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American Journal of Botany 86(11): 1563–1575. 1999. VESSEL-BEARING STEMS OF VASOVINEA TIANII GEN. ET SP. NOV. (GIGANTOPTERIDALES) FROM THE UPPER PERMIAN OF GUIZHOU PROVINCE, CHINA1 HONGQI LI AND DAVID WINSHIP TAYLOR2 Department of Biology, Indiana University Southeast, New Albany, Indiana 47150 Permineralized gigantopterid stems of Vasovinea tianii Li et Taylor gen. et sp. nov. were collected from the Upper Permian of Guizhou Province, China. They are slender and bear prickles, trichomes, and compound hooks. Internally, the stems have a sparganum cortex, eustele, and secondary xylem. The mesarch protoxylem tracheids have annular to helical thickenings, and metaxylem tracheary elements have scalariform and/or transversely elongated, bordered pits, while those of the secondary xylem have scalariform to circular bordered pits. Importantly, the inner part of the secondary xylem has large vessel elements with foraminate-like perforation plates. The hooks and other morphological and anatomical characteristics are similar to those found in gigantopterids, suggesting that Vasovinea is a member of the Gigantopteridales. The vegetative plant is reconstructed from permineralized stems and Gigantopteris-type leaves based on the anatomical similarities and intimate association. The eustele, secondary xylem, and other features support the placement of the order among the seed plants. Ecologically, Vasovinea is suggested to have been a vine or liana that used compound hooks to climb among the trees in a Permian tropical rain forest. The occurrence of vessels could have been an efficient adaptation to allow the slender stems to conduct sufficient water to the large Gigantopteris-type leaves. Key words: gigantopterids; Gigantopteris; hooks; lianas; paleoecology; Permian; stems; Vasovinea; vessels. Fossil specimens of gigantopterids have been found extensively from the Lower Permian and possibly into the lowest Lower Triassic sediments throughout southeastern Asia (mostly from China), as well as from several Lower Permian sites in Texas and Oklahoma, USA. This group was first reported by Schenk (1883) from Hunan Province, China, and currently includes more than 40 taxa (H. Li et al., 1994). The distinctive characteristics of gigantopterids are their large compound or simple leaves, which are variable in morphology, ranging from oblong to roughly round shape, entire to toothed margin, and simple to complex reticulate venation. Conventionally the group has been classified as ferns (Schenk, 1883) or seed ferns (White, 1912; Asama, 1959; X. Li and Yao, 1983). In terms of relationships to other plants, Asama (1974, 1982, 1988) proposed that the simple-leafed gigantopterids could have given rise to the angiosperms, while X. Li and Yao (1983) interpreted their reconstruction of the reproductive organs of Gigantonoclea fukienensis as being ‘‘parti-angiosperms’’ in nature. Despite 1 Manuscript received 16 July 1998; revision accepted 25 March 1999. This paper represents part of Hongqi Li’s dissertation submitted in partial fulfillment of the requirements for the Ph.D. degree at the Ohio State University, Columbus, Ohio. Hongqi Li thanks Drs. Edith L. Taylor and Thomas N. Taylor for professional help, and Dr. William A. Jensen for guidance and encouragement. The authors thank Dr. Chengsen Li and the Botanical Institute of Beijing for support in fossil-collecting; Dr. Paul Kenrick and the staff of the Swedish Museum of Natural History for loan of gigantopterid specimens; and Drs. Sherwin Carlquist, Kathleen B. Pigg, Leo J. Hickey, and an anonymous reviewer for their valuable comments on the manuscript. This research is partially supported by the Geological Society of America Research Grant 5195-93, the Graduate Student Alumni Research Award, Graduate School of the Ohio State University, and grants from the Research and University Graduate School of Indiana University, and the Academic Affairs of Indiana University Southeast. 2 Author for correspondence (e-mail: [email protected]). the distinctive leaves, anatomically preserved reproductive organs have not been found so that the systematics of the group has been uncertain. Recently, some permineralized gigantopterid leaves and axes have been reported with many anatomical characteristics that can help in determining the relationships of gigantopterids to other vascular plants. The permineralized leaves of the gigantopterid Delnortea abbottiae, reported from Texas, USA, are suggested as being structurally similar to gnetophytes (Mamay et al., 1988). In contrast, the permineralized leaves of Gigantonoclea guizhouensis, from the Upper Permian in Guizhou, China, exhibit many features similar to those of angiosperms (H. Li and Tian, 1990; H. Li et al., 1994). Two types of permineralized gigantopterid stems also are found associated with the Guizhou gigantopterid leaves (H. Li, Taylor, and Taylor, 1992, 1993). One type is the prickly Aculeovinea yunguiensis (H. Li and Taylor, 1998), which is considered a seed plant. Another type containing vessels has been reported preliminarily (H. Li, Taylor, and Taylor, 1996). Before the report by H. Li, Taylor, and Taylor (1996), the fossil record of vessels could be traced back to the Early Cretaceous. In extant plants, vessels consist of vertically linked tracheary elements with perforate end walls so that they conduct water much more efficiently than tracheids with imperforate end walls. Vessels are commonly found in angiosperms and gnetophytes, with rarer occurrences in non-seed plants (see Discussion). Vessels in living seed plants have been classified in two types, i.e., foraminate vessels in gnetophytes and scalariform ones (and/or their derived simple form) in angiosperms (Bailey, 1944; Carlquist, 1992, 1994, 1996a). Vasovinea tianii is different from the above two types, possessing unique, foraminate-like vessels in the secondary xylem and a possible scalariform-reticulate vessel in the meta- 1563 1564 AMERICAN JOURNAL xylem. Therefore, the discovery of vessels in gigantopterids is important not only as the earliest fossil record of vessels, but also in analyzing the systematic relationships and the ecological aspects of the group. To complement the preliminary study (H. Li, Taylor, and Taylor, 1996), we now provide a comprehensive description of the permineralized, vessel-bearing gigantopterid stems and establish them as a new taxon, Vasovinea tianii Li et Taylor gen. et sp. nov. With several lines of evidence from both permineralized and compression specimens, we also demonstrate that the new taxon is a member of the gigantopterids, reconstruct it together with Gigantopteris-type