Photo/Illutration Pale pink and white flowers of silk trees on Awajishima island in Hyogo Prefecture (Provided by Hisanobu Satoguchi)

The rising mercury spells the arrival of a seasonal hiatus in the blooming of flowering trees and shrubs.

Hydrangeas are mostly past their prime, leaving the stage vacant until crape myrtles enter the scene in midsummer.

Or so I thought, until I happened across a flowering silk tree (Albizia julibrissin).

Its pale pink and white flowers with prominent stamens resemble fine silk threads. They not only look extremely delicate, but also feel feather-light to the touch.

The fern-like leaves fold closed and “sleep” at night. Hence the tree’s Japanese name “nemu no ki,” which translates literally as “sleeping tree.” Variations of the name include “nebu” and “neburi no ki.”

The kanji characters used for writing these names are said to be varied, but all evoke images of leaves huddling together. For one name, even just looking at its characters can make me drowsy.

Sleeping movements, known as “foliar nyctinasty,” are observed in other plant species, too.

According to “Shitatakana Shokubutsu Tachi” (Tough plants), a book by plant ecologist Taeko Tada, different plants have different sleeping styles.

Clovers, for instance, sleep with their three leaves standing up, while the leaves of sleeping “shiso” perillas are droopy.

Why plants sleep has yet to be explained, but the law of nature is obviously at work.

In contrast, humans are animals whose sleep patterns are determined by the type of society they inhabit. They sometimes secure time to work or commute by cutting down on sleep.

According to a survey by Video Research Ltd. and Dentsu Inc., young people’s sleeping hours have increased about 10 percent over the last 10 years, and smartphones are counterintuitively believed to be one of the causes.

The researchers say many people fall asleep looking at their smartphone screens while lying down. It appears these ubiquitous little devices are not necessarily keeping young people wide awake at night.

A silk tree in a park near my home stands very close to a streetlight. I felt so sorry for the tree, worrying it wasn’t getting quality sleep.

But when I looked at it closely, the leaves were tightly folded shut. It was good to see the tree’s surprisingly tough side, which I would never have guessed from its delicate daytime appearance.

--The Asahi Shimbun, July 8

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Vox Populi, Vox Dei is a popular daily column that takes up a wide range of topics, including culture, arts and social trends and developments. Written by veteran Asahi Shimbun writers, the column provides useful perspectives on and insights into contemporary Japan and its culture.

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