Only the wind turns the heads of the narcissus

EFinally comes the season when narcissus and daffodils begin to point their golden trumpets skyward, trumpeting the arrival of spring. This appearance of trumpets in concert is due as much to their tube-shaped central crown, formed of petals welded to each other, as to the characteristic inclination of the floral head on one side which differentiates them from other liliales whose corolla remains erect, perched at the top of the stem, like tulips.

When it sprouts from its bulb and emerges from the ground, the narcissus stem initially points in a vertical direction; the force exerted by its growth and its rigidity allow it to push back some pebbles which would obstruct its way towards the surface. A couple of engineers from Long Island (New York, United States) evaluated the force required to bend narcissus stems yolks of different lengths, by applying a constraint to their two ends. Endowed with a rigidity ten times greater than that of dandelions, a stem of narcissus of a few centimeters opposes a force of the order of 10 newtons, which allows the young shoots to cross the frozen ground in winter.

The stems then lengthen; their growth is much faster under plant cover, which suggests that a process of avoiding shadow promotes their growth in order to “escape” towards the light. Shortly before flowering, the flower stalk bends at the junction with the flower, first at 90°, before straightening towards an angle of 45°. To find out if this inclination was in a preferred direction, the couple from Long Island recorded the orientation of 193 narcissus flowers distributed along a 3 kilometer road, as well as the position of buildings in the neighborhood that were possibly light screen for each of them. Most of the flowers recorded (60%) were facing east; moreover, the flowers never pointed in a direction where the light was obscured.

Facing the light

This apparent phototropism of daffodils, the precise physiological mechanisms of which are not yet known, differs from that of sunflowers and dandelions whose floral head rotates during the day to follow the apparent path of the sun. Although favoring maximum illumination, the direction here remains fixed during the day… except when the flowers find themselves buffeted by a gust of wind.

The narcissus caught in the wind was quantified twenty years ago by two researchers at Duke University (North Carolina, USA). As the flowers lean to one side, the action of the wind puts twisting and bending stress on the stems. Due to their structure and shape, whose double ogive section has a slight helical asymmetry along the axis, the rods are more prone to torsion than to bending. Under a moderate wind, they twist momentarily, leaving the flower to turn its back to the wind, which reduces its hold by 30%. On the other hand, the upright posture is preserved, as long as the wind is not too violent, by the relatively high rigidity of the rods, solid at their base and hollow at their upper end. When the force of the gusts increases, the outer corolla of the flowers folds into an increasingly tight cone around the inner crown, further minimizing wind resistance and preventing the latter from damaging the flower.

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