The anatomical structure of the internodes of bamboo culms has been intensively investigated, structural details of the nodes are hardly analyzed so far. The nodes bear special significance for the intercalary growth, the cross transport of water and nutrients in the living culm, for the liquid movement during drying and preservation as well as for the physical and mechanical properties of the culm.
To clarify the structural composition of the nodes, three pachymorph and three Teptomorph species from China were analyzed. The orientation of the complex vascular system within the nodal region becomes evident by a three dimensional model. Within the diaphragm of the nodes the metaxylem and the phloem exhibit considerable structural changes.
The typical vascular bundle structure of bamboo disappears with no differences between pachymorph and leptomorph species. Intensive vessel branching is common in the nodal region. A branching of sieve tubes has not been observed, but abundant lateral sieve areas facilitate the cross transport. An agglomeration of filliform elements connects the phloem of the axial vascular bundles to that of the branched ones. The fibre length shows a definite pattern above and below a node which can be related to the growth process.
Quite a number of investigations have dealt with the structure of the internodes of a bamboo culm, which form the main material resource. Consequently a rather good knowledge exists about the anatomical pattern of a culm wall, especially the composition of the vascular bundles the variation in fibre length as well as fine structural aspects.
The composition and structural details of the nodes, however, were hardly analyzed so far. Only few papers deal with certain aspects, Haga, 1922; Weatherwax, 1967; Grosser and Lies, 1971; Zee, 1974; Hsiung et al., 1980. Difficulties in the preparation of the material and the complex anatomical structure may haste contributed to this neglect.
The nodes bear special significance for the intercalary growth and for the function of the culm. They enable the necessary, communication for cross-transport of water and nutrients as in the internodes no such conduction cells exists because their vascular bundles arc isolated from each other by the ground parenchyma. The nodal structure is also of interest for understanding the liquid movement during drying and preservation as well as the physical mechanical properties of the culm.
Investigation of Nodal Structure in Bamboo Culms:
The nodal structure was investigated on six bamboo species from China. Three belong to the pachymorph type (Sinocalamus affinis McCl., Bambusa textiles McCl., Schizostachyum pseudolimo McCl.) and three to the leptomorph type (Phyllostachys pubescens. Mazel ex H. Lehaie. Pleioblastus maculata McCl., C.D. Chu et C.S. Chao, Sinobambusa laeta McCl.)
Samples of nodes and internodes were taken from mature three years old culms from bottom (second internode), middle (1/3 height of the culm) and top portion (2/3 height). The nodal area includes 10-15 mm below and above the sheath scar. Sections and macerated material were investigated by light and scanning electron microscopy.
Results and Discussion Related to Nodes in Bamboo Culms:
(a) Gross Morphology of Nodes:
The node of a bamboo culm consists of the nodal ridge, the sheath scar and the diaphragm. Most of the bamboos have permanent nodal ridges, which differ in form among species. The shape of the diaphragm can vary at various height levels of a culm and even more between species. If it may be plane or in its central part formed up-wards, down-wards or folded.
(b) Vascular Bundles:
A perspective of the three dimensional structure of the vascular system at the nodal region has been obtained from serial sections. Most of the axial vascular bundles pass directly through the node. In the peripheral zone of the culm they bend slightly outwards branching partly into the sheath, whereas, in the inner zone they become connected with those in the diaphragm.
Especially the vascular bundles in the upper edge of the diaphragm, many small vascular bundles exist, which turn horizontally and twist repeatedly. Some of them run from one side of the nodal culm to the other. At a cross section of the nodal culm wall the typical vascular bundle structure of bamboo disappears.
The different arrangement of fibre bundles in pachymorph and leptomorph bamboo vanishes and the characteristic isolated fibre strand in pachymorph species is absent. The position of xylem and phloem within the bundles can be variously changed because of distortion.
Whereas, in the internodes the xylem of the vascular bundles consists of the protoxylem, characterized by tracheids with helical or annular thickenings and the two metaxylem vessels with intensively pitted walls, in the nodal region this composition is partly changed. At the branching of vascular bundles abundant vessels develop. Smaller cells with an intensive reticulate pitting may completely surround the metaxylem vessels. It is assumed that these cells form an “accessory tissue” which may have special functions with regard to the water transport.
