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Paleobotany |
2Department of Ecology and Evolutionary Biology, and Natural History Museum and Biodiversity Research Center, The University of Kansas, Lawrence, Kansas 66045-7534 USA; 3Department of Biology, University of Nebraska–Omaha, Omaha, Nebraska 68182 USA
Received for publication December 13, 2001. Accepted for publication June 18, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDISCUSSIONLITERATURE CITED |
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Key Words: Antarctica • Cladoxylales • Fremouw Formation • Hapsidoxylon • stem anatomy • Triassic
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDISCUSSIONLITERATURE CITED |
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), pteridophytes (Millay and Taylor, 1990
; Delevoryas, Taylor, and Taylor, 1992
), cycads (Smoot, Taylor, and Delevoryas, 1985
), possible conifers (Meyer-Berthaud and Taylor, 1991
), and seed ferns (Meyer-Berthaud, Taylor, and Taylor, 1993
; Taylor, Del Fueyo, and Taylor, 1994
). Although megafossil remains of lycophytes have not been recovered to date, microfossils attributed to this group have been identified in the Fremouw (Farabee, Taylor, and Taylor, 1990
) and Falla Formations (Askin and Cully, 1996
). Permineralized and compression/impression specimens from the Triassic of Gondwana indicate that the dominant plant group at this time was the Corystospermales, a Mesozoic seed fern group based on the foliage type Dicroidium (Taylor, 1996
). Palynological data support the interpretation that during the Middle and Late Triassic, Antarctica was occupied by a relatively diverse flora (Kyle and Fasola, 1978
; Farabee, Taylor, and Taylor, 1990
), especially for such a high paleolatitude (70°–75° S for the central Transantarctic Mountains) (Powell and Li, 1994
).
Several major plant groups are represented by various organ genera, most unique to Antarctica. These include the cycad Antarcticycas and fern Soloropteris (Smoot, Taylor, and Delevoryas, 1985
; Millay and Taylor, 1990
), as well as the petrified stem Kykloxylon, which bears Dicroidium leaves (Meyer-Berthaud, Taylor, and Taylor, 1993
). In this report, we describe a new type of anatomically preserved axis from the Middle Triassic permineralized peat site at Fremouw Peak in the Beardmore Glacier region (Taylor, Taylor, and Collinson, 1989
). The anatomy of this axis superficially resembles the complex vascular tissue arrangement of plants belonging to the Cladoxylales, such as Cladoxylon. Cladoxylaleans range in age from the late Lower Devonian to the Lower Carboniferous and are known from both compressed and permineralized specimens. The present paper compares the anatomy of the Antarctic axis with cladoxylalean axes and other fossil and extant plants containing complex vascular tissue arrangement.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDISCUSSIONLITERATURE CITED |
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) and vertebrate remains from nearby sites (Barrett, Elliot, and Lindsay, 1986
; Hammer, Collinson, and Ryan, 1990
).
Approximately 0.5–2-cm-thick sections were cut through each peat block and cellulose acetate peels were made after etching in 49% hydrofluoric acid (Galtier and Phillips, 1999
). Appropriate peels were mounted on microscope slides for examination and photomicrography. All materials are housed in the Paleobotanical Collections of the University of Kansas Natural History Museum and Biodiversity Research Center, Lawrence, Kansas, USA. Specimen numbers are 10 327, 10 390, and 11 252; slide acquisition numbers are 15 376–15 472 and 19 970–19 988.
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SYSTEMATICS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDISCUSSIONLITERATURE CITED |
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Genus
Hapsidoxylon McManus, Boucher, Taylor, and Taylor, gen. nov.
Generic diagnosis
Stems with vascular tissue embedded in ground tissue and composed of ramifying segments occurring singly or connected near center of stem, segments extending to near stem periphery; vascular tissue consisting of secondary xylem surrounding a central core of primary xylem and parenchyma; tracheids with simple to bordered pits; uniseriate and multiseriate rays present, cambium and secondary phloem continuous; periderm with equal amounts of phellem and phelloderm; vascular traces arising from vascular segments near stem periphery.
Type species
Hapsidoxylon terpsichorum McManus, Boucher, Taylor et Taylor, Figs. 1–16 in this paper.
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Species
Hapsidoxylon terpsichorum McManus, Boucher, Taylor et Taylor, sp. nov., Figs. 1–16 in this paper.
