IAWA Bulletin n.s., Vol. 12 (1),1991: 5-22 QUALITATIVE STRUCTURAL CHANGES DURING BARK DEVELOPMENT IN QUERCUS ROBUR, ULMUS GLABRA, POPULUS TREMULA AND BETULA PENDULA* by M. Trockenbrodt Ordinariat für Holzbiologie der Universität Hamburg und Institut für Holzbiologie und Holzschutz der Bundesforschungsanstalt für Forst- und Holzwirtschaft, Leuschnerstr. 91, 2050 Hamburg 80, Germany Summary Introduction The deve10pment of bark structure of In spite of its wealth of features and pecu- Quercus robur L., Ulmus glabra Huds., Po- liarities bark anatomy is seldom used for pulus tremula L. and Betula pendula Roth is plant taxonomical considerations (e.g. Zahur being described. Profound structural changes 1959; Richter 1981; van Wyk 1985; Trocken- can be observed during the first years after brodt & Parameswaran 1986). Obtaining bark secondary growth has started. In all four spe- samples authenticated by herbarium vouchers cies the epidermis is replaced by a periderm, is laborious if not impossible because barks the cortex shows intensive dilatation growth, rarely are a part of botanical collections. and the groups of primary bark fibres are Moreover, bark tissue often is an extremely pushed apart. The collapse of sieve tube heterogeneous material, and commonly ap- members starts with the second year. With plied techniques far sampie preparation are proceeding secondary growth, the specific inadequate. However, the main reason far formation of sc1erenchymatic tissue, especi- today's limited use of bark anatomy far ally sc1ereids, and the dilatation growth are taxonomical wark is a lack of knowledge processes which strongly affect the bark about the structure of bark and its develop- structure of Quercus robur, Populus tremula ment. Contrary to wood, bark structure and Betula pendula. In addition, wide, fused changes continuously with age. Information phloem rays develop in Quercus robur. The about the variability of bark structure, especi- structure of Ulmus glabra bark is affected by ally within one individual during its growth, the formation of phloem fibre-/sc1ereid-like is essential for an estimation of the diagnostic cells and mucilage cells and by dilatation value of bark anatomical features. Up to now growth. The histological pattern of Ulmus only a few investigations have dealt with the glabra bark stabilises to a great extent after developmental anatomy of bark. Some of the first few years, the other barks investi- these are on pharmacognostic aspects of cer- gated show further developmental processes tain barks (Speyer 1907; B irn stiel 1922; Has- over many years. In all species the formation ler 1936), and some information can be found of a rhytidome is the last distinct modification in more generalliterature on the anatomy of of bark structure. bark (e. g. Hanstein 1853; Möller 1882; Thore- naar 1926; Chang 1954; Reinders & Reinders- Key words: Quercus robur L., Ulmus glabra Gouwentak 1961; Esau 1969). The varia- Huds., Populus tremula L., Betula pen- bility of certain bark features within one in- dula Roth, bark anatomy, bark develop- dividual tree was analysed by Raskatov and ment. Kosichenko (1968), Kosichenko (1969), * Dedicated to Prof. Dr. Walter Liese on the occasion of his 65th birthday. Downloaded from Brill.com10/01/2021 12:51:00PM via free access 6 IAWA Bulletin n.s., Vol. 12 (1),1991 Nicholls & Phillips (1970), Liese and Para- Table 1. Age and height of the investigated meswaran (1972), Parameswaran and Liese trees. (1974), Ghouse and Siddiqui (l976a, b), Age Height Ghouse and Yunus (1976), Ghouse and (years) (rn) Hashmi (1977), Ghouse and Iqbal (1977, Oak 37 14.5 1981), Yunus etal. (1977), Aday (1978), Ezell Oak 11 30 11.8 and Stewart (1978), Ghouse et al. (1982), III 14 Iqbal and Ghouse (1983) and Röckle (1986). Oak 6.8 Most of these papers are restricted to short Oak IV 14 4.8 descriptions of cell1ength variabi1ity. Oak V 15 2.1 The intention of this paper is to contribute Elm 24 16.0 to the broadening of our knowledge about Pop1ar 12 14.5 structural changes of bark tissue during its Birch 10 5.0 development. First, qualitative changes of the Birch 11 16 10.0 basic bark structure are described. Quantita- tive changes and the diagnostic value of sin- gle bark anatomical features will be discussed in subsequent papers. lar stern diameter intervals, and according to the intactness of the tissue. Quercus robur Material and Methods bark was sampled at 5-9 height levels which Tree species suitable for the investigation correspond to a bark age of 1-33 years, bark had to fulfill certain requirements: thickness of 0.2-9.5 mm and stern diameter of0.4-16.5 cm. Ulmus glabra bark sampies sufficient preservation of bark with age, were taken from 12 height levels representing i.e. no early formation of rhytidome com- bark age of 1-24 years, bark thickness of bined with the loss of bark tissue; 0.7-10.7 mm and stern diameter of0.4-25.0 typical representatives of different struc- cm. Populus tremula bark sampIes derived tural wood and bark types; from 9 height levels which correspond to a sufficiently complex structure, i.e. a high bark age of 1-11 years, bark thickness of number of possibly varying features. 