It is comforting, exhilarating even in these bleakish times, to learn of some wondrously ingenious detail of life, so extraordinary as almost to defy belief.
For ornithologist Tim Birkhead a fertilised hen’s egg is an “everyday miracle of nature”, a complete and self contained support system for the developing embryo within. The yolk must contain, by necessity, all the nutrients for its transformation into a fully fledged chick – the proteins to construct its body parts, sufficient energy-rich fats, a dozen vitamins and the minerals iodine, selenium, phosphorus and zinc.
Their presence in precisely the right amount and proportion is remarkable enough but more astonishing still is how the embryo acquires through its seemingly impermeable shell the one vital ingredient from the outside world necessary for its development – oxygen.
The egg’s ability to “breathe” is determined by the shell’s method of construction. The arrival of the yolk in the uterus activates dozens of tiny aerosol sprays that squirt a concentrated solution of calcium carbonate. The solution hardens to form columns of calcite, lined up like a stack of fence posts and separated by tiny vertical microscopic pores. This method of construction might seem a bit haphazard, but the “total pore area” –their number multiplied by their diameter – must be precisely calibrated to ensure the correct amount of ambient gases flowing in and out of the shell. Too much oxygen and the embryo’s metabolism goes into overdrive, too little and it will suffocate or be poisoned by the accumulation of carbon dioxide.
But that is not the end of the matter. There is an enormous variation in the size of birds’ eggs (from the quarter of a gram for the tiny humming bird to the one kilogram of the lumbering emu), the thickness of their shells and duration of their incubation. Nonetheless, biologist Hermann Rahn, in a study of the eggs of 90 diverse species (warblers, blackbirds, pigeons, kestrels, ostrich etc), demonstrated the “total pore area” for each is perfectly calibrated to ensure the volume of gases exchanged is attuned to the needs of the growing embryo within. The practicalities of how the number and diameter of those microscopic pores is so exquisitely determined, observes Birkhead, “is completely unknown” – at least to science.
The grooved structure of the cactus spine provides a further, if unexpected, exhilarating instance of the perfection of micro design in nature – as I discovered last year after visiting the surreal and stupendous Huntington Desert Garden in southern California. The popular names of the hundreds of species of cacti indicates something of their exuberant shape and colour: the “silver torch”, handsome and erect, cloaked in snow white bristles; the “hedgehog”, low-growing and inconspicuous; the “Bishop’s mitre”, flat and star-shaped like a biretta with a showy blue pompom at its centre; and the bizarre “creeping devil”, whose prickly python-shaped stems worm their way across the ground.
The fabulous aesthetic impact of these exotic forms is inseparable from the plants’ need to conserve water in the arid desert heat. This explains the three distinctive features of cacti. First they have dispensed with leaves, thus conserving the moisture that in temperate plants is transpired through the stomata (or mouths) on their undersurface. Next, their internal structure is modified to store water and carbon dioxide that, activated by the process of photosynthesis, form the tissues from which all plants are made.
As for their prickly spines, they usefully defend the succulent cactus from thirsty desert foragers. But Dr Fahrana Malik of the University of Swansea has demonstrated how they serve the further crucial function of capturing and condensing the fog rolling in from the Pacific, as the hot desert air is cooled by the ocean. Deploying time-lapse photography, Malik was able to show droplets of water forming on the tips of the spines, then moving along them before being absorbed directly into the cactus stem. The mechanics of this process are not straightforward, requiring – as revealed by photographs of the spines at 150-fold magnification – that they be etched with miniscule vertical grooves creating a “roughness gradient”, impelling the droplets towards the stem. These microgrooves are just as essential for the survival of cacti as are the micropores of eggshells for birds.
Such wonders are truly inexhaustible. “We may begin where we please”, noted the Victorian philosopher George Henry Lewes; “we shall never come to the end, our curiosity will never slacken.” And what could be more cheering than that?
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