Crossings were carried out between both subgenuses of Mentha in order to find out histologically the cause of the cross-incompatibility between them. Pollen grains could germinate on the stigma of the species belonging to other subgenuses and pollen-tubes could penetrate their style tissue. But both the germination of pollen grains and the elongation of pollen-tubes were somewhat restrained and pollen-tubes which reached the base of style diminished in number as compared with the case of crossing between species in the same subgenuses.