leaves, analyze its possible habit, and briefly discuss its systematic relationships to seed plants. MATERIALS AND METHODS Limestone and mudstone samples with gigantopterid stems and leaves were collected from a talus pile mined from the upper and middle parts of the Xuanwei Formation of the Upper Permian at the Shan-JianShu site, Yueliangtian Coal Mine in Panxian County, Guizhou, China, in 1993. Additional information about the locality and stratigraphy can be found in H. Li et al. (1994) and H. Li and Taylor (1998). Permineralized specimens from five limestone samples (L9407, PLY02, PLY03, PLY04, and L9414), and compressed specimens from three mudstone samples (L9426, L9448, and L9449) were used. All samples contain gigantopterid foliage and stem(s) in each, and some have additional compound hooks as well as other structures, although not all of these organs from each of the samples are figured in this paper. Samples PLY03 and PLY04 are small, but each contains a Vasovinea tianii stem and a piece of Gigantopteris-type leaf. Sample L9407 has numerous pieces of Gigantopteris-type leaves and several pieces of V. tianii stems. PLY02 is a large sample, weighing more than 8 kg, and contains hundreds of pieces of permineralized (usually gigantopterid) plant organs, including at least two types of gigantopterid leaves (Gigantopteris and Gigantonoclea), two types of gigantopterid stems (V. tianii and Aculeovinea yunguiensis; H. Li and Taylor, 1998), and some undescribed reproductive organs. Specimens of Vasovinea and Gigantopteris in this sample are relatively fewer in number compared to those of Aculeovinea and Gigantonoclea. The permineralized stems were sectioned and prepared using the well-known cellulose acetate peel technique (Phillips, 1976). Areas with interesting structures from the peels were trimmed, cleaned, and mounted on microscope slides for further observations. Occasionally specimens were peeled in both the transverse and longitudinal sections to show the structural correlation of both sections (Figs. 1, 6). Specimens were photographed with an MP-4 camera using 4 3 5 film or with an Olympus 35-mm camera mounted on an Olympus Steroscan dissecting microscope. To prepare scanning electron microscope (SEM) samples, several permineralized wood pieces with vessels were etched with a 1% HCl solution for ;20 min so that the diluted solution could dissolve the calcium carbonate slowly to expose cellular structures, e.g., the perforation plates (Figs. 16–19). Then they were mounted and sputter coated with gold before being examined with SEM. The compressed specimens were directly photographed with a Polaroid MP-4 camera. OF BOTANY [Vol. 86 cm in diameter, bearing compound hooks, prickles, and glandular and tendril-like trichomes. Cortex with paired vascular traces and sparganum structure in the outer part. Eustele with parenchymatous pith and mesarch protoxylem, consisting of tracheids with annular, helical, or helical-scalariform thickenings. Centripetal metaxylem tracheids commonly consisting of one to two layers, while centrifugal metaxylem usually consisting of two to three layers of tracheary elements; metaxylem tracheary elements with scalariform and/or transversely elongated bordered pits on their lateral walls. Outer portion of the secondary xylem consisting of radial files of tracheids and smaller vessels. Lateral walls of tracheary elements in secondary xylem exhibiting multiseriate, alternate, and transversely elongate to circular bordered pits. The inner portion of the secondary xylem consisting of large vessels with diameter increasing from ;150 to 250 (up to 500) mm towards the primary xylem; vessel elements vertically connected by foraminate-like perforation plates on the long, inclined (usually) to short, almost horizontal (occasionally) end walls, each plate with multiseriate, alternate, obliquely elliptical to circular pores without borders. Homocellular, heteroseriate rays occurring every one to three radial tracheary files, consisting of uni- to bi-seriate xylic rays and multiseriate medullar rays between xylem segments. Holotype—Slides L9407-C-B2, L9407-C-B16, and L9407-D-T2. Figs. 1–4, 6. Paratypes—Slides PLY02-C10-1-1, PLY02-E-1, PLY03–01, PLY03–06, PLY03–07, PLY03–11, PLY03– 34, and PLY04-B; Specimens PLY02 and PLY03. Figs. 5, 7–20, 29–30. The slides and specimens of both the holotype and paratypes have been deposited in the National Museum of Plant History of China at the Institute of Botany, Chinese Academy of Sciences, Beijing, China. Etymology—The generic name is composed of vaso([L] 5 vessel) and vinea ([L] 5 vine) indicating a liana stem with vessels. The specific name is proposed in honor of Professor Baolin Tian, the Beijing Graduate School, China University of Mining and Technology, for his contribution to the study of the Gigantopteris flora of western Guizhou, China. Locality—Yueliangtian Coal Mine, Panxian County, Guizhou Province, China. Stratigraphic occurrence—Lower and Upper Xuanwei Formations, Upper Permian. Age—Longtanian-Changxingian, Late Permian. SYSTEMATICS AND DESCRIPTION Order— Gigantopteridales X. Li et Z. Yao (1983) Family—Gigantopteridaceae Koidzumi (1936) Species—Vasovinea tianii gen. et sp. nov. Generic and specific diagnosis—Stems slender, ;1 Description—The stems are usually ,1 cm in diameter and up to 5 cm in length. One stem measured ;4 3 6 mm in transverse section (Figs. 1–3, 6), while others were compressed and calculated to be ;1 cm in diameter (Figs. 5, 29–30). Well-preserved stems have an epidermis and hypodermis. The epidermis consists of one layer of small cells that have relatively thick walls. The layer is ;10 mm thick and its cells are ;10 mm wide and 8 to November 1999] LI AND TAYLOR—VESSEL-BEARING GIGANTOPTERID VASOVINEA FROM CHINA 1565 Figs. 1–5. Vasovinea tianii stems. 1. Stem (left) longitudinal section with transverse section at bottom. Note attached basal part of a compound hook (right side in the box), oblique section of the hook in the middle bottom (arrow), and prickles (arrowheads). Slide L9407-C-B16; bar 5 4 mm, 32.5. 2. Transverse section (14 continuous peels below the transverse section in Fig. 1) showing central xylem segments, two vascular traces (marked as ‘‘T’’) outside a single lacuna (left arrow), and a prickle at right (right arrow). Slide L9407-C-B2; bar 5 1 mm, 316. 3. Transverse section (5 mm below the section in Fig. 2) with several xylem segments, sparganum cortex (arrow), and the basal part of a compound hook structure (H) in an oblique section. Slide L9407-D-T2; bar 5 1 cm, 316. 4. Enlarged portion of a wood segment and a vascular trace (at top) shown in Fig. 2. Notice the angular-shaped vessels and the mesarch primary xylem with tiny protoxylem cells (arrowhead). bar 5 0.5 mm, 336. 