The so-called boll represents a stage of development of the cotton flower at which the seed is maturing within protective bracts that have not as yet opened. Subsequently the "boll" bursts open, exposing the cotton fiber. When at the stage of development here shown, the boll is subject to the attack of the boll weevil, which has become a pest in some regions of the South.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 8 Chapter 4