This essay was published in ‘Science and Philosophy’ at Medium and that page can be accessed here: https://medium.com/science-and-philosophy/the-ginkgos-geniza-e8c048e7bae8

Our neighborhood ginkgo is holding steadfast to autumnal bloom. Well, perhaps not ‘bloom’ —  not at least in the commonly accepted sense of the word —  but its leaves, every single one of them, are still clad in brilliant chrome-yellow and the vision is breathtaking. More so when silhouetted against an uninterrupted winter-blue sky. At this time of year (late November), most of its branched companions have donned and discarded their colored coats. But, this tree, insouciant to peer pressure, stands firm in sun-bedecked gold; a botanical candelabra aflame with a thousand shimmering lights. This is how it always is with this tree. Come rain or shine, its leaves brace together as one man. What they do, they do with unity.

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Color and denudation

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Autumn announces its arrival with shorter days and cooler temperatures and this is the cue for deciduous trees to begin stocking up for winter. In their turn, their own season-scouts cue us – as early as sweltering August! – to the impending end of summer. In my neck of the woods, that scout is the Dogwood. It signals the Sun’s angling away, from the gathering north wind, with a crestfallen droop of its leaf margins. 

There is where it all starts – in leaf margins. Seasons, in growth and death, lurk there. They periodically gather armies to march out from the pavilions in a triumph that is ever short-lived and soon collapses to retreat until another dawn; another year. The spectacle might be farcical if it was not, visually and philosophically, entrancing.

And in this fashion, Autumn too starts in the margins of the leaves. From where, with a faltering start as a barely discernible change in color, it soon hits stride in a determined centripetal march turning every leaf into a flag that heralds a showy allegiance. 

Trees now become beehives of activity. Over the next many weeks, chlorophyll in leaves is broken down into its constituent nutritive ingredients for safe storage, photosynthesis slows and the green disappears unmasking the yellow and red colors¹ of underlying leaf compounds. With chlorophyll gone, and their nutrients captured, leaves become functionless and useless. As the north wind starts to gather speed, the trees begin a methodical abscission and stoically allow their leaves to tumble to the ground. Changing color and shedding leaves, they hurry to stash for winter’s lean. The trees prepare to hibernate.

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Ginkgos too display the self-same autumnal motions but at an uncommonly rapid pace that is unusual for trees. In mid-October, their green leaves turn gold in near-perfect unison. And, as if this grand showing of their seasonal debut was not enough, they dial up the drama with a stunning finale. Unlike the weeks-long gradual shedding of other trees, the ginkgo disrobes with impatience. It stages a synchronized leaf drop that happens in the span of hours; over a day or overnight.

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In late November, often prompted by an abrupt downturn in temperature, and in the proverbial blink of an eye, they defrock their leaves and strip their branches to gild the ground. Watching ginkgo leaf-fall is like watching the slow unfurling of curtains at the end of a theatrical performance: curtains of invisible gossamer, loosely sewn with sequins, descend in a fluttering incandescence into a pool of gold. ‘As if in one consent’ the poet Howard Nemerov remarked, of this phenomenon, which has in recent years, earned the affectionate moniker of, ‘the great ginkgo dump’.

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Dioecy and Sex

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There is so much more that is unique and extraordinary to a ginkgo than just synchronized shedding. It is a member of a compact grove of extant trees that have a documented² co-existence with dinosaurs in the Jurassic age. It is an astonishing detail for which ginkgos are called ‘living fossils’. This tree has no known living relatives and so it occupies an entire taxonomic order, family, genus and species in splendid solitude. Ginkgo leaves have a unique fan shape that is not replicated elsewhere – like an open fan with a cherry-stalk handle; a shape that is called ‘flabellate’ in botany. More than forty flavonoids and a variety of terpenes have been isolated from its autumn leaves from which, a processed botanical extract is used as a therapeutic supplement for cognitive disorders and neural health³. 

With such a gallimaufry of botanical marvels, interest in this tree is pampered for choice. It is difficult to pick any one of her remarkable attributes to highlight but ‘her’ is perhaps a good place to start. A ginkgo is one amongst a small group of trees that separate into male and female. Male ginkgos have male cones which produce pollen and female trees have female cones to receive the pollen which is then fertilized inside the cone to a fleshy seed called the ‘ginkgo nut’. There is a very definite division of labor here, with the male ginkgo trees investing their energy in making pollen and the female trees channeling their own energies into the production of nuts. 

So how does the male pollen get to the female cone? With the help of pollinators. In the ginkgo’s case, common pollinators are the wind and small bees. In poetic fashion, the male tree requests the wind to carry his pollen to any female who is in the wind’s path. Well, even if not physically true, this is what happens in any case. The wind carries the pollen from the male to the waiting female cones and, if he does his job well, their reproduction is pronounced a success with the female happily birthing many hundred ginkgo nuts in autumn. 

