Ginkgo biloba: brief notes
Robert W. Ridge © 1987
Ginkgo biloba: brief notes
Robert W. Ridge © 1987
Leaves are stalked an are variable in size and shape. Generally they are bi-lobed, without midrib, and irregularly crenate. Veins repeatedly fork and sometimes fuse (anastamose). Ginkgo is dioecious, and sporangia arise with the leaves on the short shoots. Microsporangia are catkins, 3-6 on a shoot, each being a pendulous axis bearing numerous stamens loosely arranged. The stamen is a short stalked knob, with 2-4 anthers which dehisce on the long axis. Macrosporangia are usually borne 1-3, more or less erect on the shoot. Each consists of a long stalk which bears an ovule on either side (sometimes three) below the apex. Ovule sessile, straight, surrounded by a collar at the base, and naked.
Fruit: a drupe-like seed, with orange flesh covering a woody shell. The embryo (sometimes 2 or 3) with 2 or 3 cotyledons. The fruit produce large amounts of butyric acid in the autumn, when they drop, and is a particularly nasty smell!
Elwes and Henry described a number of varieties in their publication in 1906. These were variegata, pendule, macrophylla laciniata, triloba, and fastigiata. I have been able to find none of these in Japan so far, although I have heard that there are two specimens of the pendulous variety in the gardens of government house in New Zealand. However, I have found three varieties in Tsukuba. These are a small yellow-leaved variety, an ordinary green leaved variety, and an intermediate of these two. The "ordinary" variety appears to be the most common in the area and in other places I have visited. There is also a variety that produces very small fruits.
Another interesting phenomenon, which may be a variety, is the hatsuki, or ginkgos which grow fruits on their leaves. These were first mentioned in the Japanese literature over 100 years ago, so I guess they've been well known for longer than that in Japan. They are very rare trees, only two are known in Ibaraki prefecture, and both have been labelled living national treasures. These trees produce fruits and leaves normally as well, and only some leaves acquire fruits. The fruits are sterile. There are no obvious attributes of ginkgo to distinguish the sexes, apart from observing the emergence of the sporangia in the Spring; but as the trees generally don't 'flower' for 50 years, its rather a long time to wait!! It is possible to distinguish the sexes by chromosome morphology, because the female lacks one tiny satellite on one chromosome. To tell the sexes apart seems to be important only in countries other than Japan and China, because of the rather bad smell of the fruit in autumn male trees are preferred. However, the Japanese people certainly don't seem to care and don't mind the smell, and many people collect the nuts for food. I have read that the male trees tend to be more upright, pyramidal, and the females more compact with lower branches and some pendulous, but I have seen the opposite of these attributes. And anyway, it would be better to be able to tell the sex from the seed. Perhaps an antibody is the answer.
Chi-chi (nipples) usually develop on old trees. The ginkgo at Kew (a male) is just starting to develop them now and it is about 200 years old. However, I've seen them quite well developed on a female tree known to be 56 years old. Chi-chi occur singly or in clusters and can take root if they reach the ground. Their function wil be described in another paper.
From this history it is clear that no other plant has a stronger claim to be called a living fossil, a term used by Darwin to designate survivors of the past. This tree was thriving 125 m years ago when dinosaurs still roamed the earth and the genus has remained from that time virtually unchanged. Ginkgo biloba is the sole living member of a once great and dominant race of plants. It is thus a most precious and tenuous link between the present and remote past.
Ginkgo now survives in the wild state only in a remote mountain region in eastern China. However, it is much cultivated throughout the world, especially in China, Korea and Japan. In Japan, very old specimens exist in the old gardens of temples, shrines and castles, and there are some famous trees, for example the tree next to the Hachiman Shrine in Kamakura (south of Tokyo and one of the old capitals) is famous because the assassin of an early Shogun hid under the tree before his infamous deed in 1290. This now enormous tree still stands next to the shrine and produces very large amounts of seed every year.
The Japanese also have many ginkgo planted in parks and as avenues, and thousands are planted every year. Tsukuba has a number of ginkgo avenues which promise to be quite magnificent in a hundred years. Why is ginkgo so popular in Japan? There are a number of reasons, one or two are my guesses. What I haven't told you so far is that ginkgo is exceptionally beautiful in the autumn, the leaves turn a yellow gold, looking like "golden ducks feet" one of the Chinese names for it. It is also a symbol of longevity, which is perhaps why it is also popular around temples and shrines. The nuts are tasty, usually eaten in an egg custard like dish called chawanmushi. Ginkgo is also a great survivor in polluted cities, growing where other trees fade away, and is exceptionally resistant to fungus and insect attack.
The nucleus of the spermatogenous cell divides without forming an intervening wall (the sperms have no walls) forming two sperms, which quickly develop their spiral motile apparatus. The mature spermatozoid is similar to the cycad sperm, but it is smaller and has only 2.5 turns of the spiral, compared to the cycads 5 or 6. Numerous flagella (10-12,000 in cycads, uncounted in Ginkgo) are attached along the spiral. The pollen tube opens and sperms are released (though they literally squeeze out) into an archegonial chamber liquid, which appears at this stage. As you must realise, the ginkgo sperm doesn't have very far so swim, compared to fern sperm for example. The sperm is very flexible, as we have seen from video shots. When it enters the archegonial neck, the sperm becomes greatly stretched.
Authors disagree as to whether the whole spermatid enters the archegonium, or whether the flagella apparatus is left behind. The nucleus of the spermatozoid separates and contacts the egg nucleus, forming a diploid zygote. It has been suggested that fertilization and embryogenesis may occur either on the tree or on the ground. In our experience at Tsukuba, fertilization occurs on the tree, but as the fruit falls soon after this period, embryogenesis probably occurs on the ground. I'm sure this may differ from region to region and according to the weather. Of course, removing the fruits from the tree doesn't affect fertilization, so it is quite feasible that fertilisation could occur on the ground.
Another inclusion, about which nothing is known, is a fibrilogranular inclusion that develops at the prothaliall end of the nucleus. This granule divides as the cell divides, so that each sperm has some part of it. Some authors have reported a similarity of staining (at the light microscope level) between the nucleolus and fibrilogranular body. Certainly there are some similarities at the EM level. At this stage plastids are actively dividing, and it we have managed to preserve the constriction ring (presumably actin) of the dividing plastids. This ring is usually only well preserved after freeze substitution. It is interesting that the plastids are active in division, because this suggest that maternal inheritance doesn't occur in ginkgo, and there would seem little point for the plastids to be dividing otherwise, as there would be no advantage in expiring energy away from the motility system. I'm not sure we can say the same thing for mitochondria. Any comments welcome.
After division, during which the blepharoplasts are associated with the spindle poles, the blepharoplasts break up to form the spiral flagella band. How the spiral is formed we have yet to work out, but multi-layered structures (MLS) structures are quickly established after the break up of the blepharoplast. The spiral band is based on an MLS structure of which little is known, but which is apparently very important phylogenetically, because, at least in the green algae, its presence or absence distinguishes the asymmetrical chlamydomonas types (in which it is absent) and the more advanced asymmetrical types, in which it is present in various forms. The MLS is also present in motile stages of liverworts, Equisetum, and of course the cycads. However, the structure of the MLS varies, being only two-layered in some ferns and (possibly) five layered in ginkgo .