5. Stem with vessels in the xylem segments and trichomes on the surface of the stem and embedded in the surrounding sediments (arrowheads). Slide PLY04B; bar 5 2 mm, 37.5. 1566 AMERICAN JOURNAL OF BOTANY [Vol. 86 Figs. 6–13. Vasovinea tianii stems. 6. Enlargement from Fig. 1 showing large vessels in both transverse section (upper) and the longitudinal section (lower) with vertically linked parenchyma cells (arrow) in the inner cortical portion and sclerenchyma strands on the right side. Slide L9407C-B16; bar 5 1 mm, 316. 7. Xylem in longitudinal section with two perforation plates (arrows) in the upper and central lower parts, a typical lateral wall with bordered pits in the right lower part, and cross-field pitting of secondary xylem tracheids on the left side. Slide PLY03–01; bar 5 50 mm, 3200. 8. Primary xylem (left arrow) and secondary xylem (right) in longitudinal section. Notice the possible vessel element in the metaxylem that has scalariform-reticulate structures on its long, oblique end wall (arrow), in contrast to the large vessel in secondary xylem on right with a foraminate-like perforation plate. Slide PLY03–06; bar 5 200 mm, 368. 9. Longitudinal section of a centripetal metaxylem tracheid (on pith side) with uniseriate scalariform pitting (lower left) and 2–3 rows of transversely elongated bordered pits (left upper, arrowhead). Also seen are two poorly preserved tracheids with annular thickenings in the middle of the figure, and a protoxylem tracheid with helical to helical-scalariform thickenings on right. Slide PLY03–06; bar 5 50 mm, 3200. 10. Enlargement from Fig. 8 showing a possible vessel element with a long, highly inclined end wall (upper arrow) that has a scalariform-reticulate structure without surrounding borders or primary walls. The lower part shows a broken piece of the lateral wall with imperforate transversely elongated bordered pits (lower arrow). Slide PLY03–06; bar 5 50 mm, 3200. 11. Enlargement from Fig. 8 showing the perforation plate (pp) with multiseriate pores and the lateral wall of the vessel with bordered pits in the lower right part. Slide PLY03–06; bar 5 50 mm, 3200. 12. Tangential section showing a long oblique perforation plate connecting two vessel elements on left and a smaller perforation plate at lower right corner. Slide PLY02-C10-1-1; bar 5 200 mm, 342. (Reprinted with permission from H. Li et al., 1996. Copyright 1996, American Association for the Advancement of Science.) 13. Tangential section of the secondary xylem showing a tracheid with circular bordered pits. Slide PLY03–07; bar 5 50 mm, 3200. November 1999] LI AND TAYLOR—VESSEL-BEARING 20 mm long. Beneath the epidermis are 2–4 layers of hypodermal cells that are commonly ;40–50 mm long and 15–40 mm in diameter, and densely arranged with the larger cells occurring to the outside and smaller ones to the inside. Attached to the stems are compound hooks (see below) and a variety of appendages including prickles and glandular and tendril-like trichomes. Appendages—The appendages are epidermal-cortical outgrowths and lack any vascular tissue. They are commonly found on stems, but some smaller appendages can be found on the compound hooks (Fig. 1, right arrowhead). The prickles on the stems are usually ;450 mm (up to ;1000 mm) long and 450 mm (up to 550 mm) wide at their bases, but a narrower one found near the base of a compound hook is only ;250 mm wide at the base (Fig. 2, right arrow). Each prickle is covered by the epidermis and internally composed of vertically elongate parenchyma cells, 20–30 mm in diameter and 40–50 mm long. A few parenchyma cells beneath the epidermal cells are narrower and have thickened cell walls. Some hypodermal cells (dark colored) beneath the base separate the prickle from the inner part, suggesting that the prickle is epidermal and cortical in origin. Structurally, these prickles are nearly identical to those of Aculeovinea yunguiensis (H. Li and Taylor, 1998), but smaller in size. Glandular trichomes can be found on both the stems (Figs. 26, 28) and the compound hooks. Generally, they are 500–2000 mm long and 500–1000 mm in diameter at their bases. Their basal parts are similar to the prickles in terms of their structure and the epidermal and cortical origin. However, their apices consist of either several large cells (Fig. 28) or a single larger oval cell, 200 mm in length and 100 mm in diameter, at the tip (Fig. 26). The tendril-like trichomes (the tendril-like structures in H. Li, Taylor, and Taylor, 1992, 1993) are long and curved in shape. These trichomes are ;300 mm in diameter and can be several millimetres long. They contain similar-shaped parenchyma cells, with the inner cells slightly enlarged. They may be preserved as tubes (Fig. 30, arrowhead) when the inner parenchyma cells are no longer preserved. In other sections, these trichomes often appear as wart-like structures extending from the stems (Fig. 5, center) or circular rings (Fig. 5, arrowheads) embedded in the surrounding sediments. Cortical histology—Beneath the hypodermis, in the well-preserved stems, the cortex consists of a sparganum structure (i.e., vertically parallel sclerenchymatous strands alternating with vertical parenchymatous tiers; see Taylor and Taylor, 1993) towards the outer side (Fig. 3, bottom) and a wider parenchymatous zone towards the inside (Figs. 2, 3). The sclerenchyma cells are ;20 mm in diameter and 1000–3000 mm long (Fig. 6, right). The cortical parenchyma cells are roughly cuboidal, 45–70 (up to 95) mm in dimension, and vertically linked (Fig. 6, arrow). They are often damaged or crushed, leaving empty spaces in the cortex (Figs. 3, 5, 6). Vascular system—A eustelic primary architecture surrounds a poorly preserved pith, which contains cuboidal parenchyma cells, 40–70 (up to 110) mm in dimension (Figs. 2, 3). The vascular cylinder is preserved in 5–9 GIGANTOPTERID VASOVINEA FROM CHINA 1567 xylem segments of different sizes that are either well preserved or somewhat crushed. In transverse section, each large woody segment appears as a sector of an annulus, with the inner side longer than the outer side, because of the larger vessels to the pith side and smaller tracheary elements to the outside. Along the periphery of the pith, primary xylem bundles and a few solitary tracheids are embedded among the parenchyma cells. Mesarch protoxylem strands can be found in the centers of some of the large bundles (Fig. 4, arrowhead at the bottom), but they are frequently difficult to distinguish. The protoxylem tracheids can be narrower than 20 mm in diameter with annular to helical or