130 IAWA Bulletin n.s., Vol. 1 (3), 1980

SOME REMARKS ON THE NOMENCLATURE OF FIBRES, SCLEREIDS AND FIBRE-SCLEREIDS IN THE SECONDARY PHLOEM OF TREES

by

N. Parameswaran Institut fUr Holzbiologie und Holzschutz der Bundesforschungsanstalt fUr Forst- und Holzwirtschaft, Leuschnerstrasse 91, D-2050 Hamburg 80, B. R. D.

The paper by Bamber and Summerville tion of Esau (1977): 'a sclerenchyma cell with (1979) in this Bulletin on the sclerified tissue characteristics intermediate between those of a in the barks of Lauraceae evokes some questions fibre and a sclereid'. It has been recently as to the terminology of the various scleren­ established (Parameswaran & Liese, in prep.) chymatous elements in the secondary phloem. that these cells in Larix and Pseudotsuga Richter (1980) has recently gathered addition­ possess a polylamellate wall structure, just as al information on the Lauraceae barks within the sclereids (Parameswaran, 1975). We tend to the framework of an extensive anatomical classify these cells as sclerotic phloem fibres, study. It appears justified at this juncture because they are elongated through apical to offer some suggestions towards a redefini­ growth and have a polylamellate wall. tion of some of the terms employed, inasmuch The situation in the case of fibre-sclereids as the variety of forms for the lignified cells in is more problematic. Holdheide (1951) con­ the bark of trees has rendered the formulation sidered his 'sklerotische Fasem', for which he of precise criteria for separating them difficult. illustrated the bark of Larix decidua, as arising The term 'sclerenchyma' has been defined from parenchyma cells, which either enlarge by Esau (1977) as a tissue 'composed of scler­ evenly or even show apical growth. He also enchyma cells and includes fibres, fibre­ cited the exceptional case of such fibres de­ sclereids and sclereids', whereby the cell veloping in the first annual growth of elm bark. types are distinguished solely on the basis According to Holdheide this definition of of form and size (see also Esau, 1969). 'Fasersklereiden' can be applied to cells oc­ The secondary phloem fibres are to be curring in Aesculus, Fraxinus, Prunus, Pyrus, characterized as a cell type developing from Sorbus, Ulmus, etc. Esau et al. (1953) also the fusiform initials of the vascular cam­ defined the phloem fibres differentiating bium, showing apical intrusive growth in the from phloem parenchyma cells as fibre-sclereids, course of development (cf. Parameswaran & which develop their secondary wall in a tissue Liese, 1974). Sclereids on the other hand which is no more active in conduction. This mostly represent a secondary development developmental context finds its extreme from the still living parenchyma cells of the expression in the explanation of the term fibre­ axial and radial systems; they are character­ sclereid 'as phloem fibres that develop from ized by a lignified polylamellate secondary parenchyma cells of non-functional phloem' wall with ramified pitting. In form they are (Fahn, 1974). mostly isodiametric and shorter than fibres. Reports of a sclerenchymatization and fibre The sclereids may also show capabilities of formation, occurring not until the sieve ele­ idioblastic enlargement and production of ments have become non-functional in Parthe­ irregular branches and spicules in the sec­ nium argentatum (Art schwager, 1943) and ondary phloem tissue (Esau, 1969). A typical, Prunus avium (Schneider, 1945), have compli­ apical intrusive growth is however lacking cated the issue, since this means that these cells in the sclereids, compared with phloem fibres. develop as a result of redifferentiation of living In this connection it has to be emphasized parenchyma cells. Evert (1960) treated this that the lignified cells occurring in the bark of problem in extenso in his work on Pyrus Pseudotsuga menziesii and Larix decidua have communis phloem. The long sclereids occurring been called fibrous sclerotic cells (Shimakura, in this taxon, said to resemble fibre-sclereids 1936), fibre-sclereids (Holdheide, 1951), fibres sensu Holdheide (1951), are referred to as (Chang, 1954) and sclereids (Srivastava, 1963); sclereids. These cell elements, along with the their ontogeny has yet to be clarified. In terms non-elongating 'short sclereids~, are said to of their elongated form they can be defined as develop from precursors overwintering as 'fibre-sclereids' in accordanc-e with the defini- undifferentiated elements in the cambial zone. IAWABulletinn.s., Vol. 1(3),1980 131

Table I

Cell type Ontogeny Wall structure Form Growth

Phloem fibre derived from fusiform normally provided with elongate apical intrusive initials of the vascular the three layers of growth present cambium (e.g. Quercus) secondary wall in addition to middle lamella/primary wall Sclerotic ? lignified, polylamellate elongate apical intrusive phloem fibres (e.g. Larix, Pseudo- wall growth present fsuga) Sclereids derived from living lignified, polylamellate isodiametric or apical in tru sive parenchymatous cells wall slightly to growth absent (both axial and radial) considerably in the functional elongate and/or nonfunctional phloem (e.g. Fagus) Lignified derived from axial/ thick-walled, lignified, isodiametric, apical intrusive parenchyma radial parenchyma not polylamellate corresponding growth absent to length of parenchyma cells

In Pyrus malus (Evert, 1963) similar cells are ondary wall alone without the formation of a suggested as arising from parenchyma cells in lamellated structure does not necessarily the non-functional phloem and termed fibre­ mean that this wall is 'sclerified'. In the sclereids; In a recent study of the fine structure literature one finds this term often used of fibre-sclereids in the phloem of Pyrus for several cell types, which only show a jamasakura Nanko et al. (1979) found a mere wall thickening with lignification. Per­ three-ply structure for the secondary walls, just haps 'lignified' parenchyma would describe the as for the phloem fibres of Populus they cell type better and obviate the confusion with studied earlier (Nanko et aI., 1977); these the terminology 'sclerified', 'sclerotic' etc., 'fibre-sclereids' are said to arise from axial which should be reserved for cells developing a parenchyma. regular polylamellate wall. In view of these different facts, it appears A redefinition of the terms discussed here that the term fibre-sclereid is not justified in thus appears appropriate and is presented in the taxa mentioned, since this cell type does Table 1, with the full awareness that some of not represent an intermediate form between the questions still remain unanswered and in phloem fibres and sclereids, as do fibre-tracheids the hope that this might stimulate further as intermediate forms between libriform fibres discussion. and tracheids. Meanwhile it is perhaps worth­ while to consider the possibility of a develop­ Acknowledgements ment of these cells from cambium initials, Thanks are due to Dipl.-Holzwirt H.G. Rich­ which lag behind in growth and which differen­ ter, Dr. P. Baas, and Prof. W. Liese for fruit­ tiate into fibre-like cells only at a belated stage ful discussions. in the non-functional phloem. It may be mentioned that such fibres, considered to be References genuine phloem fibres, occur in the phloem of Artschwager, E. 1943. Contribution to the several tropical trees (Parameswaran & Liese, morphology and anatomy of guayule (Par­ 1970). Further ontogenetic studies should thenium argentatum). U.S. Dept. Agr. Tech. clarify the situation. BUll. No. 842,33 pp. The cell type shown in Fig. 7 of Bamber Bamber, R.K. & R. Summerville. 1979. Taxon­ and Summerville's (1979) paper has been de­ omic significance of sclerified tissue in the signated as 'sclerified longitudinal paren­ barks of Lauraceae. lAW A Bull. 1979/4: chyma'. The slightly thickened lignified sec- 69-74.