Allusion has already been made in this bulletin to the importance of bees in the complete cross fertilization of fruit blossoms and to the fact that certain varieties of pears have been found to be completely self-sterile, requiring, therefore, pollen from other varieties before they can develop perfect seeds and fruits. It is interesting to study the ways in which cross fertilization of plants is secured through the visits of insects. The part that bees perform in the development and perpetuation of numerous ornamental and economic plants is thereby clearly shown. Space will only permit the introduction here of one or two examples. The willow-herb, which is an abundant secreter of nectar and thus attracts bees freely, illustrates one feature in pollination by bees. A young blossom of this plant ([fig. 44, A]) shows the stamens maturing and shedding their pollen, while the pistil remains curved downward and with closed stigmas. In the older flower ([fig. 44, B]), the stamens having shed their pollen and begun to wither, the pistil has straightened up and exposed its stigmatic surfaces for the reception of the pollen which a bee chancing to come from a younger blossom is likely to bring. Self-pollination is thus positively prevented and cross fertilization is insured.
In the mountain laurel the anthers are held securely by little pockets in the corolla, so that as the flower opens the stamens are found bent over ([fig. 50, B]) ready to be liberated ([fig. 50, C]) by the visit of a bee. When the stamen flies up the pollen is discharged from the anther and dusted on the underside of the bee. The latter as it alights on the next flower naturally touches the stigma first and rubs off some of the pollen it has brought from the last flower visited. It then proceeds to secure the nectar of the flower on which it has just alighted, and in doing this liberates the stamens of this flower and gets dusted again with pollen, which it carries to the next flower.
Fig. 50.—Mountain laurel (Kalmia latifolia). A, flowering branch. B, expanded flower: ap, anther pocket. C, section of expanded flower: ap, ap, anther pockets; s, stigma: a, anther (free); pg, pollen grains in shower: ca, calyx. D, section of flower bud: ap, anther pocket. E, stamen more enlarged: a, anther; po, pores; pg, pollen grains; f, filament. (From Cheshire.)
Fig. 51.—Apple (Pyrus malus), showing structure of flower and result of imperfect fertilization. A, blossom: s, stigmas; a, anthers; p, petal; s′, sepal; ca, calyx; d, dissepiment. B, cross section of imperfectly developed fruit: f, f, fertilized carpels; u, unfertilised carpel. (From Cheshire.)
The cross section of an imperfectly developed apple shown herewith ([fig. 51, B]) illustrates the importance of complete fertilization of fruit blossoms. The seed vessel at u shows only an abortive seed, and the side of the fruit nearest this point is also correspondingly undeveloped. This is owing to imperfect or complete lack of fertilization of this carpel, five distinct fertilizations being necessary to produce a perfect fruit. Bees being, during the period of fruit blossoms, the most abundant insects that might effect the necessary distribution of the pollen of these flowers, the importance is at once seen of having an apiary in or near the orchard. Continued rainy or cold weather may keep the bees confined to their hives much of the time during fruit bloom, hence it is advisable to have them near at hand and in numbers proportionate to the size of the orchards, so that even a few hours of sunshine will assure their making a thorough distribution of the pollen. In the absence of accurate experiments regarding the number of colonies of bees required to insure proper fertilization in the orchard, and also in view of the fact that surrounding conditions vary greatly, it is difficult to say exactly how many colonies are positively necessary for a given number of trees. However, four or five well-populated hives for every hundred large apple trees will doubtless suffice, even though no other hive bees are within a mile of the orchard. The bees of a neighbor's apiary are often quite sufficient for the orchardist's purpose, the benefit resulting from their labors being, therefore, mutual, though the orchardist doubtless derives in this case greater advantage from them than does their owner himself. Escaped swarms lodged in forest trees in the vicinity of the orchard are sometimes sufficiently numerous to perform the work well. The great value of bees as cross fertilizers makes their destruction a serious injury to the interests of the fruit grower. Therefore spraying with arsenicals during fruit bloom should never be practiced. The injurious insects can be reached quite as well before and after the blooming period.
Fig. 52.—Heath-like wild aster (Aster ericoides). (Original.)