The Breeding System in Primula Veris L.

S ummary Modern textbook accounts of within‐morph incompatibility in Primula usually assume that it is total, or nearly so. However, few published accounts can be traced concerning the efficiency of the incompatibility. These suggest that it is total in P. obconica , but rather weak in P. vulgaris, P. veris, P. auricula and P. sinensis. Our work in P. veris suggests that for one population, within‐morph incompatibility is nearly total at normal temperatures. The mechanism of incompatibility varies between the morphs. In within‐pin pollinations, pollen germination is good, and tubes short and broad, being apparently unable to penetrate stigma papillae for physical reasons. In within‐thrum pollinations, pollen germination is poor and the few tubes produced are broad and misshapen. These and the surrounding stigma papillae fluoresce strongly, and similar effects can be produced with pollen‐free water extracts. A tentative model of the control of incompatibility in Primula is based on an enzyme dimer mode of action.