One of the discouragements in plum-growing is the uncertainty which attends the setting of the fruit in some varieties even though the trees bear an abundance of blossoms. Blooming, the prelude of fruiting, had little significance to the fruit-grower until the discovery was made that many varieties of several fruits were unable to fertilize themselves and that failures of fruit crops were often due to the planting of infertile varieties. Investigations as to the self-sterility of pears, plums and grapes have shown blossoming-time to be one of the most important life periods of these fruits. The knowledge obtained by workers in this field has to some degree modified the planting of all orchard-fruits and of none more than of the plums. Indeed, it is held by many that it is hardly safe to plant any excepting the Domestica and Insititia plums without provision for cross-pollination. A variety is in need of cross-pollination when the pollen from its own blossoms does not fecundate the ovules of the variety. There is a delicate and complicated procession during the process of fruit formation and the life of the fruit may be jeopardized by any one of a number of external or internal influences. These deleterious influences are most often unfavorable weather or defects in the reproductive organs of the plants themselves. Of the latter, in the plum there are several rather common ones which cause self-sterility, as impotency of pollen, insufficiency of pollen, defective pistils and difference in the time between the maturity of the pollen and the receptiveness of the stigmas. It is held that the main cause of the infertility in plums is impotency of pollen on the pistils of the same variety. The pollen may be produced in abundance, be perfect as regards appearance, and potent on the pistils of other varieties but wholly fail to fecundate the ovaries of the variety from which it came. The most marked examples of such impotency are to be found in the native plums though the Triflora sorts are generally accredited with being largely self-sterile and the Domesticas somewhat so. The proof offered to show the impotency of plums is for most part the records of fruit setting under covered blossoms. In this method of testing the impotency of pollen there are several sources of error and the figures given by experimenters probably greatly exaggerate the infertility of plums, but since the experience of plum-growers generally affirms the results in some measure it is well to hold that the native plums at least should be so planted as to secure cross-pollination. It is doubtful if the Domestica and Triflora plums are self-sterile and yet the question is an open one as regards some varieties of these species.