Wood Anatomy of the Genus Abies Luis García Esteban*, Paloma

Home , Abies bracteata, Abies cephalonica, Abies cilicica, Abies durangensis, Abies guatemalensis, Abies hickelii

IAWA Journal, Vol. 30 (3), 2009: 231–245

Wood anatomy of the genus Abies A review

Luis García Esteban*, Paloma de Palacios, Francisco García Fernández and Ruth Moreno Universidad Politécnica de Madrid. Escuela Técnica Superior de Ingenieros de Montes, Departamento de Ingeniería Forestal, Ciudad Universitaria, 28040 Madrid, Spain *Corresponding author [E-mail: [email protected]]

SUMMARY The literature on the wood anatomy of the genus Abies is reviewed and discussed, and complemented with a detailed study of 33 species, 1 subspecies and 4 varieties. In general, the species studied do not show diagnostic interspecific differences, although it is possible to establish differences between groups of species using certain quantitative and qualitative features. The marginal axial parenchyma consisting of single cells and the ray parenchyma cells with distinctly pitted horizontal walls, nodular end walls and presence of indentures are constant for the genus, although these features also occur in the other genera of the Abietoideae. The absence of ray tracheids in Abies can be used to distinguish it from Cedrus and Tsuga, and the irregularly shaped parenchymatous marginal ray cells are only shared with Cedrus. The absence of resin canals enables Abies to be distinguished from very closely related genera such as Keteleeria and Nothotsuga. The crystals in the ray cells, taxodioid cross-field pitting and the warty layer in the tracheids can be regarded as diagnostic generic features. Key words: Abies, Abietoideae, anatomy, wood.

INTRODUCTION

The family Pinaceae, with 11 genera and 225 species, is the largest conifer family. The genus Abies, with 48 species and 24 varieties, has the second highest number of species after the genus Pinus (Farjon 2001). Distribution is exclusive to the northern hemisphere. In Eurasia the southernmost species are located in the west in Morocco (A. pinsapo var. marocana and A. pinsapo var. tazaotana) and in the east in the Taiwanese mountains (A. kawakamii), while the northernmost species is in Siberia (A. sibirica). In America, the northernmost species is A. lasiocarpa and the southernmost is A. guatemalensis (Liu 1971). In general, the wood structure of the genus Abies is very homogeneous (Jacquiot 1955; Peraza 1964) and its features are very similar to the genus Pseudolarix (Phil- lips 1948). In fact, together with Pseudolarix and Keteleeria, Abies has the simplest structure of the Pinaceae (Greguss 1955).

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The wood anatomy of Abies was studied by several researchers in the late 19th century (Castellarnau 1880; Kleeberg 1885) and early 20th century (Penhallow 1904; Jeffrey 1905; Thompson 1910; Vierhapper 1910). Studies refer to individual species (Castellarnau 1880), to species grouped according to provenance (Penhallow 1907; Wiesehuegel 1932), to their phylogeny or evolution (Jeffrey 1905) or to specific struc- tural aspects (Thompson 1912; Bannan 1936). In the mid-20th century, more complete studies were undertaken, such as by Phillips (1948), who described the characteristics of the genus and 5 species, and Greguss (1955), who also described the specific features of the genus in addition to 30 species and 2 varieties, including the most complete biometry made until that time. Subsequently, Greguss (1972) added to this work with 9 more species. Jacquiot (1955), Peraza (1964), Jane (1970), Panshin & DeZeeuw (1970), Esteban & Guindeo (1988), Schweingruber (1990) and Esteban et al. (1996) included general descriptions of the specific features of the genus Abies in their studies, as well as species descriptions. Esteban et al. (2002) described 30 species. Lastly, in its Softwood List, the IAWA Committee (2004) made many references to characteristic wood anatomical features of Abies. The objective of this work was to review and discuss existing studies on the wood anatomy of the genus Abies, and to complement this with a detailed study of 33 species, 1 subspecies and 4 varieties.

