Joseph is a fruitful bough, even a fruitful bough by a well; whose branches run over the wall: The sum of a fruitful vine - This appears to me to refer to Jacob himself, who was blessed with such a numerous posterity that in two hundred and fifteen years after this his own descendants amounted to upwards of 600,000 effective men; and the figures here are intended to point out the continual growth and increase of his posterity. Jacob was a fruitful tree planted by a fountain, which because it was good would yield good fruit; and because it was planted near a fountain, from being continually watered, would be perpetually fruitful. The same is used and applied to Jacob, Deuteronomy 33:28 : The Fountain Of Jacob shall be upon a land of corn, and wine, etc.
The daughters, בנות banoth, put here for branches, shoot over or run upon the wall - Alluding probably to the case of the vine, which requires to be supported by a wall, trees, etc. Some commentators have understood this literally, and have applied it to the Egyptian women, who were so struck with the beauty of Joseph as to get upon walls, the tops of houses, etc., to see him as he passed by. This is agreeable to the view taken of the subject by the Koran. See Clarke on Genesis 39:6 (note).