In fact, a lot of literature and historical books, including the Tale of Genji, say that a Sensu had Waka (poem) written on it or were decorated with some flowers.
具体的には和歌を書いて贈ったり、花を載せて贈ったりしたことが、源氏物語など、多くの文学作品や歴史書に書かれている。