MATERIAL AND METHODS

The material used in this study came from the wood collections of several research centres. Table 1 lists the 33 species, 1 subspecies and 4 varieties studied, with references to the collections from which the samples were obtained (Stern 1988). In the case of Abies alba (ETSIMw X2174) and A. pinsapo (ETSIMw X2175, X2176, X2177, X2267 and X2268) the samples were collected by us in their natural areas in Spain and Morocco, from 5 mature trees felled in each area. Microscopic slides were prepared in accordance with the usual methods of softening, sectioning, staining and mounting, and the anatomical descriptions were made using the standard terminology of the IAWA Committee (2004). The samples were observed using light microscopy, without staining and after staining any resin with safranin and Sudan 4 (Jane 1970). The SEM samples were prepared as recommended by Heady and Evans (2000), and observed in a JEOL JSM-6380 scanning electron microscope.

RESULTS AND DISCUSSION

Table 1 summarises the features observed in the species, subspecies and varieties studied. Differentiation of species within the genus and from other genera In general terms, there are no qualitative differences that can be used consistently to separate species of the genus Abies. Below some examples are given of specific identification keys published in the past, that appear not to be valid on closer scrutiny. Castellarnau (1880) showed that

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A. alba wood can easily be distinguished from that of A. pinsapo by the occurrence of marginal axial parenchyma with nodular transverse end walls in the former and its absence in the latter. Our observations show that this feature occurs in both species (Table 1). Krauss (1864) in Viguié & Gaussen (1928) proposed an identification key based on the rays, which distinguished A. cephalonica, A. sibirica, A. fraseri, A. alba, A. pinsapo, A. nordmanniana, A. cilicica and A. balsamea. The key was based on the number of cross-field pits and their form (oval or rounded) and the height and shape (oval or rounded) of ray parenchyma cells in the tangential section. In the present study no major differences were observed in the morphology of the ray cells which would enable the species of Abies to be distinguished. Penhallow (1896) proposed a classification of North American conifer wood in which he attributed the genus Abies with the following features: resin canals rare, recorded in A. nobilis and A. bracteata; absence of fusiform rays; absence of ray tracheids, except in the case of A. balsamea; end walls of ray parenchyma cells markedly nodular; and absence of helical thickenings in tracheids. However, resin canals and ray tracheids, used by Penhallow (1896), do not support any classification, as both features appear to be a response to wounding. Wiesehuegel (1932) also proposed an identification key for the American firs which included the presence or absence of crystals in the rays and the occurrence of partially biseriate rays. These two features are very common in all the species studied and cannot be used to separate species within Abies. The same author also distinguished A. balsamea, A. grandis, A. venusta and A. magnifica on the basis of biseriate tracheid pitting in comparison with the other American firs, in which the arrangement is uniseriate. Because the presence of biseriate pitting is closely associated with the tracheid diameter of earlywood and the diameter is closely related to climate conditions, this feature does not support any classification either. Lastly, Wiesehuegel (1932) used the number of pits per cross-field as a diagnostic feature for A. amabilis in the American firs based on the predominance of a single pit per cross-field. His results differ from those obtained in the present study, where the number of pits ranges from 2 to 4. Saint-Laurent (1932) established differences between the wood of A. pinsapo var. marocana and A. numidica on the basis of the more abundant axial parenchyma tissue in the former. In the present study no differences between these taxa were observed, as in both species the parenchyma is restricted to single marginal cells. Brown et al. (1949) proposed a classification to distinguish A. balsamea and A. fraseri from A. concolor, A. procera, A. grandis and A. magnificaon the basis of the tangential section of the ray parenchyma cells, which is oblong in A. balsamea and A. fraseri and circular to oval in the others. This feature has often been used to distinguish very similar species (Phillips 1948; Greguss 1955; Core et al. 1979; Roig 1992), but the observations in the present study show that the use of this feature is highly subjective. Similarly, Kukachka (1960) established a division of the genus based on the colour of the ray cell content, but this is also a subjective classification and is highly influenced by factors such as whether the wood is juvenile or mature, or the degree of heartwood formation. Peraza (1964) separated A. alba from the group A. pinsapo and A. pinsapo var. tazaotana on the basis of the higher frequency of rays more than 30 cells high in A. alba. However, Esteban et al. (2007), using mature wood from trunks of trees collected in their area of origin,

