Poison?

He was desperate. The witch had given him the recipe; one thimble full of snow lichen, boiled for five hours in waters gathered from the Pool of Licarni. Then after he had cooled the liquid and kept it cold for five nights, stirring constantly, and if he could persuade Mylin to drink it, the witch had said it was possible she would consider him as a mate.

But as he walked out away from the witch, his coins rattling in her hand, she asked him if he knew snow lichen. He had nodded and kept walking. He had been certain he knew.

But now, starting at the one growth of litchen he'd found, he wasn't so sure.

Snow lichen crept outwards in ever growing circles, the newest always being white. If he looked carefully he could see rings of whites and greys, each ring a year's growth. Snow lichen was black where the roots clung to the rock.

But so was spring lichen.

The difference was the grey was faintly green.

And spring lichen was poisonous.