The Ginkgo’s Geniza
By: Meena N.S
Tags: Ginkgo, Science, Ecology, Botany, Trees

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 something goes wrong with the process? Isn’t it better to have both reproductive organs close together so you can pollinate your own self as a back up? That is what many thousand 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.

Foliage as inflorescence (Pics: author’s own)

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. Most flowering plants are hermaphrodites. This feature is commonly illustrated with the hibiscus and the lily as 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 make the eyes weary 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 reasons for the phenomenon: The common explanation of dioecy is that it is a fool-proof method to prevent selfing and assuredly give 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 pollen production 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 decide to hedge their bets with a pollinator that travels farther. The male takes a chance with the wind and accordingly, increases the volume of his pollen; the female takes a chance with animals who can carry her fruit far and consequently, increases the attractiveness (nutrient and sugar content) of the nuts/fruit.

The beautifully ridged bark of a ginkgo with leaves just starting to turn in early autumn (Pic: author’s own)

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 unexpectedly sprout branches that have female cones and produce nuts. Whether due to environmental pressure, or some other reason, is still not clear.