Bur and Catkin
It always seems matter for surprise that the round burs of the chestnut should grow in catkins that seemed destined to produce fruit-custers of a quite different type. Here is a picture that emphasizes the contrast, as it shows a well developed bur in connection with the remains of the catkin. As the staminate and pistillate flowers of the chestnut are borne in different clusters, cross pollenization is very readily effected.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 11 Chapter 4