Sterility in Pears

Orchard Work.—Fertility and sterility studies with Bartlett and related pears have extended over a 12-year period. Self- and cross-pollinations were made of 13 varieties of the Bartlett group and many reciprocal crosses and self-pollinations effected between parent species and cultivated varieties. A total of 143,516 blossoms were used (1924-35) and at one time 180 trees were under survey. The Bartlett group, comprising 13 sorts, was assembled and grown to secure flowers for pollination, to develop fruit for observation and to save seed in order to raise seedlings for progeny studies. A few of the newer members of the group have been planted in the orchard but have not as yet been used!. Sterility of the Bartlett pear is not constant, but varies somewhat with season and location. Moreover, this variety is not absolutely self-sterile under all conditions since an occasional fruit sets with its own pollen, but seed formation, indicating fertility, is rare. When seed is formed it is often non-germinable. Fruit setting is more common than seed setting. Histological Work.—Most of the pollen grains of all of the varieties used were well formed and viable in artificial media. Studies of the nature of pollen tube growth in cultures indicate that pollen may be deemed to be a one-celled organism with growth responses similar to those of such simple organisms. Studies of pollen tube growth in the pistils of several varieties show that neither the pollen grains nor the pistils considered by themselves are defective. For example, in the case of self-pollinations and reciprocal crosses between Bartlett and l Guyot, made in 1930, Guyot pollen grew well in Bartlett pistils and brought about fertilization, as did the Bartlett pollen in Guyot pistils. However, when these two varieties were self-pollinated the tubes extended only part way down the styles. Thus, in the interaction between the pollen tubes and the pistils of the same variety, there are various factors involved which bring about, first, a retardation and, finally, a cessation of pollen tube growth