Pistillate Hop Plant
The hop is unique among plants having large economic importance in that the only part of it that has value is the flower. The bitter principle, called lupulin, developed in the flower, has great value from the standpoint of the brewer; and hop-growing is an important industry wherever beer is manufactured extensively. It is suggested that selective breeding might improve the quality or enhance the quantity of the essential lupulin borne by the hop flower.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 8 Chapter 5