Figure 1. Development and maturation of pollen in the anther. Schematic representation of an Arabidopsis flower with filaments and developed anther. Anther cross-section shows epidermis (E), tapetum (T), pollen sac (PS), stomium (St) and endothecium (En); these parts of the anther are explained in the Glossary. Following meiosis, a tetrad is formed with four haploid microspores. Subsequently the microspores are released by the action of callase, an enzyme that degrades the wall of the tetrad. The free microspores then undergo one round of mitosis, producing a vegetative cell nucleus and a generative cell nucleus. The generative cell divides again to produce two sperm cells.