helical-scalariform thickenings (Fig. 9). The centripetal metaxylem usually is one to two cells wide (Fig. 4), and the tracheids commonly are less than 50 mm in diameter and have scalariform (Fig. 9, left lower part) or 2–3 rows of transversely elongated, bordered pits (Fig. 9, arrowhead). The centrifugal metaxylem commonly is 2–3 cells wide (Figs. 4, 8 [left]), with the inner tracheary elements being narrow and having scalariform bordered pits. The outer elements usually are large, up to 100 mm in diameter, and have up to five rows of transversely elongated, bordered pits on the lateral walls. The elongated pits are arranged into obliquely opposite rows when two or three rows are present. In some of those with five rows, the pit openings are only 2.5 mm high but can be up to 16 mm wide. However, as the row number increases, the pits appear to be alternately arranged and pit openings become narrower. One centrifugal metaxylem element (Figs. 8 [arrow], 10), 50 mm in diameter, has two types of wall structures. The tip of the tracheary element forms a long, highly inclined end wall that has a scalariform-reticulate structure (Fig. 10, upper arrow). The lower part shows a small broken piece of its lateral wall with oval bordered pits (Fig. 10, lower arrow). Tracing downwards on the series of peels, scalariform bordered pits are found on the lateral wall of the same element. Therefore, this tracheary element has scalariform and laterally elongated bordered pits on the lateral walls. However, because there are no borders around the scalariform-reticulate structure and no primary walls appeared within the structure, the oblique end wall appears to be perforated. In other words, this element possibly could be a vessel member. The secondary xylem has tracheary elements arranged into radial files, each with an inner portion of large vessels (1–5 vessels wide) and an outer portion of smaller tracheary elements. Several stems show an inner portion that is about 3–5 vessels wide. These vessels increase in diameter from less than 150 mm to 250 mm towards primary xylem (Figs. 2–5). The vessel outlines are round (Figs. 3, 5, 14) or squarish (Figs. 2–5) in transverse section. In transverse section, the tracheary elements may appear as tangential rows of cells of similar size and shape (Fig. 4). In addition, in tangential section (Figs. 6 [left], 12) the end walls appear at similar levels. One stem has large vessels arranged into radial files that are tangentially only 1–2 vessels wide, with each vessel up to 500 mm in diameter, but outer elements with a much smaller diameter (Figs. 15–16). This latter specimen may represent a different species, but there is insufficient material to establish a separate species at this time. 1568 AMERICAN JOURNAL OF BOTANY [Vol. 86 Figs. 14–20. Vasovinea tianii. 14. Uni- to biseriate rays (r) and vessels with oblique (above) or almost horizontal (below) perforate end walls. Slide PLY03–34; bar 5 100 mm, 3170. 15. A segment of wood in transverse section with large vessels that can be up to 500 mm in diameter. Slide PLY02-E-1; bar 5 500 mm, 321. (Reprinted with permission from H. Li et al., 1996. Copyright 1996, American Association for the Advancement of Science.) 16. Another section of the specimen in Fig. 15 showing vessel elements with oblique end walls (arrows) at roughly the same level. Notice that the cell rows of the outer secondary xylem zone are interlaced, i.e., their oblique end walls are in a zigzag form. The smaller perforation plate (left arrow) is enlarged in Fig. 17, and the larger perforation plate (right arrow) is enlarged in Fig. 18. PLY02, section E; bar 5 200 mm, 340. 17. Enlargement from Fig. 16 showing an underdeveloped perforation plate with obliquely elongated borderless pits (left arrow) whose primary wall (upper arrow) is planar without margo threads or tori. bar 5 10 mm, 31000. 18. Enlargement from Fig. 16 showing a perforation plate. Notice that the smaller pores at the right edge lack borders and have a planar primary cell wall present (arrow). bar 5 10 mm, 31000. November 1999] LI AND TAYLOR—VESSEL-BEARING Overall, vessel elements can be up to 4500–5000 mm long. Their planar end walls vary from short (Fig. 12, right lower corner) to very long (up to 1200 mm long; Fig. 12, left), from oblique (Figs. 12, 14 [upper two vessels], 16 [right arrow]) to almost horizontal (Fig. 14, lower left vessel). Most of the end wall is perforated with multiseriate (up to 12 rows) pores (Figs. 7 [upper right], 8 [lower right], 11). In the central area, most pores are large and roughly circular, 8 3 10 mm (Figs. 7, 11) and up to 13 3 15 mm (Figs. 18 [middle left], 19 [right]), but some pores are much larger and elliptical, up to 13 3 21 mm (Fig. 19, lower left). Although those pores in the central area are completely perforated without remains of border membranes, the pores in peripheral regions are smaller and exhibit an incompletely dissolved primary cell wall (Fig. 18, arrow) that is smooth and shows no trace of margo threads or tori. In transverse section, the outer zone, up to 20 cells wide (6–14 cells wide in Figs. 2–4), contains tracheary elements that are usually ;40 3 50 mm (a few up to 120 mm) in dimension and arranged in radial files alternating with rays at every one to three files. One of the smaller tracheary elements, ;120 mm in diameter, has a planar end wall (Fig. 16, left arrow) with incomplete pores (Fig. 17). The plate exhibits obliquely elongated pits, which have smooth rims (Fig. 17, left arrow), and a planar, partially dissolved primary wall (Fig. 17, upper arrow). Thus, it has the same type of perforation pattern as in the large vessels, though the dissolution is not as complete. In other words, some tracheary elements in the outer zone might be vessel elements, while others could be tracheids with imperforate end walls. The lateral walls of both large and small tracheary elements exhibit the same type of alternately arranged, multiseriate, bordered pits (Figs. 7 [lower right], 11 [lower right], 13, 20). These bordered pits are obliquely elliptical, 3 3 8 mm (Fig. 20, right upper), to nearly circular, 6–8 mm in diameter (Fig. 13). Rays consist of homocellular parenchymatous cells, which are usually 8–15 mm wide (Fig. 4), 15–25 mm long, 25–50 mm high (Figs. 7 [at left], 13 [at right], 20 [at left]), and each cross-field has $3–9 obliquely arranged pits (Fig. 7, at left). The xylic rays are well preserved, usually one to two cells wide (Figs. 4, 14) and as tall as 60 cells. Between the cauline xylem segments, there are wedge-shaped gaps (Figs. 2–6, 15 [arrows]) sometimes containing cellular remains that are over three cells wide and could be medullary