This section is from the book "On British Wild Flowers Considered In Relation To Insects", by John Lubbock. Also available from Amazon: Nature Series On British Wild Flowers Considered In Relation To Insects.
The flowers of Campanula are much frequented by insects, and secrete honey at the base of the bell. The anthers are distinct, the filaments of the stamens are expanded at the base into triangular valves which serve to protect the honey; the pistil is cleft at the top into two, three, or five stigmatic lobes. The genus is widely distributed and contains numerous species.
The accompanying figures show a flower of C. medium in three stages. In the bud (Fig. 90) just before opening, it will be seen that the large, long anthers clasp the pistil, which is no longer than they are themselves. In the second stage (Fig. 91) the anthers have opened on the inner side, and shed their pollen, which adheres to the style. The anthers themselves then shrivel up, offering a surprising contrast to their former condition. Insects visiting the flower for the sake of honey, do not, as far as I have observed, generally walk on the petals, being deterred by the stiff hairs which are scattered on their inner surface. In any case, however, they are almost sure, sooner or later, to clasp the style, when they necessarily dust themselves with the pollen. In this stage the flower is incapable of fertilization. Gradually, however, the style elongates, and the lobes of the upper end separate, so that by the time the pollen is all removed the flower is in the state shown in Fig. 92, and it is evident that any bee which may have visited a younger flower, and dusted its under side with pollen, can hardly fail to deposit some of it on the stigmatic surfaces thus extended for its reception.
Fig. 90. - Section of bud of Campanula medium.
Fig. 91. - Section of a flower in the first (male) condition
Fig. 92. - Ditto, in the second (female) condition.
It had been supposed that the hanging position of Campanula and other bell-shaped flowers had reference to the position of the stamens and pistil, so that the pollen might fall from the former on to the latter. Sprengel, however, pointed out that the real advantage to the flower consisted in the fact that the honey is thus protected against rain. If the pollen fell on to the stigma, it is indeed obvious that the stigmatic surface should be turned upwards, whereas it is at the end of the pistil, and is consequently turned downwards, showing that the pollen comes from below and not from above.
The other British genera of Campanulaceae are Lobelia, Jasione, and Phyteuma.