The morphology and cytology of the three British species of poppy in Section Orthorhoeades, Papaver rhoeas L. (2n= 14), P. lecoqii Lam. (2n= 28) and P. dubium L. (2n= 42), and their hybrids were studied. The chromosome numbers of the three species suggest that they are of diploid, tetraploid and hexaploid origin respectively. Chromosome pairing during meiosis in the hybrids suggests that P. rhoeas is only distantly related to P. dubium and P. lecoqii but that the last two species are much more closely related.

The relationships between the three species and some variant, self-fertile plants, originating from Henley-in-Arden, Warwickshire, were also studied. These plants have a chromosome number of 28 and the chromosome pairing behaviour in hybrids involving them is similar to that in hybrids involving P. lecoqii. However, hybrids between the Henley plants and the other three species were completely sterile. The stunted growth form of these hybrids suggests that genie differentiation has occurred in the Henley plants which upsets the physiological processes governing growth in the hybrids.

It is postulated that the polyploid species possess a mechanism which prevents homoeologous pairing. The mechanism appears to operate normally in hybrids between the polyploid species but appears to break down in hybrids involving the diploid species, P. rhoeas.

The significance of all the results obtained is discussed in terms of the origins and past evolution of poppies belonging to Section Orthorhoeades in Britain.