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Table 1. Features of the species, subspecies and varieties of the genus Abies studied. NB. Only features that vary below the genus level are included. For anatomical features that are constant see text. Legends: +: present; –: absent. – Growth rings: a = abrupt, g = gradual early/latewood transition. – Tracheid pitting: u = uniseriate, b = biseriate, u (b) = uniseriate and occasionally biseriate. – Axial parenchyma: m = marginal, d = diffuse. – Ray width 2-seriate in part: + = number of biseriate rays greater than 1%, (+)1 = number of biseriate rays lower than 1%. – Crystals in ray parenchyma cells: + = abundant crystals located in the marginal and submarginal ray cells, (+)2 = crystals occasionally present only in irregularly shaped marginal ray cells. For wood collection acronyms see Stern (1988). ETSIMw refers to the wood collection held at the research institute of the authors, in Madrid (Spain).

Growth Resin Tracheids Parenchyma Rays rings canals

Species Wood sample Transition from early- Transition wood to latewood canals Traumatic with extensions Torus deposits Organic pitting in Tracheid radial walls Axial parenchyma Ray width 2-seriate in part Irregularly shaped ray cells Crystals in ray parenchyma cells

Abies alba Mill. a – – – u m + + + ETSIM-X2174 Abies amabilis Douglas ex J. Forbes a – – – b m + + + ETSIMw-X1216 Abies balsamea (L.) Mill. a – – – u(b) m + + – MADRw (X2173) Abies borisii-regis Mattf. a – – – u(b) m + + + MADw-17846 Abies bracteata (D. Don) A. Poit. a + – – b m – + – MADw-18347 Abies cephalonica a – – – u m + + – RBHw-7079 Loudon a – – – u m,d + + + MADRw (X1592) Abies cilicica a – – – u(b) m – + + RBHw-14036 (Antoine & Kotschy) a – – – u m (+)1 + + MADRw (X2084) Carrière a – – – u(b) m – + + MADw-17376 Abies concolor a – – – u m (+)1 + + MADRw (X1577) (Gordon & Glend.) Lindl. ex Hildebr. Abies durangensis a – – – u(b) m – + + USw-32800 Martínez Abies firmaSiebold a – – – b m – + + CTFw-11458 ch. & Zucc. 19332 ch. 32418 ch. Abies forrestii Coltm.- a – – – u m – + – Kw-18361 Rog. var. forrestii Abies fraseri (Pursh) a – – – u m – + + MADw-771 Poir. a – – – u m (+)1 + + USw-14814 Abies grandis a – – – u m – – – ETSIMw-X1526 (Douglas) ex D. Don) Lindl. Abies guatemalensis a + – – b m – + + MADRw (X1593) Rehder

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(Table 1 continued)

