North American black-fruited hawthorns .2. Floral development of 10- and 20-stamen morphotypes in Crataegus section Douglasii (Rosaceae: Maloideae)

Crataegus section Douglasii exhibits variation in stamen number per flower typical for the genus throughout North America. To understand the developmental basis for this variation we studied the early floral ontogeny of the three taxa in section Douglasii: C. douglasii (both Pacific northwest and the upper Great Lakes basin), C. rivularis, and C. suksdorfii. Crataegus suksdorfii, like all known diploid Crataegus, has approximate to 20 stamens; the two other taxa have approximate to 10 stamens, a condition associated only with polyploidy. In all taxa petal primordia and a whorl of dye pairs of stamen primordia develop from five common primordia. The 10-stamen condition results from loss of two whorls of five stamens that are subsequently formed in C. suksdorfii. Loss of these two whorls in the 10-stamen taxa is the result of neither a smaller floral apex at initiation, nor a smaller flower at anthesis. Stamen number variability, particularly in C. douglasii and C. rivularis, is the result predominantly of fewer than two stamen primordia developing between adjacent petal primordia. Pollen production in C. douglasii is half that in C. suksdorfii because of the reduction in stamen number. The results are presented and discussed in terms of morphogenetic explanations of meristic variation.