Self-sterility and self-fertility are greatly influenced by the condition under which a variety is grown. Thus, a variety is often self-sterile in one locality and not in another. Occasionally Bartlett, usually nearly or quite self-sterile, and other varieties as well, set fruit one season and not the next. All pears, the Bartlett in particular, seem to have a greater degree of self-sterility in eastern pear regions than on the Pacific slope. In general, the better the adaptation of a variety to its environment the better it sets fruit with its own pollen. It is obvious, therefore, that it is not possible to give lists of self-sterile and self-fertile varieties. Such lists can be made out only for regions and localities. Some varieties, however, more often fail to set fruit because of self-sterility than others. Among standard pears, Bartlett, Beurré d'Anjou, Beurré Clairgeau, Clapp Favorite, Howell, Kieffer, Lawrence, Sheldon, and Winter Nelis appear to be most often self-sterile. Beurré Bosc, Flemish Beauty, and Seckel are usually self-fertile. A self-sterile variety usually sets fruit when another variety is at hand to supply pollen. Several considerations determine the selection of varieties to interplant. Thus, the two varieties must blossom at the same time if cross-pollination is to be effective. The table on pages 88 to 90 shows the sorts that bloom at the same time, or nearly enough so to make cross-pollination possible. Under normal conditions, the blooming time of varieties overlaps sufficiently for cross-pollination excepting those that bloom very early and very late. If the table is used for regions much to the north or to the south of this Station, allowance must be made for a shorter blooming period the farther north; a longer one the farther south. That varieties of pears have sexual affinities is another consideration that merits some attention. That is, one variety will fertilize another sort very well, while pollen from a third may not be at all acceptable. "Affinities" can be determined only by hand crossing. Probably the importance of affinities is over-rated. The distance between varieties set for cross-pollination must not be too great--not more than two or three rows apart. For convenience in harvesting, varieties should be selected in relation to ripening. Only commercial varieties should be interplanted, as the wastage is too great if comparatively worthless sorts are set to fertilize a standard commercial variety.