Growth Resin Tracheids Parenchyma Rays rings canals

Abies hickelii Flous & a – – – b m – + – SJRw-37237 Gaussen Abies holophylla a – – – u m – – (+)2 ETSIMw-P1155 Maxim. Abies homolepis a – – – u m – – – MADRw (X2171) Siebold & Zucc. a – + – u m – + (+)2 MADw-4428 a – + – u m – + + USw-23684 Abies kawakamii g – + – u m (+)1 + – MADw-2006 (Hayata) T. Itô a + – – u m (+)1 + – USw-21248 Abies koreana a – – – u m – + – MADRw (X2171) E.H. Wilson Abies lasiocarpa a – – – u(b) m – + – MADw-25431 (Hook.) Nutt. a – + – u m – + (+)2 USw-14492 Abies lasiocarpa (Hk.) a – – – u m – + – MADRw (X1576) Nutt. var. arizonica (Merriam) Lemmon Abies magnifica a – + – u(b) m – + (+)2 MADw-6307 A. Murray bis a – + – u m (+)1 + + USw-14485 Abies mariesii Mast. g – + – u m – + (+)2 MADw-20334 g – + – u m (+)1 + – USw-24492 Abies nephrolepis a – + – u(b) m – + – CTFw-22366 ch. (Trautv. ex Maxim.) 26163 ch. Maxim. a + – – u m – + – MADRw (X2170) Abies nordmanniana a – – – u m (+)1 + (+)2 RBHw-14031 (Steven) Spach a – – – u m (+)1 + (+)2 MADw-10536 a – – – u m (+)1 + (+)2 MADw-39459 Abies nordmanniana a – – – u(b) m – + (+)2 RBHw-14034 (Steven) Spach subsp. equi–trojani (Asch. & Sint. ex Boiss.) Coode & Cullen Abies numidica de a – + – u m + + + Kw-18393 Lannoy ex Carrière a – + – u m + + + SJRw-14446 Abies pinsapo Boiss. a – + + b m (+)1 + + ETSIMw-X2175 Provenance: Grazalema Provenance: Sierra a – + – u(b) m (+)1 + + ETSIMw-X2176 Bermeja Provenance: Sierra a – + – b m + + + ETSIMw-X2177 de las Nieves Abies pinsapo Boiss. a – + – b m (+)1 + + ETSIMw-X2268 var. marocana (Trab.) Ceballos & Bolaños

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(Table 1 continued)

Growth Resin Tracheids Parenchyma Rays rings canals

Abies pinsapo Boiss. a – + – u(b) m,d (+)1 + + ETSIMw-X2267 var. tazaotana (S. Cózar ex Villar) Pourtet Abies procera Rehder a – – – u m + + (+)2 MADw-44891 a – + – u(b) m + + + USw-19149 Abies recurvata Mast. a – – – u m (+)1 + – SJRw-23054-085 Abies religiosa a – – – u m (+)1 + + ETSIMw-X1218 (Kunth) Schltdl. & Cham. Abies sachalinensis a – + – u(b) m – + – CTFw-11459 ch. (F. Schmidt) Mast. 19333 ch Abies sibirica Ledeb. a – + – u m – + – Kw-184m06 Abies spectabilis g – + – u(b) m – + – Kw-18417 (D. Don) Spach Abies veitchii Lindl. a + – – u m (+)1 + – Kw-18422 Abies vejarii Martínez g – + – u m (+)1 + + MADw-25208 g – + – u m (+)1 + + USw-32803 showed that the two taxa cannot be distinguished by the height of their rays. Kučera and Bosshard (1975) studied the presence of partially biseriate rays in A. alba. They quantified the height in number of cells and ray type, defined as ray height + number of rows of biseriate cells, and stated that these values could be used as a classification feature. The most complete identification key for Abies spp. was made by Greguss (1955), who focused on biometric features such as the number, position and size of cross-field pits and the ray height, and classified the Abies spp. in groups of species which were very similar to each other, although there is no record of whether the biometry was performed in all cases on mature wood from the trunk, or how many provenances were studied. He also recorded the presence of pitting in the end walls of the ray parenchyma cells, which, when seen in the tangential section, appear in either cribiform (sieve-like) grouping or scalariform transverse bars, and regarded this as a diagnostic feature of the species. The use of this feature does not support a clear distinction between species and in fact it does not always occur in the species in which Greguss (1955) recorded it. Although in some of the studies cited the criteria used can help to distinguish a particular species or group of species, they cannot be applied universally to the entire genus.

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Distinguishing Abies from other genera presents no difficulties, with the exception of Pseudolarix. The greatest similarities are with genera of the subfamily Abietoideae. The following features are typical of Abies and Pseudolarix: resin canals absent, axial parenchyma scarce and occurring only in single cells on the growth ring boundary, ray parenchyma cells with distinctly pitted horizontal walls and nodular end walls, presence of indentures, regular presence of calcium oxalate crystals, and ray tracheids absent. Abies can be distinguished from the other Abietoideae genera without difficulty: Ket- eleeria and Nothotsuga have axial resin canals, Cedrus has scalloped tori and traumatic radial canals, while in Tsuga ray tracheids are commonly present. Pseudolarix differs from Abies in that it lacks a warty layer.

