無常: mujô. If we were to imagine Renku's Mandala of All Creation laid out in the form of a traditional Buddhist or Hindu mandala, the categories would radiate outward, like the petals of an immense flower, but one category would occupy the center—the very heart of the matter—and that category would be impermanence.
The spiritual exercise of renku involves the contemplation of the transitory nature of all things. We meditate on things (and people), one by one, but we move forward, in the poem, and they all fade behind us. They are no longer present, here and now, in the flow of the poem. Because this is the core insight of renku (and of Buddhism), an image of impermanence within a verse (complementing the experience of impermanence as one moves from verse to verse) can be the perfect move in this poetic game.
The most iconic image of impermanence in Japanese poetry is morning dew: those glittering, splendid orbs of shining light that won't survive to noon. But this impermanence creates their preciousness and their beauty. As you explore the universe through renku, try to think of other images that remind us of this truth about all things. Even the mountain will be dust.