In "Ode on Melancholy", Keats uses the images of three poisons in the first stanza: Wolf's bane, nightshade, and yew-berries. Are these poisons simply meant to connote death/suicide, or might they have a deeper purpose?
Keats may have chosen the first two of these specific poisons because he could associate them with grapes and wine, and he may have chosen yew-berries because they look like beads. The first poison is:
Wolf's-bane, tight-rooted, for its poisonous wine,
and in mythology, Medea gave Theseus a cup of wine poisoned by wolfsbane.
The second poison is
nightshade, ruby grape of Proserpine;
Bitter nightshade is a vine with poisonous red berries which grow in clusters somewhat resembling bunches of small grapes. So again, we have an association with wine.
Yew berries are also red, although they look much less like grapes. Yew berries actually look like a little bit like beads, in that it looks like they have a hole through them (it actually only goes halfway through, the extremely poisonous seed being in the hole). Keats' line is
Make not your rosary of yew-berries,
What is the significance of this line? Rosaries are used for prayers, so possibly what Keats means here is "don't pray to death."
The theme of poisonous wine or grapes returns in the third stanza:
... and aching Pleasure nigh,
Turning to poison while the bee-mouth sips:
... save him whose strenuous tongue
Can burst Joy's grape against his palate fine;
His soul shalt taste the sadness of her might,
And be among her cloudy trophies hung.