General features While some authors advocate differences between the heartwood and sapwood of certain species of the genus Abies (Sargent 1902; Beauverie 1910; Record & Hess 1943; Phillips 1948), their colour is in fact similar. Nonetheless, fresh wood of certain species, e.g. A. pinsapo (Esteban et al. 2007), shows differences in colour, although this should not be regarded as diagnostic (IAWA Committee 2004). Growth ring boundaries in the different species of Abies studied are distinct (Fig. 1), but the transition from earlywood to latewood varies between species and can be abrupt (e.g. A. nordmanniana subsp. equi-trojani) (Fig. 2) or gradual (e.g. A. spectabilis) (Fig. 3) (Table 1). In species which grow in less favourable ecological conditions, growth rings are usually narrower and the early/latewood transition is more abrupt. The gradual transition in the wood of A. grandis, A. venusta and A. concolor was used by Wiesehuegel (1932) as a diagnostic feature, but this characteristic of some species of Abies should not be applied for this purpose as it is strongly influenced by ecological conditions (Esteban et al. 2003). At times the wood has a characteristic odour, ranging from pleasant to fetid (Record & Hess 1943; Brown et al. 1949; Core et al. 1979), although in the samples of A. pinsapo and A. alba collected from their natural areas this was only apparent in fresh wood. Analysis of these features in the samples studied shows that they cannot be used for interspecific differentiation. Neither are they representative enough to enable differences from other genera to be established, except in relation to those in which the colour of the sapwood and the heartwood is distinct. The abrupt or gradual transition of the growth rings allows distinct groups to be established, but this feature can only be used when the samples come from their natural areas.

Tracheids Earlywood tracheids are rectangular or hexagonal in outline (Fig. 4a), becoming exclusively rectangular the closer they are to the latewood (Fig. 4b). Tracheid pitting in tangential walls is common, generally in latewood tracheids (Fig. 5a) as described by Wiesehuegel (1932), Jacquiot (1955) and Peraza (1964), with well defined tori (Fig. 5b). Tracheid pitting in radial walls in earlywood is predominantly uniseriate (Fig. 6). When the pitting is biseriate, the arrangement is opposite (Fig. 7).

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The tori in earlywood pitting are disc-shaped and well defined (Fig. 8), with extensions in almost half of the species studied (Fig. 9a, b), as previously described by Greguss (1955), Willebrand (1995) and Sano et al. (1999). In all the species studied a

Figures 1–9. Tracheids. – 1: Growth rings distinct (Abies sibirica). – 2: Abrupt early/latewood transition (A. nordmanniana subsp. equi-trojani). – 3: Gradual early/latewood transition (A. spectabilis). – 4: (a) Tracheids in earlywood rectangular or hexagonal in outline; (b) Tracheids in latewood rectangular in outline (A. nordmanniana subsp. equi-trojani). – 5: (a) Tracheid pitting in tangential wall in latewood tracheids; (b) Torus well defined (A. nordmanniana subsp. equi- trojani). – 6: Tracheid pitting uniseriate (A. lasiocarpa). – 7: Tracheid pitting biseriate in opposite arrangement (A. hickelii). – 8: Well defined tori in radial wall pit (A. nordmanniana subsp. equi- trojani). – 9: (a) Tori with extensions (A. vejarii); (b) idem (A. magnifica). — Scale bars in Fig. 1–3 = 250 μm, in Fig. 4, 7 & 9b = 50 μm, in Fig. 5a & 6 = 100 μm, in Fig. 5b, 9a & 8 = 10 μm.

