) have charted the maturation in developing bamboo culms based on observations of some bamboo species. Fig. 99 summarizes the measurements by Liese and Weiner of fibre wall thickness across the culm wall. Generally, fibres at the outer culm wall have thicker and more lamellated walls than those in the inner part. Cell wall thickening of bundle sheath fibres starts from the inner vascular side and proceeds to the outer parenchyma side. Whereas at the outer culm part the fibres near the protoxylem have thicker walls than those near the phloem, the reverse is true near the lacuna with thicker cell walls in the fibre sheath between ground parenchyma and phloem. Fibre maturation can be a process prolonged over many growing seasons. Any investigation of changes Fig. 99: Fibre wall thickness across the culm wall for the fibre sheath between ground parenchyma (GP) and phloem (PH), as well as between protoxylem (PX) and ground parenchyma (GP), 1 cortex, 9 = tear lacuna - Phyllostachys viridiglaucescens FIBRE WALL THICKNESS (µm) 11 10 9 8 7 6 5 4 3 2 1 0 1 2 3456 7 8 910 FIBRE SHEATHS OVER CROSS CULM WALL fibre sheath GP-PH fibre sheath PX-GP 131 structural modifications related to ageing must also recognize the distinct pattern of differences in the structure of the fibre wall and the location in the culm wall. Nomura (1993) observed the ageing of Phyllostachys heterocycla culms from seedling stage through the first five years and found an increase off about 6.7 times in the diameter of the vascular bundle fibre sheath, 4.4 times in the metaxylem vessels and about 1.9 times in fibre length. Detailed measurements of changes with age were undertaken with culms of Phyllostachys viridiglaucescens aged up to 12 years by Liese and Weiner (1996, 1997). During the first month of the growing period, most fibres were still unlignified — e.g., at the 20th Fig. 100: Fibre wall thickness over the transverse section at the 20th internode from 3-month, 1-year, 6-year and 11-year-old culms of Phyllostachys viridiglaucescens FIBRE WALL THICKNESS (µm) 8 6 4 2 0 epidermis middle lacuna FIBRE SHEATHS OVER CROSS SECTION 1994 1993 1987 1983 132 the anatomy of bamboo culms 101 10µm Fig. 101: Fibre cells with three lamellae and without starch in a 1-year-old culm of Phyllostachys viridiglaucescens internode they had a very thin cell wall of only 1.5-1.7 µm, which increased to 2.3 µm in the fully elongated culm. At this stage, the fibre wall consisted of three lignified lamellae. Fig 100 shows, at the 20th internode, the cell wall thickness of fibre sheaths in 3-month, 1-year, 6-year and 11-year-old culms across the culm wall, near the epidermis, in the middle and near the lacuna. The 11-year old culm has thicker fibre walls than the 7-year old one. Such increases have been measured both in lower and in upper internodes. The increase in wall thickness is not caused by the thickening of the existing cell wall, but by the deposition of additional lamellae. Whereas the wall of a 1-year-old culm 133 102 10µm Fig. 102: Fibre cells with eight lamellae in a 12-year-old culm of Phyllostachys viridiglaucescens 103 5µm Fig. 103: Wall thickening owing to the development of septation in a fibre in Dendrocalamus latiflorus 134 the anatomy of bamboo culms shows three lamellae at the base (2.6 µm), a culm of 12 years has about eight lamellae (about 8 µm) (Figs. 101, 102). The polylamellation of fibre walls is especially high in cells adjacent to the ground tissue parenchyma and somewhat less pronounced in fibres near the vascular tissue as also shown by Murphy and Alvin (1997a). The multi- layered texture of different lamellae has already been described earlier. The addition of new wall layers during ageing is also reflected in the development of septated fibres (Fig. 103). These results demonstrate that cell wall thickening of fibres occurs not only during the maturation period but also in later years. Corresponding observations on developmental changes of fibre structures owing to ageing were also made on palms (Calamus axillaris, Rhapis excelsa) by Weiner et al. (1996). In a similar way, thickening of parenchyma cell walls was noted in older culms (Alvin and Murphy 1988) up to 3 years. In Figs. 104 and 105, the parenchyma tissue of an 1- year-old and a 12-year-old culm is shown, with distinct thickened cell walls in the latter one. The polylamellate parenchyma wall of a 12-year-old culm can be seen in Fig. 106; the fine structural details of the fibrillar orientation of the parenchyma cell wall were shown in Fig. 60. The wall thickening of parenchyma cells requires their state as living cells, which is shown by the storage of starch as energy reserve also in older culms (Fig. 107). A similar increase of wall thickness in fibre cells necessitates their living state at a higher age. Generally, fibres lose their protoplast soon after their wall differentiation as known from fibres in dicotyledonous woody tissue. In bamboo, however, fibres remain alive for a long period, as demonstrated by their capacity to store starch and to undergo septation (see page 70 & Fig. 107). The wall thickening may explain the increase in density observed even in older culms. For validating such observations, the presence and variability of the starch content must be considered. The structural changes influence certain culm properties, like their resistance to splitting. Hence, in well-managed Chinese bamboo forests, only culms that are at least 5-6 years old are harvested. 135 structural modifications 104 250µm Fig. 104: Ground parenchyma with vascular bundle in a 1-year-old culm of Phyllostachys viridiglaucescens The lignification of Phyllostachys heterocycla during its growth from a sprout to a 14-year-old culm was studied by Itoh (1990), revealing full lignification of the component cells within one growing season. Changes in the chemical composition of the culm wall of P. pubescens up to 7 years have been investigated by Chen et al. (1985). Older culms show symptoms of senescence which affect their functional efficiency, especially the conductivity of the metaxylem vessels for water and the sieve tubes for the assimilates (Grosser and Liese 1971). It is remarkable that these crucial pathways are formed within a few days during shoot differentiation and have to function for 136 the anatomy of bamboo culms 105 250µm Fig. 105: Ground parenchyma with vascular bundle in a 12 - year - old culm of Phyllostachys viridiglaucescens 106 10µm Fig. 106: Polylamellate parenchyma wall of a 12-year-old culm of Phyllostachys viridiglaucescens 137 structural modifications 107 50µm Fig. 107: Starch granulae in the parenchyma of a 12-year-old culm of Phyllostachys viridiglaucescens 138 the anatomy of bamboo culms 108 30µm Fig. 108: Metaxylem vessel filled with tyloses in a 12 - year - old culm of Phyllostachys viridiglaucescens 10 years or longer, with no chance of restoration as it is possible for trees using their secondary meristem.