Biology’s Fascination with Six

Yes “6”, not sex (and sometimes also “5” too)

In terms of their total number of species and number of individuals, the Hexapoda (“hexa” from the Greek “hex” and meaning “six”) are the most common creatures on Earth.  They are animals with 6 legs and because of their abundance they have to be called highly successful. They include, of course, all the insects and that appears to show that having 6 legs must have been an advantage over having fewer or more. And indeed, insects seem to have evolved from ancestors with more than three pairs of legs, but a further reduction seemed unnecessary and even unwanted, for the 6 legs, with always two on one side and one on the other on the ground, provide them with a stable tripod resting stance and a balanced gate (now often copied by robotic vehicles to operate in difficult terrain). The six legs can also buffer an insect’s fall from a height and even with one or two legs lost or injured, an insect can, albeit more slowly, crawl away.

But insects are, of course, not the only organisms in which “six” plays a role. The sea anemone subclass Hexacorallia contains six orders including the stony, reef-forming coral species, all of which possess a six-fold structural symmetry. Six-fold symmetries are also familiar from the honey-combs constructed by honey bees and from the nests of wasps and hornets. The arrangement of the facets in the compound eyes of insects is based on hexagons. The outer cuticular surface of some tiny hexapods, known as springtails, feature beautifully regular hexagons that resemble the lattice structure of graphite and, turning our attention to some internal building blocks, we find more examples of hexagonal organizations, for example in the stacks of retinal microvilli, which when cross sectioned exhibit  a honey comb pattern. The all-important carbohydrates like sugars, starch, cellulose and chitin are all based on the hexagonal shapes of their building blocks, namely a hexagon of 6 carbon atoms.

Although there are plants with flowers (or leaves like clover) with a six-fold symmetry, the number five is also popular in Nature: just look at your hands and feet! Or cut the fruits of an okra (my favourite vegetable) and take a look: you will notice its pentameric  (frem Greek “penta” meaning “five”) shape. A huge number of flowers exist that have five petals; in fact nearly 40% of the flowering plants are pentameric and a little over 10% are hexameric. The most famous examples of five-armed animals are, of course, the starfish. Belonging to the animal phylum known as Echinodermata, all its members in fact, including sea urchins and sea cucumbers are pentameric.

When I was the director of the Electron Microscopy Unit at the University of the West Indies and used our scanning electron microscope to examine various structures, I came across many examples of pentagonal shapes. There were the shapes and surface patterns of some insect eggs (also recently reported for the mosquito Psorophora albipes by Cecilia F. De Mello and co-workers), the compact structures of some pollen grains, and the outlines of some diatoms. Sadly, I never had a chance to examine samples of the unicellular, microscopic deep sea coccoliths, but they, too, are also known to contain many examples of specimens with pentagonally-shaped calcium carbonate plates. In the rare cases where a bregmatic skull bone was present in humans, it was reported to have been of roughly pentagonal shape. In pathological conditions like Alzheimers, amyloid fibrils may be developed and the latter contain pentagonal protein components.

All mammals have five “holes” in their head: two ears, two nostrils and a mouth and generally speaking we distinguish five brain regions, namely the cerebrum, diencephalon (with thalamus and hypothalamus), mesencephalon (= midbrain), metencephalon (= cerebellum) and medulla. We also speak of the “five senses”, i.e.  sight, sound, smell, taste, and touch, but also invoke a “sixth sense” when we describe an ability to perceive something that does not seem to involve the ‘regular’ five senses. Although there are actually more than 6 senses, there you are: we believe that Nature just loves sixes and fives.

© Dr V.B. Meyer-Rochow and, 2021.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

Lowering their Heads and Facing East

Why do sunflowers behave this way?

Anyone who undertakes a train journey in late summer from Helsinki to Beijing (like I did quite some years ago), will see extensive fields of sunflowers before the train leaves Europe and enters Asian territory. What is remarkable about the sunflowers is that they all face one direction and observing that, I thought the sunflowers probably orient themselves towards the sun. But according to a recent study by Horvath and co-workers, this is not so, because following a short period of full bloom during which the flowers do orient towards the sun, they then gradually bend downward and become ‘locked’ in a direction to face the East. (Now the beautiful and patriotic song “The East is Red” 东方红, never mind the words, enters my head and reminds me of the time I heard it daily while spending 3 weeks in China by invitation of the Chinese Academy: you can guess how long ago that must’ve been!). However, sunflowers, which like many fruit and vegetable plants originated in the ‘New World’, have other reasons to turn eastward.

In the past, several reasons had been advanced to explain why after the flowering period sunflower inflorescences no longer track the sun, but remain east-oriented. Yet none have been tested reliably and the new explanation for the first time takes into consideration the place of origin of the sunflower plant, the astronomical data of the sun’s position, the meteorological data of diurnal cloudiness, the time-dependent elevation angle of mature sunflower heads and the absorption spectra of the inflorescence’s front and back. An earlier suggestion was that a non-skyward orientation of mature sunflower heads would make it more difficult for birds to peck at the seeds. While true, it does not explain why the flowers should face east. Another attempt to explain the eastward orientation was that it would reduce the heat load at noon, but west-facing flowers would have the same advantage, so why ‘east’? It has also been assumed that east-facing allows greater light reception in the morning and speeds up drying of morning dew, thereby reducing fungal attack. That an easterly orientation promotes attractiveness to pollinators has been suggested, but by the time the sunflower heads get ‘locked’ in the easterly position, pollination has long been finished and the idea that an easterly orientation and the lower head temperature could be advantageous for seed maturation was not supported experimentally.