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This phenomenon of species separation into sexes is called ‘dioecy’ and such species are termed ‘dioecious’. From humans upward, the animal kingdom is replete with species that split into male and female. Fewer plants⁴ than animals display dioecy because plants are rooted and do not have the luxury (or pleasure) of meeting mates in person to work their charms. Some dioecious plants are sure to have graced kitchen spaces worldwide: date palms, papaya and spinach. Even the controversial marijuana is dioecious!

Intuitively, a physical separation into male and female does not appear to be a good survival or reproductive strategy, if your feet are rooted in the earth! If you can’t directly court a mate and need to enlist the aid of a third party pollinator, what happens if the pollinator is fickle or if something goes wrong with the process? Isn’t it then better to have both reproductive organs close together so you can pollinate your own self as a back-up? That is what many thousands of flowering plants have safely chosen. And yet, separate and dioecious ginkgo have a survival record of 200 million years and are clearly doing something right. Something that continues to elude our reasoned pragmatism.

If dioecy is rare in plants, what is common? The common is hermaphroditism, where both male and female reproductive organs coexist in the same flower. This feature is commonly illustrated with the hibiscus or the lily as textbook examples. There are also ‘monecious’ trees. They flower in one of three possible combinations wherein one tree can have: a) separate- male and female flowers b) separate- hermaphrodite and male flowers or c) separate- hermaphrodite and female flowers. Almost never does one tree – in the natural state – have all three separate flower types — hermaphrodite, male and female — because … well, what would be the point? 

These permutations and combinations can boggle the mind and weary the eye and so, let us nimbly step away from here to ask the very pertinent question: Why did dioecy evolve? When there are hermaphroditic states⁵ that allow the plant/tree a fail-safe shot at pollination; why did nature think dioecy was useful? In the words of the grand sire himself “There is much difficulty in understanding why hermaphroditic plants should ever have rendered dioecious. There would be no such conversion unless pollen was carried by insects or the wind regularly from individual to the other; for otherwise, every step towards dioeciousness would lead to sterility”⁶. 

Biology has a fundamental dictum: Cross pollination (crossing) produces more robust individuals with a significantly greater survival advantage than self pollination (selfing). Selfing, if it occurs, is almost always only as a fallback safety net. Plants actively choose crossing. The methods by which hermaphrodite flowers avoid self pollination is a fascinating subject that deserves far greater attention than this essay can provide and so, for now, let us continue on our road and save that for another day. 

A seminal paper by renowned biologist Professor KS Bawa⁷, on the evolution and frequency of dioecy in Plantae, flagged the underestimation of dioecy and reported a higher incidence in tropical and island forests. He went on to elaborate other reasons for the phenomenon: The common explanation of dioecy is that it is a fool-proof method to prevent selfing and assuredly gives rise to offspring with a reproductive advantage. But there is always the lurking danger of a disinterested pollinator or of process failure and these biological anxieties must be allayed. And so, male trees compensate by increasing their pollen production to many times more than the normal amount. The females, grateful to be free of the task of making pollen, invest their surplus energy into fleshy and nutritive fruit with which to attract animals who can carry and disperse the seeds farther. This is division of labor with a natural selection advantage and a natural impetus for dioecy. Lastly, plant reproduction is wholly dependent on pollinators. The type of pollinator closely influences the style and method of reproduction. To illustrate: a pollinator that operates in a restricted foraging space, limits the potential of pollination and dispersal. Dioecious plants hedge their bets with a pollinator that travels farther. The male takes a chance with the wind and accordingly, increases the volume of pollen; the females takes a chance with animals who can carry her fruit far and consequently, increases the attractiveness (nutrient and sugar content) of the nuts/fruit.

With close to 400,000 plant species on our planet, there isn’t one reproductive style that fits all tastes. Each species makes its own assessment for what is needed to succeed and effects that plan. At times, it gets a little more intricate. Within a species, an individual plant or tree could respond to an external circumstance or crisis with a display of behavior that is a variation from the expected. 

One of those displays is an inconstant sexual state. Confronted by a threat to reproduction, and with all else failing, a dioecious plant can switch its sexual state. With some species (Arum) this change does not require a threat and is a naturally periodic event⁷. With some others, it is dependent on the external environment and overt selection pressure. This ‘leaky dioecy’ — leaking from one sexual state to another —  is physically observable in ginkgos; in both male and female trees, but more commonly in the male⁸. Male ginkgos are known to sprout branches that have female cones and produce nuts. Whether due to environmental pressure, or some other reason, is still not clear. 

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Culture: new and old

There might be a way to find out. The fleshy outer covering of the nut emits an odor that is terribly offensive to humans. This has earned the urbane and city-bred female ginkgo the unfortunate epithet, ‘stinky ginkgo’. In the middle of the last century, this side-effect was not well publicized and cities across the US, entranced by the ginkgo’s fall fashion, planted the trees widely on pavements and in parks. When the trees matured, the malodorous stench of ripe ‘stinky-ginkgo’ nuts posed a problem to noses that were unused to this smell and reeled unhappily under its potency. In due course, city denizens started to complain. No one wanted to chop down the beautiful trees, and after a good deal of deliberation, the solution adopted was for new plantings to be of male cultivars alone and for existing females to be sprayed with ‘sprout-nip’ – a growth regulator that would cause unripe seeds to fall. Cities have been doing this for at least a decade and all newly planted ginkgos are male cultivars. It remains to be seen if they switch sex, over time, to right the balance.

