|Campanulaceae - Bellflower Family
Above: Harebell (Campanula rotundifolia) growing in calcareous grassland. The spinelike sepals,
radiating at an acute angle is a characteristic feature to look for in identifying this plant, along with the
bell-shaped corolla (tube of fused petals) which is characteristic of the bell-flower family.
Floral Formula: K5 C(5) A(5) G(3)
As I have written elsewhere, every plant has its own story to tell and campanula's story is a fascinating
one! During development the flower begins with a male phase. The five stamens initially surround the
style and when the anthers dehisce (split open) their pollen is released very early on. This pollen
adheres to and is trapped by many pollen-collecting hairs (PCHs) attached to and surrounding the
style. The empty husks of the anthers then whither and move away from the style. This arrangement of
style PCHs is called a stylar brush and is an example of a secondary pollen-presenting structure
(replacing the role of the anthers as primary pollen-presenting structures). This arrangement is
presumably more efficient at transferring pollen to the insect pollinator.
At the base of the style are five small and closely-fitting petal-like structures, their edges fringed by
hairs. These are generally regarded as staminal appendages, each one being an extension or
expansion of the base of each stamen filament. These structures form a nectar chamber which collects
and stores nectar secreted by the nectary, which is a pentagonal ring or ridge of tissue on top of the
ovary, surrounding the base of the style. The nectar is presumably secreted through modified stomata
as seems typical of this type of nectary. The staminal appendages or valves, forming the lid of the
nectar chamber, probably also serve to prevent non-pollinator insects from easily stealing the nectar. A
large enough insect will be able to push the valves apart and access the nectar.
Above and below: Nettle-leaved bellflower (Campanula trachelium). In the top flower, the
five petal-like staminal valves forming the lid to the nectar chamber can be clearly seen
around the base of the style. The five dehisced and whithered anthers are also visible.
Note the three stigma lobes at the end of the style. The flower at the bottom has visible
pollen clinging to teh style.
During this male phase, the stigma lobes begin closed together, but gradually open and curl backwards as the
female parts of the flower matures and it switches from its male to its female phase. The stigma lobes unfurl to
expose their pollen-receptive surfaces. The stigma lobes will eventually curl back far enough to touch the
style. This is a potential problem, as this increases the risk that the stigma will contact pollen from its own style
and self-pollinate and the whole point of having a male phase preceding the female phase is to prevent
self-pollination. To overcome this, the PCHs retract as the flower matures. The PCHs are extensions of single
large cells and their retraction is likened to the retraction of a finger in a glove. Each hair retracts into a pit. As
it does so, the pollen is either brushed off or taken into the pit where it does not apparently germinate. By the
time the stigma is mature, the pollen produced by the flower's own anthers is now safely out of reach of the
stigma, greatly reducing the risk of self-pollination.
When should a flower switch from being male to being female? Ideally when its pollen has been collected.
Campanula has a remarkable solution to this problem. The flower senses insects visiting it and this tactile
stimulus increases the rate of female development, accelerating the switch to the female phase. Empirical data
confirms this and it has been shown by experiment that repeated brushing of the stylar hairs triggers this
response, so the PCHs appear to also act as tactile sensors, detecting the stimulus of insects brushing the
pollen brush and signalling to the rest of the flower to advance its switching to the female phase!
Suggested Reading and References
S. Vranken; R. Brys; M. Hoffmann and H. Jacquemyn, 2013. Secondary pollen presentation and the temporal
dynamics of stylar hair retraction and style elongation in Campanula trachelium (Campanulaceae). Plant
Biology 16: 669–676.
30 Aug 2015
23 Sep 2016
Plants provide nectar more or less specifically for certain pollinators, generally those for whom the size, shape
and mechanism of the flower will ensure successful collection and delivery of pollen. Hanging down on a
slender and flexible stalk, as the Harebell does, makes it hard for many insects but favours certain agile bees.
The hiding of nectar deep inside the flower structure makes it hard for insects ineffective at pollination to steal
the nectar. In the case of the Harebell, it favours bees whose proboscis is long enough to reach to the base of
the stamens and slender enough to fit between the style and anthers.