The shape of the bamboo might simulate a Luffa (hechima in Japanese, sponge gourd, Luffa cylindrica).

No part of the plant is wasted – all of them are used as medicine, scrubbing sponge, filler, basket making etc.
The Hechima vine produces about one liter of liquid a day in summer and used as hechima-water.

Or the shape of the basket might represent a trunk of a tall erect tree growing high up to the sky, exemplifying upright rectitude in a man.