Polyploidy and Geographic Distribution in Spiraea

The genus Spiraea is widely distributed in both the Old and New Worlds, although the center of distribution appears to be in Asia, where the genus is best represented.Polyploidy -both autopolyploidy and allopolyploidy -is a common feature in other members of the Rosaceae, and Spiraea is no exception in this respect.Many species appear to cross readily, which adds to the complexity of the genus.Some cytological work has been done with Spiraea in connection with chromosome relationships in the Rosaceae and especially in relation to the origin of the Pomoideae (Sax 1932).The genus is not favorable for cytological work; the anthers are very small at the time of meiosis, the chromosomes are small and are differentiated from cytoplasmic inclusions with difficulty.The flowering period for the genus extends from April to July, so that collections must be made over a period of several months.Chromosome counts were determined from aceto-carmine smear preparations.Pollen fertility was determined for various species and species hybrids, and the date of flowering was recorded.The basic chromosome number is 9, and not 8, as previously reported.In many cases it was difficult to determine the exact number of chromosomes, especially in some of the polyploids and species hybrids.The data obtained from the various species are recorded in Table I.Most of the species of the first section of the genus are diploids with 9 pairs of chromosomes at meiosis.The two exceptions appear to be autopolyploids.Spiraea myrtilloides, a hexaploid, is similar to S.gemmata, and may be an autopolyploid form of the latter species, even though it is somewhat smaller.The other exception is clearly an autopolyploid of 5. chamaedryjolia, giving rise to the variety ulmijolia.The variety is larger and has a greater distribution than the type, and tends to reproduce vegetatively by suckers.All species examined in* the section Chamaedryon are relatively fertile.All are natives of the Old World and all are early-flowering.Only three species in the section Calospira were studied.SpiraeaMiyabei is an early-flowering diploid but is relatively sterile.Both the 1936