Downloaded from Brill.com10/04/2021 03:49:05AM via free access Esteban et al. — Review of Abies wood anatomy 239 warty layer occurs on the inner layer of the secondary wall (S3 or tertiary wall) (Fig. 10). Organic deposits were observed in tracheids adjacent to the rays in A. pinsapo (Esteban et al. 2007), most likely as a response to wounding (Fig. 11), which means

Figures 10–19. – 10–12: Tracheids (cont.). – 10: Warty layer (Abies pinsapo). – 11: Organic deposits in tracheids (A. pinsapo). – 12: Trabecula (A. magnifica). – 13 &14: Axial parenchyma. – 13: Axial parenchyma consisting of single cells on the growth ring boundary (A. pinsapo). – 14: Transverse end walls nodular (A. veitchii). – 15–19: Rays. – 15: Rays more than 30 cells high (A. grandis). – 16: Rays 2-seriate in part (A. pinsapo). – 17: (a) Horizontal walls of ray parenchyma cells distinctly pitted; (b) End walls of ray parenchyma cells nodular; (c) Inden- tures (A. cilicica). – 18: Irregularly shaped cells (A. hickelii). – 19: Cross-field pits taxodioid (A. kawakamii). — Scale bars in Fig. 10 = 5 μm, in Fig. 11, 12, 17, 18 & 19 = 50 μm, in Fig. 13 & 16 = 100 μm, in Fig. 14 = 10 μm, in Fig. 15 = 250 μm.

Downloaded from Brill.com10/04/2021 03:49:05AM via free access 240 IAWA Journal, Vol. 30 (3), 2009 that this feature should not be regarded as characteristic of Abies, as it is in Araucaria and Agathis (Esteban et al. 1996; Esteban et al. 2005). The presence of trabecula is also common (Fig. 12). Abies species do not present interspecific distinguishing features in their tracheids, but the warty layer must be taken into account as a diagnostic feature for Abies at the genus level. It is in fact one of the features which enables Abies to be distinguished from Pseudolarix. Organic deposits and torus with extensions should not be regarded as characteristic of the genus.

Axial parenchyma Axial parenchyma is very scarce, appearing in single cells with high cell content in marginal arrangement (Fig. 13). Phillips (1948), Jacquiot (1955) and Schweingruber (1990) described the presence of axial parenchyma in Abies spp. as frequent, but also noted that it was very scarce. It is normally confined to the growth ring boundary as single cells throughout the growth ring either in the first row of earlywood or last row of latewood (Wiesehuegel 1932; IAWA Committee 2004). However, in some of the species studied (A. cephalonica, A. pinsapo var. tazaotana) it is not restricted to this position but occurs evenly throughout the growth ring, although this arrangement is not usual within the genus. The transverse end walls of axial parenchyma cells in Abies are distinctly nodular (Fig. 14), as previously described by Jacquiot (1955), and the nodules are more obvi- ous in the tangential section (Yatsenko-Khmelevsky 1954). Their nodular appearance is due to true simple pitting of the secondary wall, unlike the nodules in Cupressaceae, which occur as a result of thickening of the primary wall and are not due to pitting as such (Phillips 1948). The presence of marginal parenchyma was observed in all the species studied and should be regarded as a distinguishing feature of the genus. It is also characteristic of the other Abietoideae genera (Cedrus, Keteleeria, Nothotsuga, Pseudolarix and Tsuga).

Rays The highest rays are generally 15 to 30 cells high, although in some species (A. alba, A. pinsapo) rays are more than 40 cells high (Fig. 15). Ray height in the genus Abies has often been used in attempts to distinguish species and is probably one of the few features which enables groups of species to be differentiated within the genus. Kleeberg (1885) studied the ray height of some species of Abies and recorded the following maximum heights: A. cephalonica 24 cells, A. alba 26, A. religiosa 6, A. pinsapo 14, A. nordmanniana 20, A. grandis 16, A. nordmanniana subsp. equi-trojani 20 and A. balsamea 15. Wiesehuegel (1932) considered that the low ray height in A. lasiocarpa and A. arizonica was sufficient grounds to distinguish these two species from the others. On the other hand, according to Peraza (1964) the very high ray height in A. alba can be used to distinguish it from A. pinsapo. The very low maximum cell height values recorded by Kleeberg (1885) probably resulted from the use of branch or juvenile wood, as in the case of the conclusion reached by Wiesehuegel (1932), given that in the present study A. lasiocarpa did not show particularly low rays. Similarly,

