Take a close look at Li Bo gazing at the waterfall in these two Tokugawa-period images. If you can't make out the details click here for a close up of the relevant part of the top image and here for a close up of the relevant part of the bottom image.

To compare with the Muromachi-period image, click here.

The difference should be obvious. In addition to the outstanding addition to Li Bo's added feature, notice the opening through which the water falls in the top image.

The brief poem is 李白一本開百番, which, rather literally, means "Li Bo's one prong led to a hundred others." While the poem is intended to allude to Li's erect member, what it is saying overall is that Li's waterfall poem became so famous that similar imagery became a stock element in poetry in China and Japan. Thus Li's one poem led to many ("hundreds") of others like it.