There is great difference in the quantity of pollen produced by the several groups of plums but it is very doubtful if insufficiency of pollen is a factor of any considerable importance in the self-sterility of this fruit. Yet the matter is worth attention because of its bearing upon the selection of pollinizers. Of the several botanic groups, speaking somewhat generally, the Americanas and Nigras bear most pollen; the Munsoniana plums are abundant pollen bearers; the Trifloras seldom show a shortage but bear rather less than the others named; the Domesticas produce pollen abundantly; while the hybrid groups are the most capricious of all the plums in this respect, some varieties bearing much and others but little pollen. Probably the amount of pollen which the flowers of any tree produce is somewhat modified by the climate in which the tree is growing, by the weather and by the vigor of the tree. Waugh[147] and Goff[148] have shown that self-sterile plums often have abnormal pistils or pistils too weak for the development of fruits. Not infrequently flowers of the plum are without pistils, as occasionally, but less rarely, occurs with the stamens and petals. These abnormalities cannot be very general causes of self-sterility in plums, however, as varieties, or even trees, cannot often be found which are not fruitful if other varieties are growing near them. It is very doubtful if even so much as fifty per ct. of abnormal flowers, seriously jeopardizes a plum crop, as the trees bear, if they blossom at all, several times as many flowers as they can mature plums. But a high percentage of abnormal flowers nearly always indicates a general weakness in fruit-setting. Another cause often assigned for the failure of plums to set fruit is the difference in time of maturity of stamens and pistils. It is claimed that when these organs do not mature simultaneously the plums do not set unless pollen is supplied from some other source. The task of taking notes at blossoming time on more than three hundred varieties of plums on the grounds of this Station has given abundant opportunity to observe the comparative degrees of maturity of pistils and stamens in varieties of this fruit. In general the pistils mature first, often three or four days before the stamens. Rarely the pollen is disgorged before the stigmas are receptive. But stigmas remain receptive, weather conditions being favorable, for several days and the pollen from all anthers is not shed at once and is produced with such seeming prodigality as to almost insure the retaining of a sufficient amount to pollinate late-maturing stigmas. In view of these considerations, premature or retarded ripening of either pistils or stamens does not seem of great significance in the setting of fruit.