From the statements just made it may be seen that the main cause of the failures to set fruit when trees bloom freely must be ascribed to the failure of pollen to fertilize the pistils of the flowers of the same variety. The solution of the problem of self-sterility in the main, then, is to so plant that varieties will be mutually cross-fertilized. In the selection of varieties for such cross-pollination two factors must be considered, simultaneity of blossoming and sexual affinity. It is evident, if cross-fertilization is to play an important part in fruit-growing, in planting to secure it kinds must be chosen which come into blossom at the same time as those they are expected to fertilize. The table on pages 106 to 109 shows the sorts that bloom together or nearly enough so to make cross-pollination possible. It will be found upon examining the table that, under normal conditions and during the average season, varieties of any one species overlap sufficiently for the above purpose unless it be the very early and very late varieties. Variations due to locality and to season must be expected but within the bounds of New York these will be slight. If the table is used for other regions than New York it must be borne in mind that the farther south, the longer the blossoming season; the farther north, the shorter the season. Properly interpreted the table of dates should be a useful guide as to the simultaneity of blooming. Varieties of plums seem to have sexual affinities. That is, some varieties will fertilize each other very well and some will not, even though they belong to the same species. There seems to be little definite knowledge as to the sexual affinities of plums and it is not, therefore, possible to lay down exact rules for the selection of pollinizers for individual varieties. In the current discussions of cross-pollination it is probable that the importance of “affinities” is over-rated, and yet the subject is worthy of consideration. Waugh and Kerr have given this subject considerable attention for native and Japanese plums and have recommended a list of pollinizers for the several species.[149] The Domesticas and Insititias, the above writers hold, are best cross-pollinated by varieties from the same species if cross-pollination is essential. The subject cannot be closed without the expression of the opinion that the lack of cross-pollination as a cause of the uncertainties in the setting of fruit has been over estimated in the planting of plum orchards. This expression of doubt is made because there are serious disadvantages in the planting of mixed orchards of any fruit and the question as to whether these do not outweigh the advantages must ever be considered. LOCATIONS AND SOILS FOR PLUMS.