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Wood Anatomy of Actinostrobus (Cupressaceae)

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Wood Anatomy of Actinostrobus (Cupressaceae) IAWA Journal, Vol. 26 (I), 2005: 79-92 WOOD ANATOMY OF ACTINOSTROBUS (CUPRESSACEAE) R. D. Heady 1 & P. D. Evans2 SUMMARY The wood anatomy of the Western Australian species Actinostrobus are­ narius (Cupressaceae) is described for the first time and its features are compared with those of the two other species in the genus: A. acuminatus and A. pyramidalis. Mature heartwood in A. arenarius is light-brown in colour and has an air-dry density of0.56 g/cm3. Average tracheid length is 4.3 mm. A very prominent warty layer, with individual warts commonly greater than one micron in height and large enough to be visible to light microscopy, lines the inner walls of tracheids. Callitroid thickening is commonly present in narrow (latewood) tracheids, but is absent from wide ones (earlywood). Axial parenchyma cells with dark-red resinous inc1usions are tangentially zonate in earlywood. Bordered pitting in early­ wood and latewood is uniseriate. Pit borders are circular and there is a raised torus. Average ray height is low. Cross-field pitting is cupressoid and the number of pits per cross field ranges from two to five, with a mean of 3.1. Average ray heights, ray frequencies, ray volumes, and numbers of pits present in cross fields are higher in A. arenarius than in A. pyra­ midalis, thus supporting the c1assification of A. arenarius as aseparate species within Actinostrobus. Veins of distorted xylem cells, similar in appearance to 'frost rings' occur sporadically in the sterns of a11 three species. If such rings are confined to Actinostrobus, then the combination of a very prominent warty layer, and the common occurrence of frost rings could provide a means of separating Actinostrobus from Callitris. Validation of this scheme requires further research to determine if such rings commonly occur in Callitris. Key words: Actinostrobus arenarius, A. acuminatus, A. pyramidalis, wood anatomy, callitroid thickening, warty layer, frost ring. INTRODUCTION Trees belonging to the genus Actinostrobus are small multi-stemmed conifers endemie to the south-western corner ofWesternAustralia. The contemporary 'Flora of Australia' (Hill 1998) lists three species in the genus: A. acuminatus, A. arenarius and A. pyrami­ dalis. The most-recently recognised species, A. arenarius, was formerly taxonomically inc1uded with A. pyramidalis, but was dec1ared a species in its own right by Gardner I) ANU Electron Microscopy Unit, RSBS, and School ofResources, Environment and Society (SRES). The Australian National University, Canberra, ACT 0200, Australia. 2) Department of Wood Science, University of British Columbia, Vancouver, Canada. Downloaded from Brill.com09/23/2021 02:39:40PM via free access 80 IAWA Journal, Vol. 26 (1), 2005 (1964) mainly on the basis of its cones, "the fertile scales of which are larger than those of A. pyramidalis and have erect and somewhat acute, not in-curved apices." The wood anatomy of A. arenarius has not previously been described. Descriptions of the wood anatomy of A. acuminatus and A. pyramidalis have been carried out by Baker and Smith (1910), Patton (1927), Peirce (1937), Prince (1938), Phillips (1948), and Greguss (1955). All of these accounts, however, were based on very 1imited numbers of wood sampies, mainly non-vouchered, or on twig or branch-wood specimens. For example, Patton (1927) had no sampies of A. acuminatus and does not state whether his A. pyramidalis sampies were stern or branch-wood. Peirce (1937) used one sampie each of A. acuminatus and A. pyramidalis. Prince (1938) gives no indica­ tion of his sampling procedure. Phillips (1948) had access only to twig material of A. acuminatus and had no specimens of A. pyramidalis. Greguss (1955) used "a specimen corning from a branch 6 years old", had "no material for comparative purposes" and ad­ mits that his description "requires to be supplemented" because of sampling limitations. This may explain why reports of callitroid thickening within the genus are contradic­ tory. Callitroid thickening is reported to be present in A. acuminatus (Kleeberg 1885; Phillips 1948; Barefoot & Hankins 1982) whereas Baker and Smith (1910), Peirce (1937), Prince (1938), and Greguss (1955) make no mention of its presence in their studies, and Patton (1927) reported it to be "absent". There is clearly a need for a com­ prehensive examination of the wood anatomy of Actinostrobus including that of the undescribed species, A. arenarius, in order to resolve this discrepancy. More importantly a complete description of the wood anatomy of Actinostrobus would complement re­ cent molecular phylogenetic studies of the taxonomy ofthe Cupressaceae (Gadek et al. 2000; Pye et al. 2003). These studies have confirmed the close association between Actinostrobus and Callitris (Gadek et al. 2000), and have suggested a particularly close relationship between Actinostrobus and the Western Australian Callitris species, C. drummondii (Pye et al. 2003). The aims of this study were: 1) to describe the wood anatomy of A. arenarius for the first time and