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イチョウ
Ginkgo, called ichō in Japanese, is a deciduous gymnosperm tree. Widely planted in Japanese temple grounds, gardens, parks and streets, it changes from green foliage to autumn yellow and then bare winter branches.
Ginkgo leaves spread like a fan and often have a central notch, with variation in the notch and outline. The tree carries green foliage through spring and summer, then sheds its yellow autumn leaves. Its broad leaves belong to a gymnosperm rather than a flowering broadleaf tree.
Ginkgo leaves already contain green chlorophyll and yellow carotenoid pigments. As chlorophyll breaks down in autumn, green fades and the yellow becomes more visible. This colour change generally precedes leaf fall, so a tree can carry mostly yellow leaves while still retaining them.
JMA’s yellowing date observes the colour of the whole designated tree. Leaf fall records a different change; a yellowing date does not mean that all the leaves have dropped.
Male and female structures occur on separate trees. Tsukuba Botanical Garden describes exposed ovules at the end of a stalk; these enlarge and their outer seed coat turns yellow. The fruit-like structure is a seed. Street planting often favours male trees to avoid the smell from female trees’ seed coats. Japanese commonly uses ichō for the tree and ginnan for its seeds.
In spring, wind carries pollen from a male tree to the pollination droplet on a female tree’s exposed ovule. The pollen enters with the fluid and develops a pollen tube. Months later, motile sperm are released and fertilization occurs with the egg inside the ovule. Pollination delivers pollen; fertilization is the later cellular union.
Hirase’s observation in 1896 showed that a seed plant retained this motile-sperm stage. Sperm do not swim from one tree to another: pollen first makes that journey on the wind. The photographs show visible male pollen-sac clusters and female ovules; botanical research supplies the account of what happens within.
The Japan Meteorological Agency defines the yellowing date as the first day when most leaves on the observation tree appear yellow. It is distinct from the first yellow leaf or complete leaf fall. The 1991–2020 normals range from late October in parts of Hokkaidō and Tōhoku to late November or early December across many areas farther southwest.
Cold tolerance, relatively few pests and tolerance of urban air conditions help explain ginkgo’s use along streets. Meiji Jingū Gaien describes winter pruning about every four years to maintain conical crowns. The avenue’s outline is the result of planting and maintenance, rather than a form shared by all ginkgo trees.
Ginkgo is the sole surviving species of a lineage once represented by a greater diversity of fossil forms. “Living fossil” does not mean that an individual tree is millions of years old. Nagai Botanical Garden identifies China as its origin and cultivation in Japan by at least the Muromachi period. Local planting histories have a different timescale from the lineage’s fossil record.
The University of Tokyo traces Koishikawa Botanical Garden to a shogunal medicinal garden established in 1684. It became a university facility in 1877. Cultivation and observation there linked Edo medicinal knowledge to modern botany, giving planted trees a setting different from a shrine grove or street avenue.
In 1896, University of Tokyo assistant Sakugorō Hirase discovered sperm in ginkgo, providing important evidence about its fertilization. Although it produces seeds, ginkgo has motile sperm during fertilization. The university’s account places the discovery at Koishikawa, connecting a cultivated tree to the history of modern Japanese botany.
Gaien’s account begins with seeds collected from existing Shinjuku Gyoen trees in 1908. Trees were raised in a nursery, selected from about 1,600 and shaped over time. Taller trees toward the Aoyama entrance and shorter ones toward the gallery work with the ground’s slope to create perspective. Yellow foliage and bare branches reveal different aspects of the same layout.
Ginkgo was chosen as Tokyo’s designated tree in 1966. Widely planted along the city’s streets and in its parks, its fan-shaped leaves and autumn yellow are familiar urban sights.
No. It is deciduous.
A seed, with a yellow outer seed coat.
No. Most leaves on the observation tree must be yellow.
Ginkgo sperm.
Through selection, shaping and winter pruning.
Wind delivers pollen to the ovule; months later, sperm take part in fertilization inside it.
Chlorophyll breaks down, reducing the green and revealing yellow carotenoids already present.
