Imperfection of Pollen and Mutability in the Genus Rosa

Abstract

During the winter of I915-I9i6 I made a study of all the species, of which specimens were obtainable, of the genus Rosa. This was done in connection with work on other genera of the family Rosaceae, notably on Rubus and on Crataegus; and it has been interesting to note that in all 3 genera there is indication of a large amount of hybridism, and that the multiplication of species is startlingly great. From the Arnold Arboretum of Harvard University, I have been able to obtain flower buds of 32 different species of Rosa. Of 3 of these species I have several varieties; 5 varieties of R. spinosissima, 3 of R. rugosa, and 2 of R. virginiana. The buds were taken when on the point of opening, thus making sure of the maturity of the pollen. They were then preserved in alcohol until such time as it was possible to examine them. I prepared sections of about half of the species gathered, with a view to determining how many showed sound pollen and how many showed a greater or smaller proportion of undeveloped or imperfect pollen. For this purpose the buds were first imbedded in celloidin to make sure that there would be no shrinkage of the parts. Sections were cut with the microtome, stained with Heidenhain's haematoxylin and safranin, and finally mounted in balsam. For nearly a century it has been known that one of the most important and most easily recognized characteristics of a hybrid is imperfect pollen. DUTROCIIET' in i832 recognized the morphological sterility of the hybrid and pointed out that pollen abortion is a criterion for hybridism. GAERTNER,2 in i849, speaks of the