Downloaded from Brill.com10/04/2021 03:49:05AM via free access Esteban et al. — Review of Abies wood anatomy 241 the data obtained by Peraza (1964) do not concur with those of Esteban et al. (2007), who used wood from mature trees and concluded that it was not possible to distinguish between A. alba and A. pinsapo on the basis of ray height. However, these authors established a clear distinction between two groups within A. pinsapo by using the ray height. The first of these consists of the Grazalema and Sierra de las Nieves pinsapo firs, with 40 and 50 cells respectively, and the second consists of the Sierra Bermeja pinsapo firs, with 36 cells, and the two Moroccan varieties, with 25 in var. marocana and 30 in var. tazaotana. Rays in Abies are exclusively uniseriate, but rays 2-seriate in part were also observed in nearly all the species, although never in excess of 10% of the total (Fig. 16). Kleeberg (1885) recorded them in A. cephalonica; Wiesehuegel (1932) in A. amabilis, A. concolor, A. grandis, A. venusta, A. nobilis and A. magnifica; and Greguss (1955) in A. magnifica. Panshin & DeZeeuw (1970) cited the occasional presence of biseriate rays in A. magnifica, and Jane (1970) described rays more than 30 cells high, commonly biseriate. The presence of biseriate rays in Abies, although very common in the species studied, should not be used as a distinguishing feature for the genus, as in some species it has not been confirmed (Table 1). In all the samples studied the horizontal walls of ray parenchyma cells are distinctly pitted and the end walls are nodular (Fig. 17a, b). Concurring with other authors, these aspects are regarded as characteristic of the genus (Phillips 1948; Greguss 1955; Jane 1970) and are used as diagnostic features to distinguish Abies spp. from other conifer genera (IAWA Committee 2004), except for those belonging to the Abietoideae, as this feature is common to the whole subfamily. The presence of well-defined indentures was also observed in all the species studied (Fig. 17c). Phillips (1948) described them in all the Pinaceae and Greguss (1955) recorded them in several species of the genus (A. alba, A. amabilis, A. concolor, A. fabri, A. fraseri, A. holophylla, A. homolepis var. umbellata, A. kawakamii, A. lasiocarpa, A. lowiana, A. magnifica, A. nephrolepis, A. nordmanniana, A. numidica, A. pinsapo, A. procera, A. sachalinensis, A. sibirica, A. spectabilis, A. veitchii, A. venusta and A. vil- morinii). This feature can be used as diagnostic in Abies. Although it is present in all the Pinaceae and is common in the Abietoideae, it is not so well defined in Cedrus and Keteleeria. The presence or absence of ray tracheids in the genus Abies has been the subject of debate. According to some authors, ray tracheids are characteristic of the wood structure of the genus (Penhallow 1907; Thompson 1912; Wiesehuegel 1932; Jane 1970), while others consider that they only occur as a response to wounding (Jeffrey 1917; Chamberlain 1935; Phillips 1948). Kleeberg (1885), Strasburger (1891) and Phillips (1948) noted the absence of ray tracheids in Abies spp. No ray tracheids were found in the samples studied, although the presence of irregularly shaped parenchymatous cells different from the other ray cells was very common (Table 1). These were located in the marginal rows of the rays (Fig. 18) and were similar to those described by Chrysler (1915) in Cedrus. The absence of ray tracheids in Abies can be regarded as a diagnostic feature of the genus. This feature enables Abies to be distinguished from Cedrus and Tsuga, in which

Downloaded from Brill.com10/04/2021 03:49:05AM via free access 242 IAWA Journal, Vol. 30 (3), 2009 it commonly occurs. However, it should be used with caution as, although some authors consider that the presence of ray tracheids is associated with wounding, others record it in some species without mentioning its traumatic origin. Further studies on Abies wood affected by wounding would ultimately reveal whether the Abies taxa always produce traumatic ray tracheids. In relation to cross-field pitting and concurring with other authors (Phillips 1948; Greguss 1955; Jane 1970; Esteban et al. 2002; IAWA Committee 2004) (Fig. 19), in all the samples studied the pitting is taxodioid. Pits vary in number from 1 to 6 per cross-field.

