The pollen grain is the structure used to transport male DNA to the female part of a flower.
Pollen must be strong enough to protect the male DNA on its journey. The outer wall of the pollen grain, called the exine, is composed of a very tough substance called sporopollenin. The inner layer is made of cellulose and is similar in construction to an ordinary plant cell wall. Pollen grains are microscopic - usually about 15 to 100 microns - and just a pinch of pollen powder contains thousands and thousands of grains.
How can different pollen types be recognised?
Each pollen type has its own unique characteristics which allow the species and plant family to be identified. The main features which distinguish one type of pollen from another are size, shape and ornamentation of the outer wall. Pollen grains come in a wide variety of shapes although the majority are basically spherical or oval or disc-shaped. The surface of the grain can also have a meshed, granular, grooved, spined or striated surface or can appear very smooth. The outer wall features include pores and furrows.
Which pollen types are most allergenic?
Most species of pollen can produce allergic reactions but some allergenic pollen types are more common for causing symptoms of hay fever. Grass pollen affects about 95% of all hay fever sufferers and birch tree pollen affects about 20%. Oak tree, plane tree (common in many streets in London) and nettle pollen are also well known for their allergenic properties. Ragweed is currently rare in the UK but is one of the most allergenic species in the USA, Canada and central Europe.
Wind pollinated, non-flowering plants tend to produce masses of pollen grains that cause symptoms of hay fever and asthma. Most flowering plants produce smaller amounts of pollen as these plants are pollenated by insects. These pollen species form sticky clumps which are visible to the eye. The clumps of pollen rarely reach the nose and so are unlikely to result in allergic reactions.
Which pollen types are most prevalent in the air stream?
Many pollen species appear on our pollen trap slides, of which 15 pollen species are considerably allergenic. Most of the pollen species found on the slides are airborne, produced by wind pollinated plants. The air is also full of fungal spores at most times of year.