rays. The region of the cambium and inner phloem appears as a zone of amorphous cellular remains or as an empty gap, due to the poor preservation. In several transverse sections, there is a narrow, dense cellular zone, surrounding the secondary xylem (Figs. 2–3), composed of 2–4 small cells, which are usually rounded rectangular in shape, ;15 mm in diameter, and have lumens commonly 6–8 mm in diameter. In tangential section, they are elon- GIGANTOPTERID VASOVINEA FROM CHINA 1569 gate, but their lengths are unknown because of their poor preservation. These cells might be the remains of the phloem fibers. A pair of vascular traces (Fig. 2, ‘‘T’’), each ;750 mm in diameter, are found in the cortex, outside a gap (Fig. 2, arrow) between two cauline segments. The two traces are of similar size and appear to originate from within the gap, resulting in a unilacunar two-trace pattern. The trace on the left appears to have been just pinched off from the small-celled, side part of a main cauline xylem segment. The right trace is bilaterally symmetrical and ring-shaped with its abaxial side wider than the adaxial side (only 2–3 cells wide; Fig. 4, upper). The abaxial side has seven or more tracheids, each up to 40 mm in diameter, radially arranged into files. These tracheids represent the primary and, possibly, secondary xylem. The central area of the vascular ring is usually empty, occasionally containing the remains of crushed parenchyma cells. This type of trace structure (Figs. 2, 4) may represent the vascular traces of a leaf. Compound hook structure—On the right side of Fig. 1 is a narrow branching axis, or a ‘‘compound hook,’’ which is ;4 cm long and has two pairs of opposite branches. Note that this figure includes both transverse and longitudinal sections of a stem (left) and the compound hook (right). The attachment and branching pattern of the compound hook were determined by examining the part and counterpart and a series of peels. The main axis of the compound hook, 3 mm in diameter, extends ;1 cm from the stem (in the box) through the matrix and then branches the first time to form a pair of lateral hook tips. The lower one was obliquely cut through and left an oblique section, while the upper one was preserved in the counterpart (Fig. 1, large arrow). Then, the main axis becomes thinner (1.6 mm in diameter) and extends ;1.6 cm and branches again, resulting in a second pair of lateral hook tips and a terminal tip, each ;0.8 mm in diameter at their bases. With additional peels, the longitudinal section of the main axis of the compound hook and an oblique section of the upper hook tip of the second pair were exposed. These lateral and terminal hook tips curve backwards and are arranged in roughly the same plane. More details of the attachment of the compound hook to the main axis (Fig. 3, lower right) are shown in a transverse section which is ;5 mm lower from the section in Fig. 2. Here, one large vascular trace (Fig. 3, ‘‘H’’) is relatively well preserved in an oblique section of the compound hook axis. This vascular trace is crescent shaped, bilaterally symmetric, and opens towards its adaxial side where the parenchyma cells occur. As in the stem, the main axis of the compound hook also has sparganum cortex. The tracheids in this main axis are usually 20–45 mm in diameter and have helical ← (Reprinted with permission from H. Li et al., 1996. Copyright 1996 American Association for the Advancement of Science). 19. Partial perforation plate of another vessel showing elliptic-circular pores without borders. The right side shows the plate just as dissolved out of the limestone matrix in which the primary walls and the borders are inferred to have been missed before the preparation. LPY02; bar 5 10 mm, 31000. 20. Longitudinal section showing broken parenchyma ray cells on the left and obliquely elongated (right upper) to circular (right lower) bordered pits on the lateral wall of a secondary tracheid. LPY02; bar 5 18.5 mm, 3540. 1570 AMERICAN JOURNAL OF BOTANY [Vol. 86 November 1999] LI AND TAYLOR—VESSEL-BEARING GIGANTOPTERID VASOVINEA FROM CHINA 1571 thickening, but a few tracheids are 15–23 mm in diameter and have smaller, scalariform to transversely elongated, bordered pits. Distally, the vascular cells gradually become reduced in size and number and eventually vanish and leave only a hollow center surrounded by densely packed, thick-walled parenchyma cells, as seen in better preserved, dispersed hooks, which are 0.8 mm in diameter at their basal parts (Figs. 23, 24). DISCUSSION The permineralized stems of Vasovinea tianii exhibit many well-preserved anatomical structures. We will discuss several structures before we analyze the implications of the fossil characteristics for determining the affinities, systematic relationships, and ecology. The tendril-like trichomes have been previously reported as tendril-like structures because of their curvature (H. Li, Taylor, and Taylor, 1992, 1993). We have changed the term because they lack vascular tissues, and their basal parts appear to be similar to the glandular trichomes in terms of their structure and epidermal and cortical origin. The vessels in living plants are widely believed to have evolved from vertically linked tracheids by the dissolution of the primary cell walls between pit pairs on their end walls. One type of vessel has foraminate perforation plates, which develop by the loss of the pit membranes between circular bordered pit pairs. This type is characteristic of gnetophytes, whose circular bordered pits usually have pit membranes with tori and margo threads (Carlquist, 1996a, b, c), and pores with the remains of the circular borders that may be slightly raised. Another type of vessel has scalariform perforation plates, which develop by the loss of the pit membranes (that have a uniform construction with micropores) between scalariform bordered pit pairs on the highly inclined, elongate, planar end walls (Carlquist, 1996a, b, c). This type of vessel commonly occurs in some ferns (e.g., Carlquist and Schneider, 1997; Schneider and Carlquist, 1998) and basal angiosperms (e.g., Bailey, 1944; Carlquist, 1992, 1994, 1996a), but derived angiosperms may have vessels with simple perforations that evolved from the scalariform type. The vessels of Vasovinea tianii in the secondary xylem also appear to develop by the dissolution of the pit membranes between pit pairs on the end walls, and the roughly round pores make the vessels superficially similar to Fig. 32. Reconstruction of stem of Vasovinea tianii Li et Taylor with compound hooks and actinodromous Gigantopteris-type leaves. the foraminate type rather than the scalariform type. However, on a closer examination, we found that the vessels of V. tianii are different from the foraminate type. First, the perforation plate is planar (e.g., Figs. 11, 16) and the number of pores is high (e.g., Figs. 7, 11, 14, 18, 19). Second, some pores are actually transversely elongate (not circular, e.g., Fig. 19) and lack borders (compared pores in Fig. 11 to bordered pits in Fig. 13). Third, within some pores the incompletely dissolved pit membranes are planar and lack tori and margo threads (Figs. 17, 18). Therefore we call these vessels foraminate-like to distinguish them from those with typical foraminate perforation plates. On the other hand, the possible vessel ← Figs. 21–31. The trichomes of the Vasovinea tianii (Figs. 26 and 28) and other associated gigantopterid structures. 