The families of gymnosperms
Ginkgo, Maidenhair Tree.
Vegetative. Deciduous; trees (to 30m high). Not resinous (secreting mucilage). Main branches spiral to whorled (irregularly whorled or produced at indefinite intervals). The leafy branchlets not flattened. The vegetative branch systems including highly specialised, leaf-bearing short-shoots (cf. those of Larix and Cedrus, producing leaf clusters annually for many years and sometimes subsequently becoming transformed into long-shoots bearing scattered leaves). Mature leaves more or less dimorphic (being more or less bilobed on long-shoots and entire on short-shoots); broad and flat (long-petiolate, the fan-shaped lamina with numerous, regularly dichotomizing veins); alternate. Longitudinal resin canals absent from the leaves.
Reproductive organization. Dioecious (via an XX female/XY male genetic system). The ovules not in cones (the more or less sessile female flowers each comprising a naked ovule with a basal, collar-like rim theoretically representing two megasporophylls; paired or in twos or threes terminating long, axillary peduncles on the short-shoots); terminating axes; orthotropus; 1 integumented.
Male trees produce elongated, catkin-like strobili in the axils of scales or leaves on short-shoots. These bear the numerous stamen-like microsporangiophores, each with 2 pendulous microsporangia which dehisce via a longitudinal slit. Pollen-sacs 2 per microsporophyll (pendulous male catkins borne 3–6 in the axils of scales or leaves on the short shoots, with microsporophylls each represented by a bractless, axillary main axis bearing laterally towards its apex a pair of pendulous microsporangia dehiscing via longitudinal slits). Pollen without air bladders. Fertilization involving spirally flagellate, motile spermatozoids.
Seeds and seedlings. Seeds wingless, with a fleshy investment; the fleshy investment developed from the integument (via its fleshy outer layer, which gives off a nauseating odour of rancid butter, and encloses a stony middle layer, which in turn encloses a watery inner layer surrounding the female prothallus); wingless. Cotyledons usually 2 (unlike the foliage leaves in exhibiting mesarch vascular strands).
Wood anatomy. Growth rings distinct. Latewood not conspicuous. Wood without distinct odour. Tracheids with opposite and multiseriate bordered pits; without callitroid pit-border thickenings. Margins of the tori not scalloped. Axial parenchyma present. Axial parenchyma zonate. Axial parenchyma without nodular thickenings on the transverse or end-walls. Rays exclusively uniseriate. Ray tissue without crystals. Earlywood cross-field pits cupressoid (or araucarioid). Normal vertical resin ducts absent (but mucilage canals occur throughout the plant).
Geography, cytology. Temperate; the single species is possibly native to China, but is now unknown in the wild; long cultivated in China and Japan for its edible seeds and its supposed medicinal properties.
Basic chomosome number, n = 12.
Taxonomy. 1 species (Ginkgo biloba); Ginkgo. Order Ginkgoales.
Comments. Leaves without resin canals, and differing conspicuously from Coniferales in that fertilization is effected by flagellate, motile spermatozoids - cf. cycads, ferns and other vascular cryptogams. Obvious relatives of G. biloba occur in fossil deposits up to 200 million years old. Now widely grown around the world from material originating from China, and exploited as "alternative medicine".
Miscellaneous. • Ginkgo: technical details (Sporne). • Ginkgo biloba (Dallimore and Jackson). • Jurassic ginkgophyte fossil leaves. • Wood anatomy: Ginkgo biloba.
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Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2008 onwards. The families of gymnosperms. Version: 9th April 2015. delta-intkey.com’.