comment on the taxonomic significance of findings; 2) to compare the wood anatomy of A. acuminatus, A. arenarius and A. pyramidalis, and to determine whether anatornical differences between the three species (if any) support the classi­ fication of A. arenarius as aseparate species; and 3) to fully describe certain features of the wood anatomy of A. acuminatus and A. pyramidalis that have received little at­ tention in previous studies. MATERIALS AND METHODS Wood sampies were obtained by incremental coring of trees growing wild in their natural habitat in flat, sandy scrub, with no over-storey, some occurring as remnant vegetation on roadside verges (Fig. 1). Seven core sampies, each taken from a different tree, were obtained for each of the three Actinostrobus species. In addition, several billet sections of the trunk wood of each species were obtained for assessment of wood colour, den­ sity, and appearance of growth rings. Figure 2 shows the approximate geographical locations of the sampled trees. Downloaded from Brill.com09/23/2021 02:39:40PM via free access Heady & Evans - Wood anatomy 0/ Actinostrobus (Cupressaceae) 81 Fig. 1. Actinostrobus arenarius growing on a roadside verge near Badgingarra, Western Australia. The white guide-post is approximately one metre high. Specimens were observed using light microscopy (LM) and scanning electron mi­ croscopy (SEM). Light microscope sections (approximately 20-50 !olm thick) were rnanually cut from water-soaked wood sampies using a single-edged razor blade. Sec­ tions were observed unstained or stained with safranin (to stain lignin, cherry-red), fast green (to stain cellulose green), or Sudan 4 (to stain fats and resins red) (lane 1970). AZeiss Axioskop microscope with halo­ gen illumination was used to examine specimens. Digital images of selected x =A. orelloril/S wood features were captured using a 0= A. aCl/millow.\ 'Spot' digital camera system (Diagnos­ p =A. pyramid(lfis lics Instruments Inc.). Plane surfaces for SEM were cut, prepared, and viewed by SEM as desöribed previously (Heady & Evans 2000). Fig. 2. Map showing the approximate geo­ graphicallocations of the sampled trees. Downloaded from Brill.com09/23/2021 02:39:40PM via free access 82 IAWA Journal, Vol. 26 (1), 2005 The densities of irregularly shaped stem-wood samples were determined using an Archimedian method (ASTM 1983) after soxhlet extraction of samples with acetone for 24 hours. Tracheid length was measured optically using the method of Ladell (1959). Ray heights, ray frequencies, ray volumes, and wart sizes were determined from pre­ recorded SEM images on a Macintosh G3 computer using an image analysis program (NIH Image, National Institute of Health, USA) (Evans et al. 1994). Mean ray heights, frequencies, and volumes were calculated from measurements of all rays present in one square millimetre areas of tangential longitudinal surfaces (TLS) in each of the seven samples for each species, (300-400 rays per species). The number of cross-field pits occurring in two cross fields in 10 rays within each of the seven samples was also quantified for each species. Analysis of variance was used to determine the significance of differences in ray height and cross-field pit numbers between species. To ensure that the data conformed to the assumptions of analysis of variance, i.e. normality with constant variance, diagnostic checks were carried out on the data before final analysis. As a result of such checks ray height data was transformed into naturallogarithms and analysed as logarithm of ray height. Statistical computation was carried out using Gen­ stat (Program 5, Release 3.1 Lawes Rothamstead Experimental Station, UK). Results are presented graphically and aleast significant bar can be used to explore statistically significant (p < 0.05) differences between individual species means. RESULTS Physical and macroscopic characteristics of the wood ofActinostrobus arenarius Longitudinal surfaces of dry, freshly-cut heartwood are light-brown, barely discem­ ible from the dark cream colour of the sapwood. There are no colour streaks. Growth rings on smooth transverse surfaces are discemible to the naked eye. Persistent branch traces extending through 5-15 growth rings were noted. Wood does not have a distinct odour or taste. Its air-dry density is 0.56 g/cm3 and its basic density is 0.53 g/cm3. Microscopic characteristics ofthe wood ofActinostrobus arenarius Boundaries of growth rings are 'distinct' due to an abrupt change in the radial diam­ eter oftracheid lumina at the latewood-earlywood interface (Fig. 3). Latewood is 'incon­ spicuous' according to the definition of Phillips (1948) since it occupies less than one­ quarter of the width of the growth ring (Fig. 3). Earlywood tracheids are thin-walled and rectangular in outline in transverse section (TS). Average tracheid length is 4.3 mm. Transition from earlywood to latewood is 'abrupt' (Fig. 3). Resin canals are absent. Axial parenchyma cells, filled with a dark-red resinous substance, are common. The arrangement of parenchyma cells is tangentially zonate; tending to be grouped into lines parallel to growth-ring boundaries in the earlywood (Fig.
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