21. A compressed compound hook structure with two pairs of branches. L9448–2; bar 5 1 cm, 32.4. 22. A compressed, incomplete compound hook with two hook tips (arrows) associated with a Gigantopteris-type leaf that has compound round teeth (top). L9449; bar 5 1 cm, 30.9. 23. Longitudinal section of the broken top part of a permineralized hook tip. Notice the hollow center. Slide L9414–1–1; bar 5 200 mm, 345. 24. Longitudinal section of a permineralized hook tip, perpendicular to the section in Fig. 23. The upper end shows the oblique transverse section of the tip as it curves back and the end has a small hollow center. Slide PLY02-C8(R)-2; bar 5 1 mm, 320. 25. Transverse section of the secondary vein of a Gigantopteris-type leaf showing the cortical sclerenchyma strands (arrows) and a heart shaped xylem segment enclosed by a poorly preserved cell layer. Slide PLY02-B7; bar 5 200 mm, 356. 26. A pair of trichomes, the one on the left has a large apical cell. Slide L9407-B-T2; bar 5 200 mm, 345. 27. A compressed stem (at right bottom), possibly belonging to Vasovinea tianii, associated with two pieces of Gigantopteris-type leaves that have complex reticulate venation (arrows). Note the stem with vertical ribs and grooves. L9426; bar 5 1 cm, 31. 28. A glandular trichome with a tapered apical cell. Slide L9407-3-2-2; bar 5 200 mm, 345. 29. A vessel-bearing, permineralized stem associated with a leaf of Gigantopteris meganetes Tian and Zhang (1980) that has complex mesh venation and the compound, rounded teeth. PLY03; bar 5 1 cm, 31.5. 30. Enlargement from Fig. 29 showing the vessels in the stem in oblique section (top). Arrowhead points to part of a tendril-like trichome in the matrix. Slide PLY03–11; bar 5 2 mm, 36.5. 31. A cross section of a narrow midrib of a Gigantopteris-type leaf has lamina remains (left upper) and radial rows of tracheids (possibly of secondary xylem) surrounding the tracheids of the triangle-shaped primary xylem in the center. Slide PLY02-C3-43-2; bar 5 200 mm, 370. 1572 AMERICAN JOURNAL in the metaxylem appears to be similar to the scalariform type in general structure. Another interesting and important structure is the compound hook. The hook tips are anatomically identical with the isolated, permineralized ones (Figs. 23, 24) associated with Vasovinea stems and Gigantopteris-type leaves. Also they are morphologically well matched with the compressed compound hooks (Figs. 21, 22) from the same locality. For example, the compressed compound hook shown in Fig. 21 has a 1.2 cm long basal axial part, a 1.4 cm long middle axial section, and five hook tips (the terminal tip is broken), with 2.6, 1.6 and 0.8 mm diameters, respectively. As with thorns and tendrils, the compound hooks could be modified from stems or leaves. We suggest that the compound hook of Vasovinea may be modified from a leaf, based on the following evidence: (1) it has a bilaterally symmetrical, crescent-shaped vascular trace (Fig. 3, ‘‘H’’), (2) the hook tips are arranged in the same plane, and (3) the branching pattern is similar to pinnate leaf venation. Affinity—Several lines of evidence show that Vasovinea tianii is a member of the gigantopterids. The most important evidence comes from the presence of a permineralized compound hook (Fig. 1), which is attached to the permineralized stem (Fig. 3, at right). This type of compound hook structure is unique to gigantopterids and has not been reported from other fossil or extant plants (see Menninger, 1970). Significantly, the structure of the compound hook is identical with those found from the early Late Permian Gigantopteris floras of Shanxi (5 Shansi in Halle, 1929) Province, northern China (Halle, 1929), and Fujian Province, southern China (Yao, 1983). Halle (1929) interpreted the hook-like structures as modified leaves of Gigantopteris nicotianaefolia. In 1995, one of us (Hongqi Li) borrowed and reexamined some of the Shanxi specimens from the Swedish Museum of Natural History. He confirmed that one of the gigantopterid leaves had thickened pinnate secondary veins that distally curved backwards to form hooks, but the basal part of each secondary vein still bore bilaterally a small amount of lamina. Thus, the Shanxi specimen clearly demonstrates a transitional type from a pinnately veined gigantopterid leaf to a compound hook with opposite hook tips. Our analysis of the permineralized compound hooks is in agreement. Additional features showing the gigantopterid affinity include the presence of sparganum cortex, prickles, trichomes, and the configuration of the vascular trace. A similar sparganum cortex is commonly known in many Paleozoic seed ferns, such as the Carboniferous lyginopterids, medullosans, and callistophytes. Some species, such as Heterangium kentuckyensis (Pigg, Taylor, and Stockey, 1987), Microspermopteris aphyllum (Pigg, Stockey, and Taylor, 1986), and Callistophyton boyssetii (Rothwell, 1975, 1981) even have similar prickle/trichome-like structures (called ‘‘cortical wings’’ in the first two references). However, among the plants reported from the Upper Permian flora of western Guizhou, a sparganum cortex and pickles/trichomes are only found in gigantopterids (see Tian et al., 1996). These structures are characteristic of the leaf midribs of Gigantonoclea guizhouensis (H. Li et al., 1994), the stems of Aculeovi- OF BOTANY [Vol. 86 nea yunguiensis (H. Li and Taylor, 1998), and Vasovinea tianii. The prickles were described as aculei (H. Li and Tian, 1990) and spines (H. Li et al., 1994) and have been redefined as prickles (H. Li and Taylor, 1998) because of their epidermal and cortical origin, and lack of vascular tissue. Although the prickles in V. tianii are fewer in number, smaller, and contain fewer thick-walled parenchyma cells, they are similar to prickles of G. guizhouensis and A. yunguiensis in structure and origin. Finally, a sparganum cortex also is found