GINKGO TREES ARE LIVING CORDAITES

1. ID 157670 GINKGO TREES ARE LIVING CORDAITES Donald Chesnut, Retired, University of Kentucky, Lexington, KY 40506-0107 drches01@uky.edu Ginkgo biloba Linnaeus 1771 is the only extant member of the once flourishing Mesozoic and Tertiary Ginkgophyta group of gymnosperm plants. Ginkgo trees, now abundant in temperate cities around the world, were thought to exist in nature only in a small refugium in China. Their dispersal to the rest of the world by botanists, diplomats and other interested parties began about 300 years ago. Their natural refugium may no longer exist. During the Mesozoic and Tertiary the ginkgophytes were cosmopolitan. Early members of the group can be traced to the Early Permian. The phylogenetic origin of the enigmatic Ginkgo has puzzled botanists for the last 150 years. Similarities in wood, reproductive structures and leaf venation between ginkgophytes and Cordaitales (as well as with cycads) has been noted for many years. The cosmopolitan Cordaitales existed in the Carboniferous and Permian periods. They were important constituents of the coal floras of the Carboniferous, but became extinct before the Mesozoic began. A. Trees Leaf venation of both groups is always parallel, branch dichotomously and never anastomose. Willem Meijer (personal commun., ca 2000) pointed out that cross sections of the leaves of Ginkgo were identical to those of Cordaites from Carboniferous coal balls. B. Leaves Some types of Cordaites had mangrove-like prop roots. Adventitious roots have also been noted for Ginkgo. Ginkgo Cordaites-like Ginkgophyte leaves Cordaites Ginkgo Cordaites For simplicity, all parts of the “Cordaites” tree are called Cordaites here. E. Reproduction C. Branching D. Wood In both cordaites and Ginkgo, the leaves emerge in a helical pattern. In both cordaites and Ginkgo, the bulk of the wood is secondary wood typical of the gymnosperms. . In both cordaites and Ginkgo, the male and female structures are born on separate cones. Further, in Ginkgo, the male and female cones are on separate trees (i.e., Ginkgo is dioecious). We do not know if cordaite cones were on separate trees. A, B. Sphenobaiera, Permian-Jurassic. C, D. Arctobaiera. from Stewart, 1983. Baiera, Jurassic, from Stewart, 1983. Cordaites principalis, from Stewart, 1983. A. Eretmophyllum, Jurassic. B. Sphenob-aiera, Jurassic, from Taylor 1981. Ginkgo biloba, deciduous leaves in winter. Reconstructions of Cordaites. A. From Scott, 1909. B. From Cridland, 1964. Note mangrove-like prop roots. Ginkgo at Henry Clay’s estate, Lexington, Kentucky: circum-ference 3.8 meters at one meter above ground. Note person Cordaites Ginkgo Ginkgo Cordaites Pollen-bearing strobili Paired ovules Branch with numerous spur shoots. Shoots about 1 cm diameter. . Spur shoot Spur shoot Old spur shoot B. Ovules. C. Pollen-bearing organs. Paired ovules. Pollen-bearing organs. Old spur shoot Both from General Bio. Supply, in Stewart, 1983. Two new spur shoots on old spur shoot. New branch on old spur shoot. Helical leaf scars on spur shoots. About 1 cm dia. Helically-arranged leaves and cones, from Stewart, 1983. Conclusions Ginkgophytes probably evolved from early cordaites in Late Devonian or Carboniferous times. Ginkgo is here considered to be the only extant member of the Cordaitales much as birds are considered to be the modern representatives of the Dinosauria. Long, strap-like leaves, typical in cordaites, are common in tropical, humid environments, whereas small, compact leaves, as in the modern Ginkgo, are common adaptations to drier, upland environments. Differences between ginkgophytes and cordaites may be caused by adaptations to dry-land environments. Cordaites Ginkgo Cordaites Ginkgo A Acknowledgements I am indebted to Dr. Cortland Eble, Kentucky Geological Survey, for his help in discussions about paleobotany and especially with palynology. Inspiration for my conclusions derives from discussions with Dr. Willem Meijer (deceased), University of Kentucky. The onus of any errors in content, grammar, as well as unfounded conclusions is entirely mine. The fleshy “fruit” of the Ginkgo is actually a seed-coat layer and not a fleshy ovary. Some cordaites may have had fleshy seed coats as well. Some of the wings may have been compressions of this seed coat. Pith Section of Ginkgo branch. Showing growth rings in secondary wood. Diameter 4.5 cm. Seed Dessicated “fruit” B Thin section of Ginkgo branch. From Catholic University of Leuven, Belgium. Cordaites seeds. From Newberry, 1873. Ginkgophyte pollen C References Phillips, T.S., and Chesnut, D.R., 1980, Coal balls in lower Middle Pennsylvanian strata of eastern Kentucky: Occurrences and peat composition [abs]: Geological Society of America, Annual Meeting, Atlanta, Abstracts with Programs, November 17-20, 1980, v. 11, n. 7, p. 498 Rendle, Alfred. B., 1904, The Classification of Flowering Plants. University Press, Cambridge University, 403 pp. [Although about flowering plants, this book has an excellent description of the ginkophytes, cycadophytes and related plants.] Stewart, Wilson N., 1983, Paleobotany and the Evolution of Plants. Cambridge University Press, 405 pp. Taylor, Thomas N., 1981, Paleobotany, An Introduction to Fossil Plant Biology. McGraw-Hill Book Co., 589 pp. Tschudy, R.H., and Scott, R.A., 1969, Aspects of Palynology. John Wiley and Sons, 510 pp. Pith casts (Artisia) of Cordaites. Cordaite pre-pollen A, B, C. Coal-ball peels from Pennsylvanian of Kentucky. C. Growth rings in cordaites wood. All from Phillips and Chesnut, 1980. Coal-ball peel from Pennsylvanian rocks. P, pith. From Rothwell in Stewart, 1983. Pith Cycadopites, Permian,from Hart in Tschudy and Scott, 1969, p. 278. Cycadopites, Triassic,from Chaloner in Tschudy and Scott, 1969, p. 303. Ginkgo biloba pollen from Tschudy in Tschudy and Scott, 1969, p. 11. Cordaitina, Permian,from Hart in Tschudy and Scott, 1969, p. 276. Florinites, a mono-saccate pollen. From Stewart, 1983. Ginkgo and cordaite pollen are monosaccate (one bag). Ginkgophyte pollen is further characterized as monosulcate (it has a single furrow). Tschudy (his Table 2-2, p. 15) placed Cordaitales pre-pollen in the monosulcate category too (I’m not sure why). Cycadopites probably represent ginkgophytes, cycadophytes and related groups. Florinites, Carboniferous,from Kosanke in Tschudy and Scott, 1969, p. 248