These showy flowers have an unusual strategy for spreading pollen as far and wide as possible to maximize their chance of survival.
I’m always fascinated by the inner landscapes of flowers and the mysteries they hold. Looking deep into the mouth of this glorious purple Canterbury bell, Campanula medium, is like looking into someone’s soul to get a sense of who they are and what makes them tick. Although all the standard flower parts are there, I was intrigued by what appears to be a thick layer of pollen clinging to the outer surface of the central pale green style, part of the female reproductive anatomy, and only a small amount on the anthers that produce and display the male seed for collection and distribution by pollinating insects. My curiosity led me on a journey of discovery about these fascinating flowers and their unusual mechanism for spreading pollen to maximize their chance of survival.
Native to Italy and southern France but naturalised over much of Europe and North America, Canterbury bells are variously known as bell flowers, Coventry bells, and even ‘cup and saucer’ owing to the resemblance of the fused petals to a cup and the semi-fused deeply fluted green sepals at their base to a saucer. The botanical appellation Campanula is derived from the Greek campanulatus meaning, well, bell-shaped!
They make a riotous display during the summer months with multiple stems up to a metre or more tall, each covered with masses of deep purple wide-mouthed bells pointing in all directions. A single bloom tops each stem like the star on a Christmas tree, initially pointing to the sky and then gently tilting to one side as it relaxes into a comfortable position. If all the bells chimed sweetly in the wind, it’d be like the tinkle of fairy bells wafting in the window on a warm summer evening.
Whilst we most often grow and enjoy flowers for their beauty and to express our sentiments, flowers are crucial to the survival of angiosperms or flowering plants, and have evolved some amazing mechanisms to perpetuate themselves. Over probably thousands of years, Canterbury bells have developed a process called ‘secondary pollen presentation’ where more pollen is made readily available to insect visitors by depositing it on other parts of the flower than just the anthers on the stamens. When the petals of flowers in the Campanula species are closed before ripening, the pressure of the petal lobes presses the stamens with their pollen laden anthers, against the outer surface of the central pale green style which is covered in masses of tiny hairs. These are designed to capture the pollen so that when the petals mature and the flower opens, the style is covered with a thick layer of pollen.
It is then that the survival strategy becomes particularly clever. In order to spread as much male seed as possible to maximise the number of other bellflowers fertilized, the action of an insect rubbing against the pollen laden style causes the hairs to retract on that section and release those pollen grains which then attach to the insect’s body for transportation to other blooms. In this way, only a small amount of pollen is released by any single insect at any given time. It’s like the plant has learnt to eke out the available resources to make them last the longest possible time to ensure the most prolific spread and best reproductive outcome.
In the meantime, the stamens have already wilted, even before the flower opens, and dropped much, although not all, of their pollen load. While the flower is blooming, they lie limply within the bell waiting for bees, butterflies, moths and beetles to collect pollen directly from their anthers when they pass on their way to the sweet treat in the large nectary at the base of the flower.
Botanists believe that the Campanula species probably developed the secondary pollen collection mechanism when the male seed was in some way compromised. However, it seems to me that what they have labelled as a secondary mechanism appears to be the primary method when the quantity of pollen attached to the style is compared to that left on the anthers when the flower opens for business. Isn’t it amazing that the mysteries of nature keep us searching for answers to these seemingly imponderable questions, just like the endless layers we work through as we seek to understand the depth and breadth of our own human soul.