I’ve always found the leaves of Ginkgo trees (Ginkgo biloba) very appealing with their delicate fan shape and unusual veination and knew a little about the tree’s status as the only extant genus of a once-widespread group of plants, so was intrigued to be given a book entitled simply ‘Ginkgo’ for my birthday by my good friend Sue. Peter Crane’s book turns out to be a fascinating read, covering the tree’s lengthy pedigree and unique features but also the way its symbolic significance for humans over recent millennia has ensured its dispersal throughout much of the world.
Gingkos are amongst the earliest seed bearing plants, sitting somewhere between ferns and conifers in evolutionary terms. Whilst the very earliest seed plants emerged in the Late Devonian, around 360 Million years ago, it was probably a second wave of seed plant evolution starting in the Permian, around 300 Ma, which gave rise to Ginkgos via seed ferns such as Glossopteris (see What has Scott of the Antarctic got to do with the theory of plate tectonics?). Ginkgo leaves have a unique structure amongst tree leaves which makes them relatively easy to identify in the fossil record. Instead of the network of splitting and merging veins (reticulation) which characterises most leaves, Ginkgo leaf veins radiate out from two primary veins in the leaf stalk or petiole. Each primary vein supplies one half of the leaf.
Reticulate veins on an apple leaf and the parallel veins of Ginkgo
By the Early to Middle Jurassic, 190-170 Ma, ginkgo-like plants seem to have been both abundant and diverse. One of the reasons for the excellent fossil record may be Ginkgo’s preference for a waterside habitat which meant that seeds, leaves etc. often ended up falling into mud and sediments where they could be preserved. Another is the incredibly tough nature of the leaves, with a thick waxy cuticle and resin deposits between the leaf veins. This recalcitrance poses a real problem for the authorities in places like Japan, where enormous numbers of Ginkgos adorn city streets.
Many beautiful fossil leaves have been found, including one from Afghanistan named Ginkgo cordilobata by Schweizer & Kircher (1995). Similar species have been found in North America, from the Arctic to Mexico, in the UK, South Africa and in many parts of China. The leaves may differ a little in appearance; longer and thinner, shorter and wider, more deeply or more shallowly lobed, but all share the characteristic veination which marks them out as Ginkgo relatives. It can be difficult to be sure how much of the apparent variation between leaf samples is intra-specific and how much really represents different species. Sometimes the leaves are associated with seed-bearing structures and pollen catkins but it is very rare to find all the bits of a plant together, in order to compare it properly with our extant Ginkgo. It’s quite likely that fossil leaves and other plant parts, such as catkins or seeds, are given different names when they are actually from the same species – a common problem for paleobotanists.
Both the abundance and number of distinctive types of ginkgo-like leaves in the fossil record declines again towards the end of the Cretaceous, 100 Ma. By this time Ginkgos would have been competing with increasingly-successful flowering plants, as well as coping with climatic changes brought about by accelerated continental drift. By 65 Ma the only survivors looked pretty much like Ginkgo biloba – our only extant species today – and were restricted to some areas of present day China. We can only speculate why this species survived and thrived whilst all its ancient relatives disappeared.
So what’s so special about Gingkos? Many things, as you might expect, for a tree that has survived in a form we recognise today for more than 250 million years. It is not just the veination of the leaves that is unusual, but also the shape of the tree itself. Those we saw growing in Kashmir in July looked almost like conifers from a distance.
Ginkgo growing in Shalimar Bagh Mughul garden, Kashmir
Crane’s book tells me that this is because of the tree’s unusual growth form. Most new growth is in the form of long shoots, with widely-spaced leaves, which grow up to 25 cm a year from their terminal bud. However, these long shoots have many short side-shoots growing from them, which don’t really grow in length but simply grow tufts of new leaves each season which fill in the canopy. The tree is able to produce plenty of new leaves each year without the need for lots of new woody tissue, so it’s an energy efficient strategy.
Leaf arrangements on long shoots and short side shoots of Ginkgo – the colour difference is incidental
I was also puzzled by the fact that many of the leaves didn’t seem to be very deeply lobed for a tree with the specific name ‘biloba’. I learned from Crane’s book that it is usually only the leaves on long shoots which are lobed – those on the short shoots have intact margins because they are well protected and develop slowly inside tightly a closed bud over the winter.
Ginkgo trees are dioecious; there are separate male and female trees and two trees of the opposite sex are needed for seeds to be produced. Nearly all the Ginkgo trees planted in England happen to be males, though I’m not sure why this should be the case. Ginkgo cells don’t carry obviously different sex-determining chromosomes, as we do, and it is not entirely clear at what point sex is fixed – there may be at least some environmental component involved. Interestingly, spontaneous partial sexual switches have been recorded on some old male ginkgo trees. These trees (known as ‘leaky’ males) produce just a few ovules as well as pollen and may be acting as a kind of failsafe to unsure some seeds are produced, rather as some flowering plants will eventually self-pollinate if no pollen is available from other individuals.
Male Ginkgo trees produce their pollen in cones on the short side shoots at the same time as the new leaves appear.
Ginkgo biloba male flower. Image by Ginkgob – Licensed under Creative Commons
The pollen sacs within split open over a period of just a few days as the cones elongate, and release enormous quantities of pollen into the air – as many as 59 million grains from every one of the 17 000 or so short shoots on an average tree! The pollen is carried on the wind – a chancy business at the best of times. If pollination is to be successful, synchronisation of its release with the ovules produced on nearby female trees is critical.
Ovules on the female trees are carried in a structure unique to Ginkgos – a stalk with two young ovules at the tip (though some earlier relatives may have had up to six ovules on each stalk).
Ginkgo biloba female flower – two ovules are on stalks. Image by Ginkgob – Licensed under Creative Commons
A sticky pollination drop at the tip of each ovule helps trap incoming pollen grains and produces a sealed cavity inside which these germinate. The droplet is absorbed at the end of each day and restored in time for the next, until one ovule is pollinated. When the pollen germinates produces a pollen tube which enters the ovule and produces a network of fungal-like filaments to supply it with nutrients. As in mosses and ferns, swimming sperm released from the pollen tube carry out fertilisation, harking back to a time in evolution when plant reproduction depended on water. Intriguingly, Marie Stopes started life as a paleobotanist and was one of the earliest people to study these Ginkgo sperm!
Though pollen transfer takes place in spring, the fusion of egg and sperm is delayed until late summer/early autumn and the seeds are shed in autumn, ready to germinate the following spring. Only one seed matures on each stalk. There is evidence from the Almont fossils found in N. Dakota, USA, that the same was true in the ginkgo type plants which grew there 57 million years ago. The mature seeds look like small plums and are a delicacy in China and Korea – something of an acquired taste though, given that they smell of butyric acid (aka vomit!)
The retreat of Ginkgos to a few parts of East and South China during the last glaciation, followed by their dispersal by humans over the last 50 000 years, is an interesting story for another time. Perhaps after I’ve visited Scalby Ness, just north of Scarborough, to see if I can find some fossil Ginkgo leaves for myself…
I was delighted to wander into Bute Park in Cardiff at the weekend and stumble across a whole avenue of my current favourite trees.
Crane P.R. (2013) Ginkgo. The tree that time forgot. Yale University Press.
Schweizer H.J. & Kircher M. (1995) Die Rhäto-Jurassischen Floren des Iran und Afghanistan: 8. Ginkgophyta. Palaeontographica Abt. B, 237, 1-58.