GINKGO TREES ARE LIVING CORDAITES

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. 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. 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.

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.