1. Cytological examination of the male meiotic divisions of Hieracium yields the following results: a. Hieracium venosum, H. paniculatum, and H. scabrum of the subgenus Stenotheca are characterized by a diploid chromosome number of 18 and exhibit perfectly normal reduction divisions. b. Hieracium canadense, H. murorum, and H. smolandicum of the subgenus Euhieracium are triploid species. Various degrees and combinations of non-pairing, lagging, extrusion, semiheterotypic divisions, polycary, polyspory, and pollen sterility are found in these species. c. Hieracium pratense of the subgenus Pilosella is a tetraploid species and reveals lagging and extrusion in the meiotic divisions. d. Hieracium flagellare is a polyploid species, and the reduction divisions show irregularities such as incomplete pairing, lagging, extrusion, polycary, polyspory, and a marked difference in the size of the pollen grains. 2. The species of Euhieracium and Pilosella exhibit the cytological peculiarities of known hybrids, as well as being polyploid. 3. The species of Euhieracium and Pilosella develop embryos apomictically. 4. The presence of taxonomic difficulty in the subgenera Pilosella and Euhieracium, together with the facts of irregular meiosis, polyploidy, and the apomictic development of embryos, is considered as evidence for the hybrid origin of the species of the subgenera here discussed. 5. Hybridism is probably one of the ways by which polyploidy arises, as well as one of the ways by which a multiplication of species is brought about.