Cytogenetic Studies in Paeonia I. the Compatibility of the Species and the Appearance of the Hybrids.

INCE 1916, the senior author has carried on an extensive series of S crossing experiments in the genus Paeonia, with the object both of obtaining new horticultural forms and of securing evidence concerning the interrelationships of the species and the processes of evolution within the genus. Some accounts of the hybrids have already been published (SAUNDERS 1928, 1933a, 193313). The junior author began cytological work on the species and hybrids in 1932, continuing that started by the late Dr. G. C. HICKS, whose work along with additional data obtained by the junior author has also been published (HICKS and STEBBINS 1934). Since that time the cytological as well as the morphological study of the hybrids has progressed steadily, except that it was somewhat interrupted by the departure of the junior author for California in 1935. The following limitations prevent a cytogenetic study of this genus comparable to that carried on in other genera, such as Crepis, Nicotiana, and Datura. Peony seeds are slow of germination, and the plant takes several years to mature; from the date of hybridization to the season of first blooms is usually a gap of six or seven years. This makes it difficult for one individual to raise many successive generations. An added difficulty is that most interspecific hybrids in Paeonia are completely sterile for a t least two or three years after they first begin to bloom; older plants of almost all the hybrids, when they have established themselves as strong clumps, set occasional seeds, usually not more than one or two to an entire plant. The size of the plants and the cost of their care through so long a period of years also make large scale experimentation for statistical data impracticable. There are in this collection a t present several thousand hybrid plants of blooming age, representing more than thirty distinct strains. The range of attempted crossings was much wider, but as was to be expected many of them persistently failed to produce seed.