Specific diagnosis
Stems 1–2 cm in diameter and variable in length, anastomosing vascular segments consisting of centrally positioned zone of homogeneous parenchyma (2–3 cells wide) surrounded by primary xylem and then secondary xylem; secondary xylem tracheids generally increase in diameter centrifugally; tracheids square to rectangular and up to 51 µm in diameter, pitting circular to elliptical bordered; rays uniseriate or multiseriate, up to 18 cells high and 1–3 cells wide; bifacial cambium of thin-walled parenchyma; secondary phloem of elongate cells with slightly swollen end walls; external periderm consisting of phellem 5–9 cells thick with dark contents, and internal phelloderm up to seven cells thick radially; periderm cells slightly flattened radially; circular traces produced at tips of vascular segments up to 4 mm in diameter and surrounded by periderm.
Holotype
Specimen in slab #10 327, B–E, is here designated as the type specimen (represented by Figs. 1, 4–6, and 14).
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Repository
Paleobotany Division, Natural History Museum and Biodiversity Research Center, University of Kansas, Lawrence, Kansas, USA.
Type locality
Col (saddle) north of Fremouw Peak, Queen Alexandra Range, central Transantarctic Mountains, Antarctica (Smoot, Taylor, and Taylor, 1985
).
Stratigraphic position
Upper part of the Fremouw Formation.
Age
Early Middle Triassic (Anisian).
Etymology
The specific epithet terpsichorum refers to the appearance of the vascular segments in serial sections (Terpsichore [Gr.] = the Muse of dancing).
Description
The specimens consist of stem segments ranging from 1.35 to 1.72 cm in diameter; the longest axis is approximately 12 cm. Surrounding the entire axis is a periderm. In a single transverse section the vascular segments occur as separate segments or are connected together in the center. These segments anastomose and separate at various levels within the axis (Figs. 1–5) and also produce traces; this is evident in successive transverse sections (Fig. 16). Segments are embedded in parenchyma and extend within 0.12–0.22 mm inside the periderm (Figs. 1–3). All segments include secondary tissues surrounding a central area of parenchyma that is 2–3 cells wide (Figs. 4–6). Smaller cells in the position of primary xylem occur in the center of some of the arms among the parenchyma cells (Fig. 6, arrows).
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A continuous bifacial vascular cambium formed by thin-walled cells is located external to the secondary xylem (Fig. 7, bracket a). This zone is approximately 2–4 cells thick and extends around each segment. Secondary phloem is located external to the vascular cambium but is difficult to distinguish in all areas of the stem due to poor preservation of the cells. The secondary phloem is made up of cells that appear circular in diameter and additional cells that appear crushed. The presence of secondary phloem is also indicated by the extension of rays beyond the secondary xylem and cambium; ray cells expand tangentially in the phloem region. In longitudinal section, the secondary phloem is characterized by elongate cells with slightly swollen end walls, similar to sieve cells; no sieve areas have been observed (Fig. 9).
The vascular tissue in Hapsidoxylon is embedded in parenchymatous ground tissue that appears to be approximately equal in volume to the vascular tissue. The ground tissue is relatively homogeneous throughout the stem, although some cells are filled with dark contents of unknown composition. Cells are isodiametric and range up to 102 µm in diameter with cells between vascular segments or between a segment and the periderm often crushed. We have found no evidence of sclerenchyma or other thick-walled cells within the ground tissue.
At the periphery of the stem are radial files of cells that are interpreted as periderm up to 0.5–0.68 mm thick (Fig. 14). The outer zone is approximately 5–9 cells thick and probably represents phellem. Individual cells are rectangular but radially flattened and contain dark contents of unknown nature. Some cells at the periphery of the stem appear to have been sloughed off. The region of cells internal to the phellem, presumably phelloderm, is approximately seven cells thick. Cells are rectangular, radially flattened, but do not contain dark contents. Between the phellem and the phelloderm is the phellogen, a region of thin-walled cells that is rarely preserved (Fig. 14, left).
At various levels of the axis the vascular segments give off traces. The vascular tissue in the traces appears to arise from the conducting tissue at the periphery of segments in the axis and in some instances a connection between the two vascular systems can be seen as the trace is produced (Fig. 2). Vascular tissue in the traces consists of radially aligned secondary xylem, rays, vascular cambium, and a few cells external to the cambium that may represent poorly preserved secondary phloem (Fig. 15). A relatively thick periderm delimits cortical tissues surrounding the traces and arises from the periderm of the stem. Primary xylem cannot be distinguished in the traces, possibly due to crushing. Traces range from 0.516 to 3.37 mm in diameter.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDISCUSSIONLITERATURE CITED |
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Maturation of the primary xylem is difficult to interpret due to the poor preservation of the primary vascular tissues and the inability to obtain any complete longitudinal sections due to frequent anastomoses. The region located within the vascular segments in transverse section is regarded as primary xylem based on the size and random arrangement of the tracheids. Based on the available material the primary xylem maturation is probably endarch; however, without the precise location of the protoxylem in longitudinal section, this interpretation can only be tentative.