What appears to be crucial is that there is a 10-50% surplus energy absorbed by an east-facing sunflower inflorescence compared with other directional orientations. This could indeed accelerate the evaporation of morning dew, but what is the easterly orientation due to? It has seemingly something to do with the region the sunflower plant evolved, namely Boone County in North America, which regularly encounters cloudy afternoons. If afternoons are cloudier than the mornings, then east-facing inflorescences have an energy advantage of around 10% over west-facing flowers and an up to 50% radiation excess over south-facing flowers, taking into consideration absorption spectra of the inflorescence and the back of the heads. Maximum radiation absorption should be advantageous for seed production and maturation. The easterly orientation seen even in domesticated European sunflowers is likely to be a genetic trait that evolved in response to the meteorologic conditions of cloudy afternoons in the region that sunflowers evolved in North America.

Given that solar panels are usually directed south and direct sunlight is most intense at noon, are sunflowers ill-adapted, or do sunflowers perhaps ‘know more’ than solar panel engineers? Sunflower heads are tilted, looking downward and under such conditions the lower angle of the sun in westerly and easterly position is crucially important. But adding afternoon cloudiness into the calculation is what then causes the East to turn into ‘the winning formula’ for Helianthus annuus (the sunflower).

© Dr V.B. Meyer-Rochow and, 2021.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

Seed or fruit

Can there possibly be a dispute?

At Ohaupo Elementary School one of my sons had to learn this verse from one of Dr. Seuss’ children’s books: “I am the Lorax. I speak for the trees. I speak for the trees for the trees have no tongues….”.  But I’d like to add (and it even rhymes) “But trees do have seeds” and with few exceptions like bananas and a few other domesticated species, the trees need them to produce new trees. And this brings me to this week’s BIOFORTHEBIOBUFF topic, for I collected something that some people call seeds and others fruit. The latter are typical of Angiosperms, i.e. the flowering plants. The fact that they appeared rather abruptly about 100 million years ago and are not known from before that time troubled the great Charles  Darwin to an extent that he called the evolution of the Angiosperms “an abominable mystery”.

On the campus of Andong National University in Korea, we have many Ginkgo biloba trees, but they are not Angiosperms, but Gymnosperms. I remember a thing or two about them from my high school bio-classes with Dr Ruppolt, e.g. that the species is dioecious and that therefore separate male and female trees exist, that the species is ancient and has survived in China unchanged for 200 million years and now has been introduced to many countries, because it is such a tough and long-lived tree. Its pollen grains are wind-dispersed and, unique among all of the different species of trees and all flowering plants, contain sperm cells with cilia. Although the ginkgo tree is related to needle trees, it does not have needles but possesses characteristically shaped, parallel-veined, bilobed leaves (hence the specific name “biloba”), which turn beautifully golden in autumn and drop to the ground.

What I was not taught was, that its cherry-sized, yellowish  -let me call it for the moment- “fruit”, has a reputation to smell like vomit or rancid butter. This was the reason why my wife chased me out into the garden with my big bag of ginkgo “fruit”, which I had collected at the university and proudly presented to her when I got home from work. Although the ginkgo seeds, known as ‘nuts’ are edible when freed from their soft and yellowish coat, it takes time to wash, dry and then boil or lightly roast them before they can be consumed. But does the ginkgo tree actually produce “fruits”?

The tree, as mentioned before, belongs to the Gymnosperms, i.e. needle trees like conifers, which are characterised by an absence of flowers and the presence of naked seeds. The berries that a few of the Gymnosperms such as ginkgo, yew and juniper produce, look like real fruits, but they are derived from the ovule’s outer layer around the seed inside the ovary and are not, as in the flowering plants (known as Angiosperms), the result of the plant’s entire ovary. The latter is what in the Angiosperms enlarges and then becomes a proper fruit. Technically speaking, therefore, ginkgo, yew and juniper do not produce fruits, but only coated seeds that lack the ovary’s protective surround. In case of the yew, its red and sweet seed coat is edible and delicious, but its seed is deadly poisonous and should never be swallowed; the juniper berry is used for meat dishes (and gin), and regarding the ginkgo nut, only the adequately prepared seed without its fleshy coat is used.

What I found strange is that I do not find the ginkgo “fruit” smelly at all, while everybody, even those in Japan and Korea who love to eat the nuts, say so. I may have what the sensory physiologist John Amoore called “specific anosmia”, which applies to people who have an otherwise normal sense of smell, but are unable to smell certain specific odours. The Chinese writing for ginkgo 銀杏 (also used in Japan) translates into “Silver Apricot” (and not into something like ‘vomit berry’ or ‘stinky nut’). Perhaps the one who invented this Chinese character also was a lucky person who had specific anosmia and was not bothered by the ginkgo nut’s smell.

© Dr V.B. Meyer-Rochow and, 2021.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.