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Ginkgo still on the go

2013; Elsevier BV; Volume: 23; Issue: 23 Linguagem: Inglês

10.1016/j.cub.2013.10.020

1879-0445

John A. Raven,

Ginkgo biloba and Cashew Applications

Ginkgo biloba, with its characteristic fan-shaped leaves, is a commonly planted street tree in cities in the temperate zone, and has religious significance in the Far East, so it is familiar to many people. Botanically, Ginkgo is a gymnosperm — a seed plant with naked seeds — but is not clearly a close relative of any other extant gymnosperm. As a genus, Ginkgo is a true living fossil — a fossil species (Ginkgo cordilobata) was found in rocks of Early Jurassic age, about 190 million years ago. After the Jurassic, Ginkgo and its relatives diversified, and 100 million years ago (Cretaceous period), Ginkgo and its relatives, as well as Ginkgo-like foliage, occurred in many temperate habitats worldwide. Since the Cretaceous, there has been a loss of diversity and a contraction of range of the Ginkgo alliance — today there is just one species, Ginkgo biloba, native to China. The extent to which human protection has ensured the survival of Ginkgo biloba until the present is not clear. My first day-to-day contact with Ginkgo biloba was as an undergraduate where a female and a male Ginkgo tree were trained up the back (south) wall of the Botany School, the home of what was then the University of Cambridge’s Botany Department. As a postdoc there, I performed my only minimal experiment on Ginkgo, published in 1972. I developed an interest in, and an affection for, Ginkgo, but am hardly an expert. However, I eagerly agreed to review Peter Crane’s book Ginkgo: The Tree that Time Forgot, despite my wondering about an entire book devoted to one tree species out of the quarter million or more species of seed plants, of which about 100,000 are trees. Peter Crane’s scholarly and entertaining book triumphantly justifies this investment of cellulose, presumably from trees other than Ginkgo, in its seven sections and 37 chapters. Crane assumes relatively little botanical background in his readers, and after a three-chapter prologue he discusses Ginkgo in the context of the physiology and growth of trees in general, emphasising the particular features of Ginkgo. Importantly, he neatly demolishes the idea that Ginkgo, whose water-conducting tissue has relatively frequent cross walls, does not make the conducting system inferior to that of most flowering plants with much less frequent cross walls. Growth is of no evolutionary use unless it results in multiplication of the number of organisms, and Crane devotes three chapters to reproduction. He details the discovery of Ginkgo’s motile male gametes, and emphasises the occurrence of separate male and female plants, and why older male plants sometimes produce functional female structures, but not vice versa for female trees. Crane also points out that pollination and fertilization do not commonly occur after the ovules have dropped from the tree, contrary to what I recall from my undergraduate lectures 53 years ago. The next two parts of the book deal with the origins, subsequent evolution and present state of the Ginkgo group. Peter Crane is, among other things, a paleobotanist and an evolutionary biologist, and he is clearly in his element here as he ranges from the enigmatic origin of the Ginkgo alliance at least 220, and probably 245, million years ago, through its evolutionary radiation to become an important tree in appropriate habitats in temperate regions worldwide, to its subsequent decline in diversity and geographical range resulting in the single surviving species, Ginkgo biloba. What caused this decline in the range and diversity of the Ginkgo alliance? Crane mentions the possibility that it was loss from the area inhabited by Ginkgo of the mammals that disperse the seeds that underlies the contraction of range of Ginkgo and its relatives. More generally, Crane discusses the role of chance in the extinction of species, rather than a lack of competitive ability. Ginkgo survived the K–T boundary event that marked the extinction of the dinosaurs, and, less widely mentioned in television documentaries, the cycadeoids, a clade of vegetatively cycad-like gymnosperms. The next section addresses the history of the interactions between humans and Ginkgo; taking a suitably critical approach, Crane places the earliest reliable written mention of Ginkgo in China, the country in which the relict natural population occurred, at just over a thousand years ago (980 CE). This earliest historical record is probably less than the age of the oldest Chinese Ginkgo alive today, although there are problems with ageing the old giant trees. Crane also considers the role that humans could have had in preventing the extinction of Ginkgo, and the transfer of the tree to Korea and to Japan where it was no longer native. Ginkgo has long been planted at religious sites in eastern Asia — Crane mentions that protection of a large old Ginkgo tree at a shrine in Korea extends to erecting a lightning conductor beside it to limit the possibility of damage from lightning strikes. The naming of Ginkgo is, as Crane points out, a complex issue, as are so many aspects of the plant, not least because it is not easy to determine the plant to which a given name in old documents refers. Regardless of the origin of its western name, Ginkgo is known to have been cultivated in the western world since the 18th century. The book then turns to the non-religious uses of Ginkgo in gardens and as street trees, as well as the use of the nuts in human nutrition and of various parts of the plant in pharmacy. As with so many plant products, there is still debate on the efficacy of Ginkgo leaves in human health. Crane ends by considering the possible future of Ginkgo, in the context of more general considerations of conservation. Crane considers the conifer Wollemia nobilis, discovered in 1994 in the Blue Mountains of NSW, Australia. The native population of only just over a 100 trees means that it is very prone to ‘bad luck’, such as the extensive bush fires of October 2013 in the Blue Mountains. Speedy action by the authorities in NSW secured the native sites, and through vegetative propagation there are now tens of thousands of Wollemia growing in many parts of the world. Crane also cites cases in which rare plants remain at great risk. Work on Ginkgo biloba and its fossil ancestors has, of course, continued since Peter Crane’s book was published in early 2013. An example of a recent study concerns a hanging fly (Mecoptera), whose wings appear (to humans, at least) very similar to the deeply divided leaves of a ginkgoacean plant from the same strata in the Middle Jurassic. The book is clearly and engagingly written, with several points exemplified from Peter Crane’s own experience. It has an excellent index. The text is very well referenced in the numbered footnotes which are combined at the end of text, and its half-tone illustrations are much to my taste. I thoroughly recommend this scholarly and entertaining book to anyone who is interested in not just Ginkgo, but more widely in plants, fossils (living and otherwise), and how humans interact with plants.