around the vascular bundles in Gigantopteris-type leaves (Fig. 25). The leaf traces (;750 mm in diameter; Figs. 2 [‘‘T’’]. Fig. 4 [upper]) in the stem are identical to the vasculature (;350 mm in diameter; Fig. 31, central part) in a section of a narrow, Gigantopteris-type leaf midrib, in terms of the radial rows of tracheids surrounding the triangleshaped primary xylem. The reduction in the diameter of the vasculatures is reasonable, considering they grow out from the stem cortex through to the midrib. Some thicker, secondary veins of a large-sized, Gigantopteris-type leaf (possibly a different species from that of Fig. 31), have a heart-shaped vasculature (Fig. 25), and the tracheids (each up to ;40 mm in diameter) are also arranged in radial rows. All these radially arranged tracheids are separated by parenchyma cells, so they appear to belong to secondary xylem. This distinguishes them from those Uor V-shaped vasculatures in Gigantonoclea guizhouensis leaf veins, which consist only of primary xylem. Therefore, these similar vasculatures suggest that Vasovinea and Gigantopteris-type leaves belong to the same kind plant. The biological relationship of Vasovinea tianii stems, Gigantopteris-type leaves, and the compound hooks also is supported by their intimate association. More than a dozen compression leaf species of three gigantopterid genera have been reported from the Permian flora of western Guizhou (Gu and Zhi, 1974; Tian and Zhang, 1980; Zhao et al., 1980). Except for the rare Linophyllum xuanweiensis, the other species belong to Gigantonoclea (with simple net veins) or Gigantopteris (with complex reticulate veins). These three genera all have a midrib (primary vein) and pinnate secondary veins. Based on our collections from the Guizhou Flora, we have recently recognized an actinodromous type of gigantopterid leaf that has either three or five primary veins (H. Li and Taylor, 1997a). Both Gigantopteris and the actinodromous leaves have complex reticulate venation, and both are so large that it is difficult to tell from which type a leaf fragment with complex netted veins comes (e.g., Figs. 22, 27). Therefore, we prefer to use the term Gigantopteris-type for all gigantopterids with complex reticulate venation. A gigantopterid leaf similar to Gigantonoclea guizhouensis has been reconstructed together with prickly Aculeovinea yunguiensis stems (H. Li and Taylor, 1998). Gigantopteris-type leaves are frequently associated with compressed stems that are similar to compressed stems of A. yunguiensis in having tiny ribs (Fig. 27), but with fewer black dots than the latter. The tiny ribs and the dots appear to be the remains of a sparganum cortex and the broken bases of trichomes or prickles of Vasovinea, respectively. Gigantopteris-type leaves also are frequently associated with the compressed hooks, e.g., the compound hook in Fig. 21. Another specimen has two hooks November 1999] LI AND TAYLOR—VESSEL-BEARING (Fig. 22, arrows) and is associated with a leaf that has complex reticulate venation and compound rounded teeth, similar to the leaf in Figs. 29–30. In permineralized materials, Gigantopteris-type leaves are frequently associated with Vasovinea stems as demonstrated in Figs. 29– 30. In one of our specimens (L9407), all of the wellpreserved leaves have complex reticulate venation, and all of six or more anatomically preserved stems have vessels. Although associational evidence strongly supports the inference that Gigantopteris-type leaves, compound hooks, and Vasovinea tianii stems could belong to the same plant species, whether the reticulate leaves are actinodromous or pinnately veined is still uncertain. We reconstructed the V. tianii stems with the actinodromous Gigantopteris-type leaves (Fig. 32) at this time, because in sample PLY04 the only leaf that is associated with the single V. tianii stem is of the actinodromous type. We also included the tendril-like trichomes, prickles, and compound hooks, since these structures are attached to the stems. We placed the hooks in a subopposite arrangement with the leaves because the location of the transverse section with the hook trace (Fig. 3) is ;5 mm below the transverse section with the possible leaf traces (Fig. 2). However, this reconstruction may need to be further refined in the future in terms of whether the Vasovinea stems may actually have actinodromous or pinnately veined leaves, or both. Systematic relationships—Although Vasovinea tianii is placed in the Gigantopteridales, it is still uncertain to which higher category the order belongs. Gigantopterids were first considered to be ferns (Schenk, 1883) and then seed ferns (White, 1912; Asama, 1959). The latter view is supported by X. Li and Yao (1983) who reported an impression specimen with both seed-bearing taeniopteroid and Gigantonoclea fukienensis leaves. However, whether the axes of those leaves were organically connected or just overlapped together appears unclear. Although there are no permineralized reproductive organs to clarify whether gigantopterids belong to seed ferns or other groups, the anatomy of Aculeovinea yunguiensis suggests that the gigantopterids were seed plants (H. Li and Taylor, 1998). The combination of characters in Vasovinea tianii, such as vessels, a eustele, sparganum cortex, and secondary xylem, separates it from the pteridophytes. These characteristics are typical of Paleozoic seed ferns, such as Heterangium, Microspermopteris, and Callistophyton (Taylor and Taylor, 1993; Pigg, Stockey, and Taylor, 1986; Pigg, Taylor, and Stockey, 1987), although none of these genera share the full suite of characters of Vasovinea. In particular, none of them have reticulate-veined leaves and vessel-bearing stems. Although vessels also are found in some extant ferns and fern allies, such as some ferns (Jeffrey, 1917; Bliss, 1939; White, 1961; Carlquist and Schneider, 1997; Schneider and Carlquist, 1998), Selaginella (Duerden, 1934), and Equisetum (Bierhorst, 1958), these vessels commonly have only scalariform perforations and none of these plants have both a eustele and secondary xylem. In extant seed plants only gnetophytes and angiosperms typically have vessels, and their vessels are of the foraminate and scalariform/simple types, respectively GIGANTOPTERID VASOVINEA FROM CHINA 1573 (see above). There is considerable controversy over whether the two types of vessels represent two different evolutionary lineages or had a single origin in gnetophytes and angiosperms (Bailey, 1944; Young, 1981; Muhammad and Sattler, 1982; Carlquist, 1992, 1994, 1996a). However, the foraminate-like vessels of Vasovinea tianii differ from both those of gnetophytes and angiosperms. Although