Warp, Bend, and Crinkle

The irresistible power of water

Those of us who live in the temperate zones are treated to an annual drama in which trees, finally admitting that in the coming cold, dark days of winter, their leaves will be more of a burden than a benefit, and cut off exchange in preparation for shedding them until they grow new ones in the warmth and light of the spring. Their source of water cut off and their chlorophyll removed, the leaves turn yellow, gold, red, attracting crowds of tourists before they shrivel, dry, and fall off. Their planarity, useful for capturing light, now vanishes as they warp and bend.  This process is not limited to the temperate zones, but is part of the lives of plants all over the world. Simply put, when a leaf no longer produces a profit, it is dumped. It’s just that in the temperate zones, there is mass dumping in the autumn. This dumping goes on in the tropics too, most obviously in the dry season.

Northern red oak, winter leaves. Most oaks do not shed their dead leaves until they grow new ones in the spring. Drawing by WRT.

 

Looking up through a Cecropia tree. Photo courtesy of Peter Marting

While in Guatemala visiting my brother, I became intrigued not only with the large, palmate green leaves of Cecropia trees, but also with their dead leaves, which often curl into a sort of ball shape.  There is nothing rare about Cecropia, a tree common in disturbed areas throughout the American tropics where they often provide housing for ants. But while drawing this dead leaf, I began to think about why things bend or curl.  A live leaf may be perfectly flat and green, but when the leaf dies and dries, it shrivels into the shape of a hand with the fingers curled upward.

My drawing of a dried, shriveled Cecropia leaf. Colored pencil.

            The basic physics of such curling is not really very complicated, and its general action is visible in many places.  Whenever you glue or otherwise firmly attach two sheets of material with different coefficients of expansion together, the sandwich will bend, cup or warp as conditions change because the two sheets change size at different rates.  In bi-metal thermometers, two metals that expand and contract differently in response to temperature are attached to each other.  As the temperature changes, the sandwich bends. Arranging a long bi-metal strip into a coil as in the image below greatly amplifies the motion. When the bending moves electrical contacts, as in old-fashioned thermostats, the furnace is turned on or off. 

A bi-metal thermometer. The coiling and uncoiling of the bi-metal strip indicates the temperature.

            All large, biological molecules such as cellulose, starch, DNA, protein, etc. (aka biopolymers) are studded from end to end with chemical groups that attract, bind, and release water, and when they do, the volume of the biopolymer changes.  This uptake and release of water from humid air or aqueous liquid at the molecular level causes anything built of these biopolymers to swell and shrink, with details of molecular and physical structure affecting the amount of swelling and shrinking. 

So, getting back to our Cecropia leaf, the upper surface of the leaf shrinks more upon drying than the lower surface, causing the leaf to curl upward. In essence, the area of the upper surface decreases more than the lower, but because the surfaces remain attached, the only way to settle the argument is for the upper leaf surface to become the inner (and therefore smaller) surface of a sort of shaggy sphere.  The details of curvature, the curls, invaginations, and projections of the dead Cecropia leaf are super-sensitive measures of the differences of shrinkage among the veins, the upper and lower surfaces, played out on a leaf-region scale.  Think of it as a fine instrument for measuring relative humidity, not point by point, but a whole complex surface at a time.

            The same principle applies to boards--- all lumber cut from fresh logs shrinks, but the contraction upon drying is greater in the tangential direction than the radial.  Therefore, upon drying, changes in shape of wood cut from various parts of the log cross section is shown in the image below.  Every sawyer knows that flat-sawn tangential boards are likely to cup, while quarter-sawn (radial) boards are not.  Savy woodworkers pay a lot of attention to these shape changes, some of which can render nice boards useless.  In addition, sapwood shrinks more than heartwood, so boards with sapwood usually spell trouble. 

A schematic cross section of a log showing how the boards from different regions change shape as they dry and shrink.

            More interesting examples of water-induced shape changes include the long, coiled awns of storkbill and cranesbill seeds. The inner side responds more strongly to absorption and desorption of water, so the awn uncoils and coils with the day-night humidity cycle, or with rain, drilling the seed into the ground--- a self-planting seed!  The seed even has backward pointing hairs, so it doesn't pull back out easily.

The Anza Borrego Desert at Borrego Springs, California. Erodium cicitarium is a common plant in the flats here, especially after rains.

Left: a storksbill plant (Erodium cicitarium) with flowers and seed heads. Right: a single Erodium seed in the dry state. You can watch it plant itself here.

Blow your moist breath over any of these seeds, and the awn unbends or uncoils, then bends or coils again as it dries.  If you attached the seed to an immovable surface, the awn would indicate the relative humidity.  Strictly analog and much cheaper than a digital hygrometer (a device for measuring air humidity).

The same thing happens with the kinked awn of bushman grass in the Namib Desert in southwestern Africa. I have not tested the response of bushman grass to water, but I assume that the kink unbends and bends again, humping the seed into the sand because the plumose end resists movement.

An aerial view of the Namib Desert of Namibia, the habitat of bushman grass. The bare gaps are “fairy circles” that form in a continuous matrix of sparse grasses of this region. Their origin is controversial, but is most likely the result of competition for water.

Left: a seed head of bushman grass, with several projecting seed awns. Right: Two seeds showing the lateral bristles and the feathery awns.

For that matter, many hygrometers, including one I used to own, are (or were) based on the contraction and relaxation of hairs (e.g. horse or human, made of the protein keratin). Below is an antique hygrometer built on that principle. There probably are not a lot of women who are unaware of the effects of humidity on hair. I can still hear graduate student Fay’s southern lady voice intoning, “Ah just washed mah hay-uh (hair), an’ ah cain’t do a thang with it!”

An antique hydrometer that indicates the relative humidity as the hair (left of the central support) contracts or extends (catalogue.museogalileo.it).

            Since all these changes depend on molecules, you might think that it should be possible to build molecular machines that move levers, spin, dance, and do all kinds of complex motions.  Indeed, nanotechnologists have built little walking robots smaller than a Paramecium--- under repeatedly changing conditions, the robot warps and unwarps, hobbling forward in its microscopic world.  The power of water is irresistible at all scales.