0.6-4.7 mm and stern diameter of 1.0-14.0 Accordingly, the ring-porous hardwood cm. Betula pendula bark was sampled at 12- species Quercus robur L. and Ulmus glabra 13 height levels with a corresponding age of Huds. as weil as the diffuse-porous hard- 1-16 years, bark thickness of 0.4-12.0 mm wood species Populus tremula L. and Betula and stern diameter of 0.3-24.0 cm. Sections pendula Roth were chosen. Initially, five in- from all levels were prepared and analysed dividuals of Quercus robur were analysed. with a light microscope and a serni-automatic The investigation revealed no tree-to-tree dif- image analyser. ferences in their basic bark structure and de- The sampies included bark, cambial zone, velopment. One individual of Ulmus glabra and mostly a narrow zone of adhering xylem. and Populus tremula and two of Betula pen- They were fixed in formalin-acetic acid- du la were examined. For details about the aicohol, penetrated with polyethylene glycol age and height of the selected trees see Table 1. 1500, and sectioned on a sliding microtome, A sampie selection following biological often with the help of adhesive tape. The sec- and mathematical rules (cf. Kucera & Bariska tions were double-stained with astra blue and 1982) is not practicable for working on bark, acridine red-crysoidin. Additional sampies because all structural information is stored in were embedded in glycol-methacrylate (cf. a very small area, and tertiary tissue changes Ruetze & Schmitt 1986). Macerations were impede the removal of exactly defined sam- prepared with Jeffrey's solution (cf. Gerlach pies. Thus, the sampIes were taken at regular 1969). distances along the stern, regular age intervals The terminology follows Trockenbrodt (determined from xylem growth rings), regu- (1990). Downloaded from Brill.com10/01/2021 12:51:00PM via free access Trockenbrodt - Structural changes during bark develüpment 7 ---~------------------------- Results With secündary grüwth proceeding (Fig. 3) additiünal tangential bands üf secündary Quercus robur L. phlüem fibre grüups üf up to. 8 cells in depth All five üak trees analysed reveal a similar have develüped. They also. are acco.mpanied develüpment üf bark structure. Differences by chambered crystalliferüus cells. Uniseriate are müre üf a quantitative nature than devi- phloem rays traverse the fibre grüups. The atiüns from a basic pattern. Therefüre the first broad phlüem rays develüp when the bark develüpment üf üaks I-V is described cambial initials between 3-5 uniseriate rays tügether. are eliminated. This fusio.n is stro.ngest where The yüungest sampIes represent the shüüt the cambium bends distinctly. Süme fibre shürtly after secündary growth has started groups with a radial width üf 15-20 cells (Fig. 1). The cüurse üf the vascular cambium can be füund elüse tü the broad rays. The is still irregular. Groups üf primary bark rays are subject tü slight dilatatiün, i.e. the fibres are arranged parallel tü the vascular cells enlarge slightly and round o.ff, but they cambium. The individual grüups are linked dü nüt divide. by slightly enlarged selereids. Inside the pri- The cürtex selereids o.f ülder bark develüp mary bark fibre groups, primary phlüem sülitarily or in spherical grüups (Fig. 4). The elements are lücated füllüwed by the first sclereid grüups between the groups üf pri- secündary phlüem elements. The phlüem mary bark fibres enlarge. The fürmatiün üf rays are exelusively uniseriate. The cürtex selereids in the secündary phloem increases, üutside the band üf primary bark fibres cün- especially between adjacent phlüem fibre sists üf an inner zone üf iSüdiametric cürtex groups. The transfürmatiün üf phloem paren- parenchyma cells, slightly enlarged due tü chyma cells intü sclereids is üften initiated by dilatatiün growth, and a narrüw üuter zone üf üne cell, and it proceeds ce'ntrifugally (Figs. sm all cürtex cüllenchyma cells. The üuter- 5 & 6). In transverse sectiüns the sclereid müst layer üf the shüüt is the intact epider- groups üften appear spherical ür stretched mis. Immediately beneath the epidermis the tangentially (Fig. 6), in radial sectiüns also. fürmatiün üf the periderm has started. spherical but stretched axially. Sülitary scle- Caused by the progressing secündary reids are cümmün, too. The sclereids' fürms growth, bark structure already changes with- and dimensio.ns vary a lüt. As a rule, scle- in the first year (Fig. 2). The epidermis is reids in the secündary phloem are larger than ruptured, and remnants adhere tü the inten- the ünes in the cürtex and in the band üf pri- sively developing periderm. The cortex col- mary bark fibres and sclereids.