More information on this topic can be found: F.C. Stern: Study of the Genus Paeonia (here)

There is a very beautiful form of this paeony known as P.X emodoff which has been distributed from the Glasnevin Gardens in Dublin. It grows up to 100 cm. (3 ft. 4 in.) in height and makes a big bush in the garden. Sir Frederick Moore informs me that it is said to be a cross between P.officinalis alba and P.emodi and that P.officinalis alba is the female parent. Comparison with dried specimens of P.emodi and with plants raised from seed of the wild P.emodi reveals no differences except that it is larger in all its parts and grows to a greater size. Again, it comes true from seed and is a diploid like P.emodi so that it is highly improbable that it can be a hybrid with P.officinalis, a tetraploid, as its seed parent. I, therefore, conclude that it is no more than a specially robust form of P.emodi