If you look at a Foxglove very carefully you can see how the
process of evolution has resulted in a number of adaptations to the Foxglove so that it
can attract insects such as Bumble Bees.
adaptations or modifications mean that the plant can attract the right type of insect so
that the flowers can be pollinated. Pollination is essential if plants are to produce
seed. If the flowers are not pollinated, the ovary which is contained in each flower is
not fertilized. Seeds can not be produced unless fertilization has taken place.
Of course before the Bumble Bee lands on the flower, it has
to find it. This is made easy for the insect as the Foxglove produces many flowers on a
tall stalk. Often many Foxgloves grow near each other and this results in a highly visible
display of colour.
It is no accident that the flowers are purple as Bumble Bees
which are the main pollinators are particularly attracted to this colour.
Insects visit the flowers for rewards. In most cases, the
reward is nectar produced in the base of the flower. The actual flower of the Foxglove has
a number of physical modifications. These ensure that once the Bumble Bee has found the a
foxglove which is in flower, it is able to land and get the nectar. In so doing the plant
has evolved a fail safe method of transferring the pollen on to the insect. When the next
flower is visited the pollen rubs off on the female part of the foxglove flower.
Let's take a closer look at some of the adaptations. To make
it easier for a Bumble Bee to land on the flower the Foxglove has evolved a wide open
mouth and a large lip to aid landing. A number of spots are present on the landing
stage which help the insect to identify where to land. These spots then lead through the
main part of the flower to the nectar. The mouth is bell shaped and this helps to close
the bee's wings as it enters the flower. The main part of the flower is tunnel shaped and
the reproductive parts of the flower are located in the roof the tunnel. A large insect
like a Bumble Bee can only just squeeze along the tunnel. This means that as it travels
along, the pollen which is on the stamen is brushed against the back of the bee. If the
bee has already visited another foxglove then the pollen from that flower is brushes off
onto the stigma pollinating it.
Of course other insects are attracted to the flower and some
of them are much smaller than the Bumble Bee. In order to deter them, the landing area is
covered with fine hairs called guard hairs which act as a physical deterrent. Smaller
insects are undesirable because they would be able to take the nectar without brushing
against the flowers reproductive parts.
Sometimes the weather is cold and wet during the flowering
period. This means that insects are far less active. To ensure that the plant can
reproduce successfully it produces many flowers on its long stalk. These do not open at
the same time. The flowers at the bottom open first. It will be some weeks before all the
flowers all the way to the top of the stalk have opened. This means that flowering takes
place over a long time and therefore some will be open during the right weather
conditions. This adaptation is therefore security against cold or wet periods when insects
are less abundant.
There is another advantage to flowering from the bottom of
the stalk to the top. These top most flowers will still be visible above other vegetation
which has grown up over the summer period. As Foxgloves are pioneer species, that is they
readily colonise open areas, this is important as grasses and ultimately brambles will
colonise the area. Plants which can not grow above these species will not survive.
Once the Bumble Bee has unwittingly transferred the pollen to
a stigma, a pollen tube germinates. It then grows down the style of the flower and into
the ovary. Fertilization takes place and seed production starts. When the seeds have grown
to maturity, the ovary dries and opens, allowing the small, light, seeds to be carried in
the wind. The plant's tall, thin, flexible, stalk is easily blown about in the wind and
this also helps to launch the seeds.