Derivations from the normal tetrasporangiate, dithecal pattern of angiosperm stamens are presented in a comparative study of numerous taxa and a literature survey. Regularly occuring polysporangiate stamens (with more than 4 sporangia) are known from 18 families, all dicotyledons, regularly occurring disporangiate stamens from c. 50 families. Most polysporangiate stamens have retained thecal organization with two longitudinal dehiscence lines: each of the 4 pollen sacs is simply transversally and sometimes also longitudinally subdivided into numerous sporangia. Thecal organization is lost in some Viscaceae, Polyporandra (Icacinaceae), and Mitrastemonaceae, where each sporangium individually dehisces by a pore, and in Rafflesia, where all sporangia merge into one large pore. These deviating stamen forms are often correlated with specialized pollination biology: athecal polysporangiate anthers with pollen presentation in mucilaginous masses through pores and fly pollination; dithecal disporangiate anthers with (1) also pollen presentation in mucilaginous masses through relatively small openings and often fly pollination, (2) precise secondary pollen presentation on the gynoecium, (3) cleistogamy, (4) opening with valves. In these cases efficient wide gaping of 2 mirror-symmetrical halves of each theca is not possible. The biological context and the systematic distribution strongly support the hypothesis that the cohesiveness of the basic dithecal, tetrasporangiate pattern is at least partly due to functional constraints at the time of pollen release from the mature anthers.