Ginkgo: The Tree That Produced Eggs

It's a tree that stands out in our parks and gardens. Its small bilobed leaves which turn yellow in the fall have a curious, fan-shaped venation, unique in the world of trees. They also allow us to identify it at first glance: it is the Ginkgo biloba, a tree which is out of the ordinary and which has long fascinated us because of its many oddities.

[Article issu de The Conversation, écrit par Catherine Lenne Enseignante-chercheuse en Biologie végétale, Université Clermont Auvergne (UCA)]

A prehistoric tree?

The first is big: the ginkgo is unique on the planet! Belonging to a very ancient family of plants dating back 270 million years, the Ginkgoales, it is the only and last living representative today. Moreover, it looks exactly like its distant, extinct and fossilized cousins, so closely that it was long believed that it had remained unchanged for millions of years, as if time and evolution had not changed. had no control over him. Darwin invented the concept of “living fossil” to refer to these immutable beings and today's media perpetuate this idea by calling the ginkgo a “prehistoric tree”.

A false idea, of course, the notion of a living fossil being nonsense, since a fossil is by definition a dead organism whose organic structures have been preserved by mineralization. The ginkgo has indeed evolved like any living species but this is not visible at first glance. Scientifically speaking, this tree is a relict species and its apparently unchanged form over time is called panchronic.

The tree that survived the atomic bomb, but not the only one

Second, the ginkgo has a reputation as an “indestructible” tree. It is indeed exceptionally resistant to diseases and pollution and these extraordinary capacities help to explain its longevity easily exceeding 1000 years and more, in its natural area. But it is not the only one on the podium of trees which reach canonical ages. The oak can also reach the millennium, the olive tree several and certain pines of the American Rockies (the bristlecone pines), born before the pyramids of Egypt, hold the world record for the oldest trees in the world, approved at more than 5800 years old.

The ginkgo's reputation for immortality is reinforced by the fact that it survived the atomic bomb that pulverized the city of Hiroshima on August 6, 1945. However, it is not the only tree to survive the apocalypse this year. that day. Around twenty other trees such as ailantes, willows, eucalyptus, catalpa and others, even closer to the epicenter than it, also came back to life, producing vigorous shoots from their charred stumps after the disaster but strangely, the popular memory only remembers him. This somewhat overrated “superhero” label ultimately masks the real reasons that make the ginkgo a tree apart, and among these, its extraordinary sexuality. Unlike deciduous or coniferous trees, and like birds, the ginkgo is a tree that lays “eggs”.

Male gingkos and female gingkos

His entire sex life is in fact very original. First, because it is a dioecious species, that is to say whose sexes are separate. So there are male ginkgoes and female ginkgoes. It is rather rare in trees (only 6% of flowering plants), even if this separation of the sexes is not specific to ginkgo, also existing in deciduous trees such as poplars, willows, holly, etc., and in some conifer species such as yews. The general rule among trees is monoecy, in which the male and female sexes are carried by the same individual, in the form of cones in conifers or in the form of flowers in deciduous trees.

As a reminder, in these majority cases, the stamens present in the male cones or in the flowers produce the pollen grains which transport the male sex cells (spermatozoa) to the female organs. These are the ovules, a sort of box containing and protecting the female sexual cells (the oospheres) and they are carried by the scales of the female cones or they are enclosed in the belly of the pistil of a flower. The sexual behavior of the ginkgo therefore differs from that of the majority of other trees.

A naked egg cell scented with butyric acid

Next, the ginkgo's sexual organs are rather unusual. The pollen-producing stamens are gathered in a sort of tiny spike that we call a catkin because it resembles a cat's tail. This organization is comparable to that of the male cones of conifers or even to the male flowers of many deciduous trees. Think, for example, of the golden, dangling catkins of willows or hazelnut trees in spring. On the male side, therefore, originality is weak.

On the other hand, the female sexual organs are neither cones nor flowers, but enormous yellow and fleshy balls, hanging at the end of long peduncles like large plums on a plum tree. In the fall, they fall to the ground and rot, releasing butyric acid with a powerful and unpleasant odor, halfway between vomit and rancid butter. It is impossible to misunderstand the identity of the female ginkgo, the identification is olfactory and unforgettable!