Two types of secondary growth occur in the stem: secondary vascular tissue produced by a bifacial vascular cambium and the production of periderm. The radial alignment of tracheids is not by itself a valid criterion for the existence of secondary growth, as this can represent oriented metaxylem (Leclercq and Bonamo, 1971
; Cichan and Taylor, 1990
). However, vascular rays are normally absent in plants with radially aligned metaxylem, indicating a lack of cambial activity. The occurrence of rays is generally regarded as an indication of cambial activity, and they are found in both the secondary xylem tracheids and phloem of Hapsidoxylon.
The phylogenetic relationships of this stem are uncertain because there are no other organs associated with Hapsidoxylon, such as leaves or reproductive organs. Also, no Triassic plants with this type of vascular tissue arrangement have been reported, and the only axes with this type of arrangement occur far earlier in the Devonian and Lower Carboniferous. However, comparisons of the anatomy with axes known to be lycophytes and cladoxylaleans may provide some clues regarding putative phylogenetic relationships.
Fossil lycophytes reached a peak in diversity during the Carboniferous with both herbaceous and arborescent types present (Thomas, 1978
). Even though the herbaceous habit survives to the present, the arborescent forms are extinct. There are only two extant species in which secondary growth occurs, Isoetes and Stylites. Carboniferous lepidodendralean stems consisted of a protostele, mixed protostele or siphonostele, surrounded by limited secondary xylem (Taylor and Taylor, 1993
); their underground organs also possessed secondary xylem. They typically had a massive cortex and periderm surrounding the secondary xylem. The vascular tissue arrangement of Hapsidoxylon differs from that in known Carboniferous lycophytes because it contains a complex arrangement of vascular segments that extensively anastomose. In addition, the vascular cambium in Carboniferous lycophytes is unifacial (Eggert and Kanemoto, 1977
), producing only secondary xylem, while in Hapsidoxylon the cambium appears to have been bifacial. Finally, the cortex and periderm in lepidodendralean stems were much more extensive than in Hapsidoxylon.
Most living lycophytes contain a protostelic vascular cylinder with exarch primary xylem maturation; no leaf gaps exist (Ogura, 1972
). The main body of Isoetes is a tuberous form that is more or less ellipsoid. The upper half is covered by a crown of elongate leaves, and fibrous roots are located on the lower half. The vascular bundle is anchor-shaped and consists of primary and secondary tissues, the secondary tissues formed by lateral and basal meristems (Karrfalt, 1982
). Secondary growth in Stylites is similar to Isoetes, but the corms are more elongate and the root-producing meristem is upturned (Rauh and Falk, 1959
). Hapsidoxylon differs from extant Isoetes and Stylites based on the greater amount of secondary growth found in Hapsidoxylon.
Lycoxylon (Srivastava, 1945
) is a Jurassic lycopod with anatomy that is nearly identical to the living genus Lycopodium. The vascular cylinder is a plectostele with variously shaped and anastomosing xylem plates, but no secondary tissues are produced. Although superficially Hapsidoxylon is similar to the plectostele found in Lycoxylon or in extant Lycopodium species, upon closer examination differences become apparent. Hapsidoxylon contains ground tissue parenchyma between the arms of vascular tissue rather than alternating plates of xylem and phloem and additionally, Hapsidoxylon contains secondary phloem, which is absent in Lycoxylon and Lycopodium. Hapsidoxylon's vascular tissue anastomoses more extensively over a shorter distance than in Lycoxylon or extant Lycopodium species.
The Cladoxylales (Hirmer, 1927
; Scheckler, 1974
) are interpreted as a group of fernlike plants ranging from the Lower Devonian to the Lower Carboniferous and are known principally by their anatomy. The general characteristics of the order are stems containing a complex vascular system composed of single or anastomosing xylem segments. The segments located toward the periphery of the stem tend to be radially arranged and U, V, W, or Y-shaped as seen in transverse section with the sinus directed toward the periphery. Other segments are more circular. The peripheral strands usually have a parenchymatous loop developed to a greater or lesser degree, though some taxa do not show this characteristic. The term "peripheral loop" was originally coined by Williamson (1874)
as a characteristic within the Zygopteridaceae and has since been applied to various Devonian plants (Stein, 1981
), including cladoxylopsids. Though some authors (Leclercq, 1970
; Scheckler, 1974
) use the peripheral loop as a systematic feature, Stein (1981)
suggests that until there is additional knowledge of the histology of the Devonian peripheral loops, the definition is too ambiguous and difficult to differentiate from normal protoxylem strands found in many extinct and extant plants and therefore should not be used as a systematic characteristic. Genera and species within the family Cladoxylaceae are determined by characters such as the position, number, and form of the vascular segments observed in transverse section, the nature of the parenchymatous loop, and the presence or absence of secondary xylem (Leclercq, 1970
; Gensel and Andrews, 1984
).