superficially similar to those of the gnetophytes, the perforation plate of Vasovinea is planar, has many more pores (which are borderless and can be transversely elongated), and has pit membranes (when they exist) without tori or margo threads. This type of foraminate-like perforation plate is not known from angiosperms, although the scalariform bordered pits and the possible vessels in the metaxylem of Vasovinea resemble those of angiosperms. Therefore, although these vessels are very important in tracing their origin, we cannot draw a comprehensive conclusion of the systematic relationships of the gigantopterids based on vessel features alone. Preliminarily results, based on phylogenetic analyses of a broad suite of characters (H. Li and Taylor, 1997b) and molecular fossil data (Taylor et al., 1998), suggest that gigantopterids are embedded well within the seed plants and may be another member of the anthophyte clade. Paleoecology—Halle (1929) reported some hook-bearing gigantopterids from central Shanxi and suggested that the gigantopterids grew as lianas in a tropical habitat. Yao (1983) reported additional hooks and also considered the gigantopterids as lianas growing in a lowland tropical forest. Geographically, Guizhou has been suggested to have been in a tropical forest biome during the Late Permian (Lin, Fuller, and Zhang, 1985; Tian et al., 1990; Nie, Rowley, and Ziegler, 1990; Isozaki, 1997). Our material strongly supports these paleoecological predictions and provides some additional ecophysiological interpretations. The morphology, anatomy, and reconstruction of Vasovinea tianii support the interpretation that it grew as a vine or liana. In extant lianas, hooks, thorns, tendrils, and spines/prickles function as attachment organs so the slender plants can climb. In particular, hooks only have been found in vines, where they are considered very efficient climbing organs (Menninger, 1970), and define a group called the hook climbers (Putz and Holbrook, 1991). The tendril-like trichomes and prickles of V. tianii might also have provided an additional climbing mechanism. Anatomical features such as a segmented xylem and the distribution and structure of its vessels also support the inference that Vasovinea tianii was a liana. Just like many living lianas, V. tianii contained xylem segments dispersed among parenchyma cells. The inflexible xylem segments within soft parenchyma tissues function like ‘‘multistranded cables’’ so that the liana stems could have withstood considerable deformation while maintaining the conductive function (see Carlquist, 1991; Putz and Holbrook, 1991). Vasovinea tianii stems have large vessels abruptly occurring at the inner portion of the secondary xylem and small vessels in the outer portion (and possibly in the metaxylem), an arrangement resembling that found in certain extant lianas (Carlquist, 1991). The pores of Vasovinea have completely lost their borders and are elongate (8 3 10 to ; 13 3 15 mm; Figs. 7, 11, 18– 1574 AMERICAN JOURNAL 19) and larger than bordered pits on lateral walls (Figs. 11 [lower right], 20). In extant lianas, vessel end walls tend to be more thoroughly perforated than those of tree and shrubby genera in the same family. The vines may have simple perforations with reduced borders, rather than scalariform perforations, or have large vessels with simple perforations and small vessels with scalariform perforations, instead of all scalariform vessels, or may have scalariform perforations with fewer bars than in trees (Carlquist, 1991). Presumably, to overcome the large resistance to water flow in tracheary elements and to complement the rapid water loss from the relatively large leaves, the plants need an extremely efficient waterconducting tissue in their stems (see Ewers, Fisher, and Chiu, 1989). Other vessel parameters can also be correlated to habit. Lianas typically have larger vessel diameters (from an average of 157 mm to 558 mm in diameter; Carlquist, 1975) than closely related woody trees in both Gnetum and dicots (Ewers, 1985; Ewers and Fisher, 1989; Ewers, Fisher, and Chiu, 1990; Fisher and Ewers, 1995). Bamber and Welle (1994) reported that many liana species from the Queensland rain forest have vessels 14–57% larger than tree species of the same genera, and the largest vessel was 610 mm in diameter. In living lianas, the widest vessel members tend to be the longest (Ewers and Fisher, 1989; Bamber and Welle, 1994). Similarly, the vessel members of Vasovinea, up to 500 mm in diameter and 4500–5000 mm in length, are in the range of the largest and longest among all plants, both living and extinct. The reconstruction of the thin Vasovinea stems with the large Gigantopteris-type leaves also resembles many extant lianas that usually possess high leaf-area to stemdiameter ratios (Carlquist, 1975, 1991). Many cordate, actinodromous gigantopterid leaves are larger than 400 cm2, with some observed in the field up to 1600 cm2, but these large leaves are associated with slender stems, commonly no more than 1 cm in diameter. It is also possible to predict the placement of Vasovinea within the forest structure. Extant lianas and vines with large, cordate leaves and long petioles usually grow in sunny areas, while those with small, narrow leaves and short petioles normally grow in more shady environments (Givnish and Vermeij, 1976). Some Gigantopteris-type leaves have been found with well-differentiated palisade and spongy cells (Fig. 25, left) and guard cells sunken in the stomata (Guo, Tian, and Chang, 1993). The mesophyll differentiation is related to plant species and to habitat with increased palisade differentiation related to increased exposure to light (Esau, 1965). This is in contrast to the undifferentiated mesophyll as in Gigantonoclea guizhouensis (H. Li et al., 1994). We suggest that both Gigantonoclea and Gigantopteris lived in a similar habitat based on the co-occurrence in the sediment, but that the Gigantopteris-Vasovinea plant grew in the sunny canopy, while the Gigantonoclea-Aculeovinea plant grew in the shady understory. In summary, Vasovinea tianii is mainly characterized by its compound hooks, sparganum cortex, eustelic primary vascular architecture, foraminate-like vessels in the secondary xylem, and possible vessel elements with scalariform-reticulate structures and tracheids with scalari- OF BOTANY [Vol. 86 form or reticulate bordered pits in the metaxylem. These morphological features suggest that V. tianii represents a unique seed plant taxon, but its phylogenetic relationships to seed ferns, gnetophytes, and angiosperms remain to be further analyzed. 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