Periwinkle seeds sown in sandy-loam soil

They say in eastern Flanders that periwinkles are used as bookmarks by anyone who enjoys reading in the fields.
Years later, when these readers come across their book again, they discover that the pages in which they slipped a periwinkle have disappeared behind a blue stain. But they all remember word for word what was written on them, for we usually put down our book when we are really overcome by what we have just read.