Swamp Wings

The cattails sprouting exuberantly now in Alaska's roadside bogs are remarkable plants. These big, homely reeds provide shelter for swamp-dwelling creatures and food for many kinds of animal life, from tadpoles nibbling near their roots to humans foraging for wild fare in the wetlands. Their dried remnants get incorporated into many waterfowl nests, and their slender leaves inspire aeronautical engineers.

OK, so I made that last part up. But it should be true. Recent analysis of the structure of cattail leaves shows these plants could have taught engineers a thing or two, and perhaps saved them some decades of trial and crashing error.

The cattail study was uncommonly interdisciplinary, involving contributions from botanist Ursula Rowlatt of Britain's Kew Botanical Gardens, and engineer Henry Morshead of the Department of Aeronautics of the Imperial College of Science. They may have found inspiration in the vicious windstorms that have hit Britain in recent years, storms that have left centuries-old trees broken and uprooted. Cattails, however, endure the storms unscathed. Understanding why a thin leaf perhaps eight feet long but only an inch or two wide can deal with gusts that snap off feet-thick oak trunks challenge the scientists, and they joined forces to work it out.

The engineer found much that was familiar in cattail design, both on the inside and outside of the leaf. Cattail leaf blades internally are essentially stacks of little shoeboxes laid endwise, rectangular compartments arrayed in multiple columns. This arrangement provides what he knew as "closed-box rigidity," a strong but lightweight structural arrangement.

Adding to the overall strength is the network of membranes separating the rectangular columns---the diaphragms serving as the ends of the shoeboxes. Here Morshead was struck by the cattail's adherence to good engineering practice. The diaphragm thicknesses are strictly ordered in a sequence of eight, which---and I'll have to trust him on this---gives maximum strength. The first third, fifth, and seventh diaphragms are thin; the second, fourth, and sixth are thicker; the eighth is always the thickest.

Sheathing all this is a network of parallel vertical fibers, running vertically the length of the leaf and giving it a finely ribbed surface. The whole leaf is flat on its inner surface, rounded on the exterior. Looking at a cross section of a cattail leaf is eerily like looking at a cross section of a modern airplane wing in miniature. The curve, the light but strong hollow structures within, even the fibrous sheathing all could be taken as high-tech design.

So, thanks to its internal arrangements, a cattail leaf should be able to deal with the effects of wind about as well as an airplane wing can. But its external form is just as important in explaining its ability to endure high winds. From base to tip, the blade twists upward and clockwise, performing one and a quarter to one and a half turns (that is, if one full turn is 360 degrees, the cattail leaf turns through 450 to 540 degrees). The researchers believe this partial twist is what really gives the cattail its ability to withstand wind. Going by their measurements and calculations, Morshead and Rowlatt conclude that the twist so minimizes the lift and drag forces generated by air moving over the blade that the net sideways force exerted on the leaf is close to zero.

Finally, in circumstances where external shape and internal strength might not suffice, the cattail can call on its well-designed mounting. The plant's tangled rhizomes absorb much of the wind energy striking its above-ground vegetative mass.

All in all, the common cattail leaf functions very well both botanically and aerodynamically. Next time you see a swampful of these wonderful weeds, think of them as an array of green wings---designed to let the wind fly right on by.