Maidenhair tree or Ginkgo biloba is the only living
species representing Ginkgophyta. Representatives
of the order Ginkgoales date back to the Permian, but the
genus Ginkgo makes its first appearance in the
Jurassic. In fact, Ginkgophytes reached
their greatest diversity during the Jurassic. Fossil evidence
indicates that at least 16
genera of ginkgophytes made up a significant part of the
Mesozoic vegetation (Willis & McElwain, 2002, p. 139). Ginkgos
declined in the Paleogene and Neogene, becoming nearly extinct
(Tidwell, 1998, p. 102).
From Ancient Diversity to a Single Species
biloba is often referred to as a "living fossil". Ginkgo leaves
are common fossils in Jurassic and Cretaceous sediments
of the Northern Hemisphere. The Ginkgo family (Ginkgoaceae)
was thought to be extinct in the wild. In 1956 a small population
of Ginkgo biloba was discovered in southeast China.
The Ginkgo tree has been cultivated in the sacred
gardens of China for centuries. Ginkgo has been
planted worldwide and is valued for its beauty and medicinal
& Davis, 2004, p. 148). Although Ginkgo biloba is
referred to as a "living fossil", the fossil
leaves of Ginkgo species show great variation in
leaf morphology (Willis &
p. 139). The fossil wood and reproductive structures are
rare as fossils. Ginkgo wood is prone to early decay.
general, Ginkgo leaves are fan-shaped and may be
deeply lobed (notched) to unlobed (entire). The veins appear
to be parallel, but actually exhibit open dichotomous venation
(each vein divides).
Ginkgo biloba is a deciduous tree. The large numbers
of Ginkgo leaves
in the fossil record suggests that the ancestors of Ginkgo
biloba were also deciduous.
Ginkgo biloba is
dioecious (separate male and female trees). Male trees have
stalked pollen sacs that are clustered together at the base
Female trees have pairs of unprotected ovules at the end
of long stalks attached to the base of leaves.
Ginkgo biloba is wind pollinated. Ovules are fertilized
by free-swimming gametes. This fertilization process occurs
on the tree. Ginkgo biloba and cycads are the only living
seed plants that produce flagellated male gametes (Taylor,
Taylor, and Krings, 2009, p. 744). One ovule gets pollinated
and a fleshy false fruit forms over
the seed. Butyric acid is produced by the false fruits after
they drop to the ground producing a nasty smell. Landscape
architects prefer to plant male Ginkgo trees
for this reason.
Ginkgo trees have a constellation of characteristics,
making their origin difficult to discern. The
Ginkgo tree grows to 30 m. The vegetative structures
of Ginkgo would remind one of a conifer or cordaite.
The stem is a eustele with large amounts of wood (secondary
xylem). Ginkgo wood is very conifer-like; however,
in cross-section the tracheids are highly variable in size,
disrupts the radial arrangement (see micrograph below).
has large parenchyma cells scattered among the tracheids. Conifer
tracheids in cross-section are of equal size, except for
and exhibit an orderly radial arrangement. The leaves remind
one of a deciduous flowering plant. The reproductive structures
of the Ginkgo are more like that of a cycad.
A Change in the Flora
diversification of Ginkgophytes during the Mesozoic
helps to mark a significant change in the world's flora.
Paleozoic flora was dominated by ferns and
clubmosses (Paleophytic flora). The Paleophytic flora
gave way to a Mesophytic flora during the Triassic
period. Woody seed-bearing plants and their relatives
dominated Mesophytic flora. Thus, the change from Paleophytic
to Mesophytic represented a change in reproductive
strategy; from spore producers to seed producers. Conifers,
cycads, and ginkgos diversified during this time and
dominated the landscape (Kenrick & Davis,
2004, p. 143).
Science Olympiad Fossil Event
The 2016 Science Olympiad Fossil List includes the
genus Ginkgo within the phylum Ginkgophyta.
Gymnosperms ("naked-seeds") include plants
that usually bear their seeds in cone-like structures
to the angiosperms (flowering plants) that have seeds
enclosed in an ovary. Gymnosperms include the following
Pinophyta, Ginkgophyta, Cycadophyta, and Gnetopyta.
P. and Davis, P. (2004). Fossil Plants. Smithsonian
T.N., Taylor E.L. & Krings, M. (2009). Paleobotany:
The Biology and Evolution of Fossil Plants [2nd
Ed]. New York: Academic Press.
W.D. (1998). Common Fossil Plants of Western North
America. [2nd Ed]. Washington: Smithsonian Institution
K.J. & McElwain, J.C. (2002). The Evolution
of Plants. New York: Oxford University Press.