Table 1 summarizes the species of Cladoxylales that have tracheids arranged in radial files in their vascular segments. The published species' descriptions either indicated presence or absence of rays, or did not mention rays, in which case N/A is recorded. We infer the presence of secondary xylem based on the occurrence of vascular rays. Arnold (1940
, 1952
) and Lemoigne and Iurina (1983)
describe Xenocladia medullosina as containing aligned secondary xylem, though there is no evidence of vascular rays in either transverse or longitudinal sections. We follow the interpretation of Leclercq (1970)
and Scheckler (1974)
, who describe X. medullosina as containing aligned metaxylem only. Of the cladoxylalean species containing radially arranged tracheids, only the following species appear to have secondary xylem: Steloxylon ludwigii, Cladoxylon kidstonii, C. mirabile, and C. taeniatum; all contain vascular rays.
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; Solms-Laubach, 1897
, 1910
) is a stem from the Devonian that is poorly understood and was originally placed with the medullosans. In tangential section, Steloxylon ludwigii contains numerous anastomosing vascular segments that are circular to ellipsoid with some elongated radially. Hapsidoxylon contains elongate vascular segments rather than circular as in S. ludwigii, and the stem diameter in Hapsidoxylon (1.35–1.72 cm) is much smaller than the diameter of S. ludwigii (13–15 cm).
Cladoxylon kidstonii (Solms-Laubach, 1910
), tentatively placed in Cladoxylales (Seward, 1917
; Leclercq, 1970
), is a Carboniferous stem based on a fragment 3 cm in diameter that contains five vascular segments and most likely contained many more. The segments are oval in cross section and contain a narrow band of primary xylem surrounded by a broad zone of secondary tracheids that are usually more developed to one side of the segment, assumed to be towards the center of the stem. Hapsidoxylon differs from C. kidstonii by containing secondary xylem evenly distributed around segments, the majority of the segments are elongate rather than oval, and the segments anastomose.
The vascular segment arrangement in the Carboniferous Cladoxylon mirabile (Unger, 1856
) is superficially similar to Hapsidoxylon. Cladoxylon mirabile is described as a stem 2–2.5 cm in diameter containing 12–20 vascular segments in transverse section. The vascular segments are elongate and either occur separately or joined with other segments. Some segments that are joined at the center of the stem form a U or V shape that opens toward the periphery of the stem. Vascular rays are described as present among the tracheids but no secondary phloem has been observed. Peripheral loops are also present within the vascular segments. In comparison, Hapsidoxylon produces much more secondary growth, contains a bifacial cambium, and lacks peripheral loops.
Cladoxylon taeniatum (Unger, 1856
; Solms-Laubach, 1897
), a Carboniferous stem, is 3.5 cm in diameter with 12–24 vascular segments. The segments are radially elongated, but rarely anastomose. Metaxylem is well developed, and there are large peripheral loops of protoxylem and parenchyma. Circular vascular segments are located in the center of the stem, with elongate segments organized toward the periphery. Hapsidoxylon contains vascular segments that frequently anastomose, and the circular segments, if any, are located towards the periphery rather than in the center of the stem; no peripheral loops are present. Hapsidoxylon contains a narrower area of parenchyma and primary xylem in the center of the vascular segments, as well as a smaller cortical region, than those found in C. taeniatum.
Overall, Hapsidoxylon produced much more secondary xylem than the cladoxylalean species described above and contains a bifacial cambium, a character that has not yet been found or is absent in these taxa. Hapsidoxylon also lacks peripheral loops. Because of these anatomical differences, and the stratigraphic occurrence in the Middle Triassic, we have not included Hapsidoxylon in the Cladoxylales.
The presence of a bifacial cambium in Hapsidoxylon may suggest a relationship with the seed plants. Generally, nonseed plants are thought to contain a unifacial vascular cambium and seed plants a bifacial one (Wilson and Eggert, 1974
). Exceptions to date include the fossil taxa Sphenophyllum, a sphenophyte stem (Eggert and Gaunt, 1973
), and Astromyelon, a calamitalean root (Wilson and Eggert, 1974
). Both of these taxa produce secondary xylem and phloem by way of a bifacial vascular cambium; however, the issue of homology with a seed plant cambium is still unresolved.