The extant species of ginkgo was named ‘Ginkgo biloba’ for the curious bilobed character of its leaves. However, not all leaves wear the cleft. It only graces the ones that spring out of the main-stem branches. The rest sprout in bunches like inflorescences from the short shoots of these stems. This is the natural leaf pattern of the tree. Yet, most city trees are sparsely populated with bilobed leaves. For a tree that bears its name, it is decidedly odd to have to rummage through the foliage for a bilobed leaf specimen.

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My love for the ginkgo is some decades old. As with most others who feel the same, with me too, it was mesmerism at first sight. And it would have stayed that way with me, as an unschooled attraction, had it not been for a chance encounter with Goethe that happened a few years ago in the cavernous deep of a much-loved library. While reading Hafiz in an essay on Persian poetry and translation, I stumbled upon ‘West-Eastern Divan’ and Goethe’s stirring appeal to Marianne von Willemer through the ginkgo¹⁰ . Remarking on the bilobed leaf and seeing in it a metaphysical allusion to his own impossible love, he writes:

A little more than two centuries later; this poem, in its turn, was the spur to Peter Crane’s resplendent hagiography of the ginkgo (see bibliography). Trees command the adulation of poets. Few become the subject of an entire book. Few more command the adulation of poets and inspire resilience in survival. Very few continue to be venerated and worshipped as God. 

In Hiroshima, they are called Hibakujumoku —the trees that survived the bomb¹¹. Six ginkgos were amongst them; four in temple grounds. All six Hibakus are still standing. Timeless and enduring. 

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Bibliography

Peter Crane. (2013). Gingko: the tree that time forgot. Yale, CT: Yale University Press

End notes and references: 

1. Common leaf pigments that are unmasked by the disintegration of chlorophyll are called Flavonoids (orange and yellows) and Anthocyanins (reds and pinks). We visualize colors in leaves and flowers not just due to the pigments but also due to plant optics — structural characteristics of leaves that cause light to reflect and refract in ways that allow us to perceive shades of color. Flavonoid compounds in the ginkgo have been explored for medicinal and anti-aging benefits
2. The documentation exists in the form of fossil records. As astonishing leaf fossils that show the same characteristic leaf shape and venation. The oldest fossil was found in Ishpushta, Afghanistan (Ref: Crane, P. Ginkgo, p. 83). Cycads and conifers are other living fossils like the Ginkgo. And there is too another extraordinary tree (or is it a plant?) in Namibia, called welwitschia which requires a recitation all its own
3. The extract is called EGb 761. It is made up of a combination of ginkgo flavonoids (22–25 percent) and terpenes (65–67 percent) (Ref: Crane, P. Ginkgo pp. 246–249). Its value in cognitive health is reinforced by a large meta-analysis that covered a thousand studies and by other reviews of RCTs conducted worldwide. (Ref: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3679686/). It is prescribed in the West as a natural supplement for memory for senescence and for the cognitively impaired. In the East, it is the nuts, not the leaves, that are used for their medicinal properties and for a wider variety of ailments that include respiratory and digestive complaints
4. The number that is commonly quoted for the incidence of dioecy is 4–6% in angiosperms and 50% in gymnosperms. These figures have been effectively challenged as underestimates (Ref: Bawa, K. Evolution of dioecy in flowering plants Ann Rev. Ecol. Syst. Vol 11 (1980): pp. 16–21 https://www.annualreviews.org/doi/10.1146/annurev.es.11.110180.000311).
5. The vast majority of flowers are hermaphrodites. They possess both male and female reproductive organs. Naturally therefore, both self and cross pollination is possible in hermaphrodite flowers. Although it is important to remember that nature actively prefers crossing and goes out of the way to prevent selfing. Every flower has some mechanism – structural, biochemical and temporal – either singly or in combination that functions an interdiction for selfing. 
6. Darwin, C. The different forms of flowers on plants of the same species (1877). London: John Murray. (Ref: Bawa, K. Evolution of dioecy in flowering plants Ann Rev. Ecol. Syst. Vol 11 (1980): p.16 https://www.annualreviews.org/doi/10.1146/annurev.es.11.110180.000311)
7. Bawa, K. Evolution of dioecy in flowering plants Ann Rev. Ecol. Syst. Vol 11 (1980): pp. 15–39 https://www.annualreviews.org/doi/10.1146/annurev.es.11.110180.000311
8. This is called sequential dioecy or sequential hermaphroditism
9. Crane, P. Ginkgo. pp. 53–65
10. The poet Johann Wolfgang von Goethe, the author of Faust, was also a poet and polymath; one of the greatest figures of the Romantic age. His poetry can be read at Poetry Foundation: https://www.poetryfoundation.org/poets/johann-wolfgang-von-goethe
11. The legacy of Hiroshima’s Hibaku trees: http://glh.unitar.org/

[ All pictures in this essay are the author’s own except where specified. ]

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