Details of the vascular pattern of the branching points of the vascular bundles in the nodes of the Gramminae were not known so far. By means of maceration, the special structures of the conducting tissue can be recognized. A remarkable feature appears to be the intensive branching of the vessels. The protoxylem tracheids show shorter up-to longer forks in the nodal region.
The metaxylem vessels in contrast possess several large simple perforations on their side walls for the contact between vessels in addition to the normal simple perforation of the end walls. Their cell wall is often covered with numerous small size pits with an alternating arrangement. These pits appear open giving direct contact between vessels. Sometimes they concentrate on a part of the cell wall and form an aperture area. Many small vessels are deformed.
A striking feature of the protoxylem tracheids in the nodes is their tyloses. Whereas, in internodes of leptomorph bamboo these cells are blocked with many small tyloses, in the nodes they can be filled with only few large ones. This phenomenon appears in leptomorph but also in pachymorph bamboo species where tyloses have not reported so far, Grosser and Liese 1971; Jiang and Li. 1985; Wen and Chou, 1985; Hsich et al. 1986.
In the diaphragm the xylem consists of only metaxylem vessels, protoxylem vessels are not developed. In the phloem a branching of sieve tubes has not been observed, but abundant lateral sieve areas facilitate the cross-transport in the nodes. At the branching point agglomeration structures of filiform elements connect the phloem of the axial vascular bundles to that of the branched ones.
In a longitudinal section the cells are arranged in a storied like pattern with three to five sub-units and the individual cells are interconnected by numerous pits. Axial sieve tubes directly connected with these cells change from their plane simple sieve plate with regular arranged sieve pores into a bulb form with irregular arranged sieve pores.
This structure enables the connection between sieve tubes and the small filiform cells. Behnke 1965 a. b. calls similar structures in the node of Dioscoreaceen as “Phloem Beckcnzcllcn”. Braun and Sauter, 1964 found such cells containing more phosphatase and assume a special significance for the assimilate transport.
(c) Cell Structures:
Finally, size and form of the various cell types in the nodal area were determined in sections and macerated material. The metaxylem vessels appear quite different in the nodal region. They are much shorter than in the internodes (210 µm versus 670 µm). Also their average diameter is smaller (115 µm versus 140 µm).
In the longitudinal view the ground parenchyma of the internodes can be differentiated in two morphological types, i.e., rectangular elongated cells and fewer shorter ones. In contrast, the parenchyma cells of the nodal region are often irregular, partly with bizarre forms.
The parenchyma around the vascular bundles has a special structure. One layer of parenchyma surrounds the metaxylem vessel. These cells of only about 8% diameter are intensively pitted towards the vessel with large pit apertures, so that most of the wall is covered with pits. Braun 1984, considered such cells around vessels in hardwoods as “contact parenchyma” or “accessory tissues”. One layer of contact parenchyma cells around metaxylem vessel and protoxylem tracheida exists also in the Rattan palms. Net like pitted cells were also observed surrounding the phloem.
The parenchyma cells at greater distance to the vessels bear lesser pits, although such pitting is quite different from the few and small pits of the ground parenchyma around the vascular bundles in internodes. In the early stage of development in the nodes, transfer parenchyma was reported. In the mature culms of our material no such cells were found.
The fibres across a culm wall are shortest at the outer part, longer at the center and decrease again at the inner part. This pattern is present within the internode as well as above and below the node which is agreement with other studies. The shortest fibres are always above and below a node, as found also for other bamboo species.
The same trend exists in Rattan palms. During the expansion of an internode due to intercalary growth Hsiung et al. (1980), distinguish five stages. It is assumed that the fibres immediately above a node are the youngest and therefore shorter. Such assumption does not explain the shorter fibres immediately below the node. Further investigations should clarify whether growth processes occur also in this region.
At the nodal level the fibres are considerably shorter than within the internode and shortest at the diaphragm. Table 1 illustrates the results for two species Phyllostachys pubescens as leptomorph and Bambusa textiles as pachymorph.
The shortest fibres are the diaphragm at all levels – (bottom, middle, top). The fibre length at the diaphragm of approx. 340 % is only about 1/3 of the length at the nodal part of the wall. Mechanical elasticity is reduced due to the shorter, thicker and also forked fibres in the nodal part, so bamboo culms under tension often break at the node.