Medullosa is a seed fern stem genus that ranges from the Lower Carboniferous to the Permian. It is characterized by axes containing several vascular segments (formerly called "steles") that run throughout the stem and consist of primary xylem surrounded by secondary xylem, a bifacial vascular cambium, and secondary phloem (Stewart and Delevoryas, 1956
). Protoxylem is located near the outer margin of the stele, and development of the secondary xylem within the vascular segments is endocentric. The vascular segments are embedded in parenchymatous ground tissue and a periderm is located in the cortical region. Even though Hapsidoxylon shares some characteristics with species of Medullosa, such as a bifacial vascular cambium and varying numbers of vascular segments, the extensive anastomosing habit of the vascular segments in Hapsidoxylon and the fairly uniform development of the secondary tissues precludes it being placed in the Medullosaceae.
The Triassic flora of the Fremouw Formation consists of seed ferns belonging to the Corystospermales and Petriellales, along with conifers, cycads, true ferns, and sphenophytes (Taylor, Taylor, and Cúneo, 2000
). The flora is especially diverse, but Hapsidoxylon is the only genus with this type of complex vascular tissue arrangement recorded to date. While it is impossible at this time to assign Hapsidoxylon to a major group of vascular plants, we hope that future research on this unique permineralized flora of Triassic age will uncover the reproductive organs of this enigmatic plant offering more resolution in its placement among vascular plants.
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FOOTNOTES |
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4 Author for reprint requests, current address: Ecology and Evolutionary Biology, University of Connecticut, 75 North Eagleville Road U-43, Storrs, Connecticut 06269-3043 USA (hilary.mcmanus{at}uconn.edu
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSYSTEMATICSDISCUSSIONLITERATURE CITED |
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Arnold C. A. 1952 Observations on fossil plants from the Devonian of eastern North America VI. Xenocladia medullosina Arnold. Contributions from the Museum of Paleontology, University of Michigan 9: 297-309
Askin R. A. T. J. Cully 1996 Permian to Jurassic palynological collections in the Shackleton Glacier area. Antarctic Journal of the United States 31: 3-5
Barrett P. J. D. H. Elliot J. F. Lindsay 1986 The Beacon Supergroup (Devonian-Triassic) and Ferrar Group (Jurassic) in the Beardmore Glacier area, Antarctica. In M. D. Turner and J. D. Splettstoesser [eds.], Geology of the central Transantarctic Mountains, 339–428, Antarctic Research Series, vol. 36, no. 14. American Geophysical Union, Washington, D.C., USA
Bertrand P. 1935 Contribution á l'étude des Cladoxylées de Saalfeld. Palaeontographica 80B: 101-170
Cichan M. A. T. N. Taylor 1990 Evolution of cambium in geologic time—a reappraisal. In M. Iqbal [ed.], The vascular cambium, 213–228. Wiley & Sons, New York, New York, USA
Delevoryas T. T. N. Taylor E. L. Taylor 1992 A marattialean fern from the Triassic of Antarctica. Review of Palaeobotany and Palynology 74: 101-107
Eggert D. A. D. D. Gaunt 1973 Phloem of Sphenophyllum. American Journal of Botany 60: 755-770
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. Scale bar = 0.25 cm. 2. Trace still attached indicated by arrow in center of photo. Arrow in right upper corner indicates another trace given off previously, slide #15 377, specimen #10 390 D1 top #17. Scale bar = 2 mm. 3. Arrows indicating traces separated from stem. The large trace indicated by arrow to the right of the stem is the same trace shown attached in Fig. 2, #15 378, specimen #10 390 D bottom #13. Scale bar = 2 mm. 4. Vascular segments in cross section with vascular tissue joined at arrows, slide #15 379, specimen #10 327 E #5. Scale bar = 0.5 mm. 5. U-shaped vascular segment in cross section just inside the periderm, slide #15 380, specimen #10 327 E #24. Scale bar = 0.5 mm. 6. Center of a vascular segment showing parenchyma (P) in center and rows of tracheids alternating with vascular rays (R) on either side of the parenchyma. Arrows indicate small diameter tracheids that are interpreted as primary xylem, slide #15 381, specimen #10 327 E #1. Scale bar = 100 µm.
3. Scale bar = 150 µm. 13. Tangential section showing vascular rays, slide #15 383, specimen #10 390 D1 side 