Flowers are the reproductive structures produced by plants which belong to the group known as Angiosperms, or 'Flowering Plants'. This group encompasses an enormous variety of different plants ranging from buttercups (Ranunculus spp.) to Oak trees (Quercus spp.) and grasses. There are in the region of 250,000 known species.
A flower is basically made up of four concentric rings of structures. There is an outer ring of modified leaves called sepals. These provide protection to the flower before it opens and are usually green. This outer ring is known as the calyx. Inside the sepals is another ring of modified leaves called petals which are often brightly coloured. This layer is known as the corolla. Within the corolla are one or more stamens containing pollen, which are the male reproductive structures. In the very centre of the flower are the female reproductive organs.
Flowers do not always have the two outer layers of calyx and corolla. These two layers have developed to the most marked degree in plants which are pollinated by insects. The corolla, or petals are often brightly coloured with markings attractive to insects. In insect-pollinated plants, there are also usually nectaries which secrete sugary nectar, located within the flower. These provide an incentive to insects to visit the flowers. In the search for nectar, the insects will often get pollen grains caught on their bodies. This may then brush off onto the stigma of the next flower visited and in this way the flowers are pollinated. Many flowers have evolved very specific associations with a particular insect species or group of species. In these cases flower structure may be very specialised (e.g. orchids). Foxgloves (Digitalis purpurea) and Primroses (Primula vulgaris) show a range of adaptations to insect pollination. Wind pollination is also common in flowering plants. In this case, because there is no necessity to attract insects, the calyx and corolla of the flowers may be absent.
Once a pollen grain reaches the stigma of another flower of the same species, it will produce a pollen tube. This grows down through the style until it reaches an ovule. Fertilisation then takes place, resulting in a seed.
When pollen from one flower fertilises the ovule of another flower, it is called cross pollination. If an ovule is fertilised by pollen from the same flower, it is called self fertilisation. In evolutionary terms, this is generally not particularly favourable, as it leads to inbreeding. Most species therefore tend to be cross pollinated. They are thus reliant on external agents to transfer the pollen. The need to either attract external agents such as insects or birds, or to take advantage of wind or water currents has led to the extraordinary variety of shapes, colours, scents and arrangements of flowers seen today.