PLANT OF THE MONTH
November, 2008
Plant of the Month Feature
by Mike Kearney
Arboretum Roots Board Member
Ginkgo biloba
Ginkgo
When I was a student at Washington University in St. Louis, I had to walk by some trees that had a particular odor when they dropped their fruit. As a result, many people on campus advocated that the trees be cut down. Barry Commoner and others in the Botany department (who many thought were a bit weird) argued that they were very special trees and should be left alone. Their point of view ultimately prevailed. As a result I learned for the first time that trees could be male or female. This led to further curiosity and an ongoing interest in the tree described below.
Ginkgo (Ginkgo biloba; frequently misspelled as "Gingko", is a unique species of tree with no close living relatives. The ginkgo is classified in its own division, the Ginkgophyta, comprising the single class Ginkgoopsida;, order Ginkgoales;, family Ginkgoaceae, genus Ginkgo and is the only extant species within this group.
Ginkgos are very large trees, normally reaching a height of 20-35 m (66-115 feet), with some specimens in China being over 50 m (164 feet). The tree has an angular crown and long, somewhat erratic branches, and is usually deep rooted and resistant to wind and snow damage. Young trees are often tall and slender, and sparsely branched; the crown becomes broader as the tree ages. During autumn, the leaves turn a bright yellow, then fall, sometimes within a short space of time (1-15 days). A combination of resistance to disease, insect-resistant wood and the ability to form aerial roots and sprouts makes ginkgos very long-lived, with some specimens claimed to be more than 2,500 years old: A 3,000 year-old ginkgo has been reported in Shandong province in China.
The leaves are unique among seed plants, being fan-shaped with veins radiating out into the leaf blade. Two veins enter the leaf blade at the base and fork repeatedly in two; this is known as dichotomous venation. The leaves are usually 5-10 cm (2-4 inches), but sometimes up to 15 cm (6 inches) long.
Reproduction
Ginkgos are dioecious, with separate sexes, some trees being female and others being male. Male plants produce small pollen cones with sporophylls each bearing two microsporangia spirally arranged around a central axis. Female plants do not produce cones. Two ovules are formed at the end of a stalk, and after pollination, one or both develop into seeds. The seed is 1.5-2 cm long. Its fleshy outer layer (the sarcotesta) is light yellow-brown, soft, and fruit-like. It is attractive in appearance, but contains butanoic acid and smells like rancid butter (which contains the same chemical) or feces when fallen.
Cultivation and uses
Ginkgos adapt well to the urban environment, tolerating pollution and confined soil spaces. They rarely suffer disease problems, even in urban conditions, and are attacked by few insects. For this reason, and for their general beauty, ginkgos are excellent urban and shade trees, and are widely planted along many streets. The ginkgo is the official tree of the city of Kumamoto, and two leaves form the symbol of the University of Tokyo, the main campus of which is famous for its numerous ginkgos.
About the Author: Mike Kearney is a member of the Bickelhaupt Arboretum Roots Advisory Board and also serves as a Clinton City Councilman.