Photographed at Ookakoen, Yokohama (Copyright Heritage of Japan)
“The keyaki (Zelkova serrata) is one of Japan’s most familiar trees. Even at a great distance the distinctive shape, like an upturned broom, is easily recognized. This tree grows wild in the local woodlands, but is also a very popular street and park tree. Many farmers also plant one or two around their yards.
There are six species in the genus Zelkova, distributed from southern Europe across south and east Asia. Botanists believe that they were once more widespread, but were pushed southward by the glaciers. One rare species (Z. sicila) grows only on the island of Sicily. The keyaki (also called Japanese zelkova) is Japan’s only native species, and is also found in China and on the Korean Peninsula.
Zelkovas are classified in the Elm Family, or Ulmaceae (nire-ka in Japanese), but are not true elms (genus Ulmus). Several species of true elm do grow in Japan, but these can be easily distinguished from the keyaki by their characteristic winged seeds, called “keys” or “samaras” in English. The thin membranes surrounding these seeds are designed to serve as sails, helping the seeds disperse on the wind.
The keyaki is a deciduous tree, and the branches will be bare in winter. Still, the silver-gray trunk, pocked with numerous scars where the thin bark has flaked off, is incredibly beautiful, especially when lit up by the last rays of the sinking sun. Also, a few seconds searching the ground under the tree will turn up some leaves for study. These are oval or egglike in shape, with a pointed tip and deep, sharp serrations along the edges.
Surprisingly, although almost all Japanese can recognize a keyaki, very few take notice of this tree’s unique strategy for dispersing its seeds. Mixed in among the leaves will be some interesting objects. These consist of a short twig, with three or four small leaves (about half the size of the regular leaves) still attached. You have to look close to see the tiny, nutlike fruits, only a few millimeters in diameter, tucked into the joints where the leaves attach to the twig.
The keyaki’s fruits are hard seeds covered by a very thin skin, a type of fruit that botanists call an achene (souka in Japanese). These achenes are inedible and thus do not attract birds the way soft, succulent berries do. They also lack the wind-catching wings of their true elm cousins. By themselves would have no way whatsoever of getting away from where they have fallen at the base of the parent tree.
by Kevin Short
But the keyaki seeds do not drop off by themselves. They remain attached, and the entire twig drops off as a single piece. As soon as the small leaves dry out they act as sails, catching the wind and sending the twigs sailing all over the place. You can find plenty of keyaki seed-twigs in the leaf litter stacked up against a curb or wall, hundreds of meters from the nearest parent tree!”
Source: “Sit Beneath the Zelkova Tree”