Incompatible pollen in this homomorphic species is inhibited on the stigma which suggests that determination of the pollen reaction might be sporophytic. On the other hand, the genetical evidence so far obtained suggests that the reaction of the pollen is under gametophytic control. Furthermore, the pollen is binucleate and callose appears to be laid down inside the grains of incompatible pollen which exhibit a zone of intense fluorescence when viewed under the microscope. The implications of these findings are discussed.