Cross-Section of a Cactus Blossom
This is what is called a perfect flower-that is to say one that has both stamens and pistil. The stamens are grouped in a circle about the pistil, as in case of the poppy, this being the typical arrangement. This picture shows the seed case or ovary at the base of the pistil. Each ovule must receive the nucleus of a pollen grain or it will not develop. Where the stamens are thus clustered about the pistil, cross-fertilization is usually prevented by the maturing of the two sets of organs at different periods.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 3 Chapter 3