Word of the Week: Samara

samara [ samer-uh, suhmairuh ] noun: a winged achene 

 

The samaras of native Liriodendron tulipifera (tuliptree) are aggregated in a cone-like structure. The seed cavity is within the distinctive keel-like projection at the samara base. Photo © Mary Free

In some plants, the pericarp of an achene extends into a papery, wing-like tissue longer than the seed. This fruit is called a samara. The wings enable the wind to carry the seeds farther from the mother plant than wingless seeds. Some samaras hang alone or in clusters from tree branches. Others, like those of Liriodendron tulipifera (tuliptree), Alnus (alder), and Betula (birch) are borne in a cone-like infructescence called an aggregate fruit.

The distinctive, keel-like projection at the base of a tuliptree samara contains the seed cavity that houses the achene. The samaras are aggregated in a “cone,” which the wind (or hungry wildlife in search of seed) break apart late fall through winter, leaving behind a base whorl of samaras on the branches. Individual samaras (up to 1¾-inch long) spin or flutter as they travel on the air currents sometimes up to two football fields away. Their seeds can remain viable for several years. The fruit of Alnus serrulata (smooth alder, hazel alder) matures in the fall, but even after the wind (or water) disperses the tiny, narrowly winged seeds, the ¾-inch, cone-like clusters of scales that enclosed them can persist through winter. The samaras of Betula nigra (river birch) are aggregated in a hairy cone-like structure, up to 1½-inch long, but when the fruits ripen, the samaras break apart and disperse, disintegrating the cone.

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The samaras of Acer (maple) and Fraxinus (ash) also have wings on just one side of the achene. These appear on maple trees usually in joined pairs–double samaras–hanging from the branches. As the two samaras dry, a crack forms between them and they eventually separate. When they split apart, each disperses like a “helicopter or whirly bird.”

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Some of us may have fond memories of our fascination with maple samaras as children. A Master Gardener once relayed a story in which a pilot had expressed surprise at her knowledge about autorotation. But just like a helicopter that is descending without engine power, a maple samara wing acts like a propeller rotating the seed slowly to the ground. Research has shown that this creates a leading-edge vortex, similar to that created by hovering insects and hummingbirds, which “lowers the air pressure over the upper surface of the maple seed, effectively sucking the wing upward to oppose gravity, giving it a boost.” (Lentik 2009) This allows the seed to travel far from the mother tree where a soft landing may give it a chance to grow and thrive.

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The samaras of  Ulmus (elm) and exotic invasive Ailanthus altissima (tree-of-heaven) have wing-like tissue that entirely surrounds the achene. The elm samara is almost round and notched on one end. The 1-to-2-inch-long samaras of tree-of-heaven twist at the tips. The samaras hang in clusters from the tree branches and flutter or spin as they fall to earth.

References

Lentink D. 2009. Maple Seeds and Animals Exploit the Same Trick to Fly. California Institute of Technology. [accessed 2021 Jan 9].

 

 

 

 

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