These autumnal stink balls are the “fruits” of the ginkgo tree, but this formulation is botanically incorrect. Because a fruit is the result of the transformation of a flower after its fertilization, and as the ginkgo has no flowers, there cannot therefore be any fruit. The ball of the female ginkgo is in fact a simple but large ovule, “naked” because it is not protected by any structure as is the case of deciduous ovules buried in the pistils of female flowers, or to a certain extent the case of conifer ovules, carried by the scales of the female cone (the “pine cone”), pressed against each other when the cone is young.

Seeds that aren't quite seeds

Since it produces ovules, a structure that appeared around 350 million years ago, gingko is part of the group of Spermaphytes which are also called seed plants since the fertilized ovules turn into seeds. The ginkgo is therefore a tree that produces seeds, like all current trees. So far, no particular originality, except that its seeds are not quite original.

To be a real seed, you have to tick four boxes. Firstly, it must obviously contain an embryo, the future plant, resulting from the union of the two male and female sex cells during fertilization, the spermatozoon brought by the pollen grain and the oosphere, the female gamete of the plants, lurking in the ovule.

Secondly, this embryo must be embedded in a nutritious tissue full of carbon reserves which will fuel the first stages of its development during germination. These carbon reserves are only produced after fertilization, if and only if an embryo is born.

Thirdly, the seed is protected by a hard protective envelope and finally, fourthly, the whole is in a state of slowed life, a sort of sleep which allows germination to be postponed until the environmental conditions are favorable for growth, i.e. that is to say the following spring in our temperate climates (which allows the passage of the bad winter season without incident).

But in the case of ginkgo, the four conditions are not all met and its “seeds” are false, they are also called “pre-seeds”. What are they missing? If the ovules of the ginkgo are so large, it is because they are filled with large quantities of nutrient reserves but which have accumulated well before fertilization. This represents a considerable expenditure of energy for the ginkgo and a very unprofitable investment because all these filled ovules will not be fertilized and the precious reserves will be lost to it when they fall in the fall. On the other hand, they will rot and enrich the soil at the foot of the tree, which will ultimately nourish it later.

The ginkgo thus lays “eggs” quite similar to those of the chicken, the reserves of which accumulate during transit through the genital tract, without any need for fertilization. These chicken eggs are rarely fertilized, unless the rooster has come across the hen in the barnyard. The ginkgo is therefore a strange bird since it is a bit oviparous!

Fertilization closer to that of algae

Finally, another oddity of sexuality makes the ginkgo a decidedly unusual tree… When fertilization takes place, the process remains archaic, closer to that of algae than that of trees. Indeed, during the evolution of plants, the invention of aerial fertilization completely freed this crucial stage from the presence of water, unlike the ancestral mode of fertilization of algae or mosses and ferns. .

In true aerial fertilization, that of conifers or deciduous trees, the spermatozoa are not swimmers, they have lost their flagella, this type of vibratile filament which allows them to move in the water. They cannot therefore move to join their female oosphere partner in the ovum. They are then brought close to it by a siphon system formed by the germination of the pollen grain deposited on the cone or on the flowers. This pollen tube allows fertilization completely free from external water, which is called siphonogamy.

But in the case of the ginkgo, which is indeed a tree adapted to the aerial environment, fertilization remains aquatic. The ovule is hollowed out by a pollen chamber filled with a liquid which overhangs the head of the oospheres (diagram). The pollen grains enter the still very small ovule in the spring, through a small hole, the micropyle, which beads up a drop of sticky water retracting inside. Pollen grains entering the ovule then germinate a short pollen tube which anchors itself in the wall of the chamber and it is only a few weeks later that the grain releases its contents into the liquid, swimming spermatozoa because they are provided of vibrating eyelashes. They swim towards the oospheres at the bottom of the pool to unite with them.

The presence of water and swimming spermatozoa is characteristic of aquatic fertilization or zoidogamy (from “zoid”, swimming cell and “gamia” = marriage), almost unique among trees or almost… cycads, plants with a habit palm trees and close brothers of the ginkgos, also have swimming spermatozoa and aquatic fertilization!

Finally, this archaic method of fertilization of the ginkgo, inherited from the distant past of plants born at the bottom of the oceans, justifies its nickname of “prehistoric” tree a little better!