Abstract
Cytological studies on a species hybrid between two diploid Rosa-species (R. multiflora _??_ × rugosa _??_) were carried out. The hybrid is found to have 1-7 (mostly 5-7) pairs at first metaphase and the bivalents exhibit normal chromosome behavior throughout the course of the maturation divisions. The univalents split longitudinally in the first anaphase and they are distributed to the poles at random in the second anaphase.
There should be no pairing of chromosomes at meiosis in this hybrids, as the parents belong to the different fundamental diploid species of Hurst (1928). After the classification of Hurst R. multiflora and R. rugosa have A- and C-septets respectively.
The hybrid produced only empty pollen grains and the artificial pollination with multiflora-pollen gave no fruit. The sterility seems to be caused by physiological conditions affecting growth of the micro- and macrospores during the gametogenesis.
It must be mentioned in this connection that Erlanon (1931) has made cytological studies on a species hybrid between R. rugosa (CC) and blanda (DD). Contrary to the expectation from Hurst's theory seven pairs of chromosomes at diakinesis were observed and the hybrid set good fruit.
These findings do not support Hurst's theory of five differential septets in Rosa.