Figure 20 & 21. Intercellular canals and crystal inclusions. – 20: Traumatic axial resin canals in tangential rows as a response to wounding (A. veitchii). – 21: Prismatic crystals in ray parenchyma cells (A. nordmanniana subsp. equi-trojani).

Resin canals The genus Abies has no resin canals, although traumatic axial resin canals in tangential rows as a response to wounding were common in some of the species studied (Fig. 20) (Table 1). Jeffrey (1905), Chamberlain (1935) and Jane (1970) noted that these canals are usually surrounded by subsidiary cells with crystals and dense cell contents. Some authors have discussed the presence of non-traumatic resin canals in Abies wood. Penhallow (1907) recorded normal resin canals in A. concolor, A. bracteata, A. nobilis and A. firma and Vierhapper (1910), although noting the absence of resin canals in Abies wood, considered that they normally occur in A. concolor, A. nobilis and A. bracteata. Despite these references, the absence of resin canals in Abies is not subject to debate. In all the samples studied in which resin canals were observed, their arrangement in tangential rows shows their traumatic nature. Therefore, the absence of resin canals should be regarded as a particularly useful feature of the genus for distinguishing it from genera which are phyletically and anatomically very close, such as Keteleeria and Nothotsuga, both of which have normal axial resin canals. In relation to the other Abietoideae genera, Abies is distinguished from Cedrus as the latter has both traumatic axial and radial resin canals, while in Tsuga and Pseudolarix it is not possible to make a distinction on the basis of this feature.

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Crystals The crystals in the samples observed are located in the marginal and submarginal ray cells. Castellarnau (1880) recorded the presence of calcium oxalate crystals in A. alba in the ray parenchyma and Phillips (1948) recorded them in A. grandis. Greguss (1955) regarded them as a special feature in Abies which should be taken into account when identifying its species. He situated them not only in the ray parenchyma cells but also in the axial parenchyma (A. veitchii) and the axial tracheids (A. chensiensis, A. nephrolepis, A. nordmanniana, A. pinsapo, A. veitchii), although the reference to the tracheids is probably due to crystals being carried over during slide preparation. Jane (1970) situated calcium oxalate crystals in Abies spp. in marginal ray parenchyma cells. Core et al. (1979) indicated that the existence of rhomboidal and rectangular calcium oxalate crystals is very common in Abies spp. and is at times used for diagnostic pur- poses. Esteban et al. (1996) described them in the axial parenchyma of A. numidica. IAWA Committee (2004) noted the occurrence of prismatic crystals in marginal and submarginal ray cells of certain species of Abies spp. Some authors have used this feature to differentiate between species: Wiesehuegel (1932) distinguished A. magnifica, which he regarded as having a constant presence of crystals, from A. concolor and A. venusta, in which he considered their presence variable. He also established that crystals do not occur in the other American firs (A. amabilis, A. arizonica, A. balsamea, A. lasiocarpa, A. nobilis) except only occasionally in A. fraseri and A. grandis. Observation of the samples studied showed that the presence of prismatic crystals in the ray parenchyma cells is very common (Fig. 21) and although it can be regarded as a feature of the genus, it should not be used to distinguish species. Therefore, the wood of Abies is so similar in the species studied that it is only possible to establish differences between certain groups of species through the biometry of some of their elements, e.g. ray height, and specific qualitative features, provided that the species are compared using samples of mature wood from the trunks of trees collected in their regions of origin.

ACKNOWLEDGEMENTS

The authors are grateful to all the institutions which collaborated by providing the samples that made this study possible.

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