THE POLLINATION OF PLUMS.
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.
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.
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.