What is pollen?
Alder Pollen - Photo M. Smith
The pollen grain is the structure used to transport the male gamete (i.e. male DNA) to the female part of a flower. Pollen must be strong to protect the male gametes on their journey. The outer wall of the pollen grain, called the exine, is composed of a very unusual substance called sporopollenin which is very tough. 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?
Birch pollen grain showing three pores
- Photo M. Smith
Each pollen type has its own unique set of characteristics which means that the species or plant family can usually be identified. In the case of grass pollen, the different species are more recently evolved and their pollen grains are very similar to each other and so are usually only identified as grass in the count. 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 outer wall features include pores and furrows, e.g. grass has one pore and no furrows, birch has three pores and no furrows, oak has three furrows with a pore in the middle of each one. The surface of the grain can also have a meshed, granular, grooved, spined or striated surface or can appear very smooth.
Which pollen types are most allergenic?
Grass pollen - Photo M. Smith
Most species of pollen have some level of allergenicity but some are particularly notorious for inducing 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. One of the most allergenic species on an international level is the wind-pollinated ragweed. This is currently rare in the UK but is frequent in parts of the U.S.A., Canada and Europe. It produces a huge amount of pollen - up to 8,000 million pollen grains can be released in just 5 hours from the giant ragweed species. Wind pollinated plants do tend to produce masses of pollen to ensure that at least some of it reaches the right target.
Ragweed pollen (Ambrosia sp.)
- Photo M. Smith
The majority of flowering plants are insect-pollinated and so their pollen does not need to be dispersed on the wind and they therefore produce smaller quantities of it. The pollen from these insect-pollinated species is often sticky to adhere to the bodies of insects and can form clumps making it visible to the eye which often makes people assume that this is the pollen type causing their symptoms. While such pollen does have allergenic properties, the chances of it reaching the nose are usually slim. So, it is the wind-pollinated species with their insignificant flowers (usually greeny-yellow and small) producing millions of pollen grains that mainly cause the hay fever symptoms and trigger asthma in those susceptible.
Which pollen types are most prevalent in the air stream?
In all, about 15 species that are known to be considerably allergenic are seen each year on the slides from the pollen traps in the UK. There are about another five that are regularly seen but considered to be less of a problem. The traps also collect very small numbers of some other species, usually those flowering locally. Overall, the traps collect only a very small proportion of the hundreds of different species of shrubs, weeds, grasses and trees that flower throughout the growing season in the UK, because, as mentioned above, most of them do not get airborne. Most of the pollen species that appear on the slides are pollinated by the wind and are often produced in great quantity to ensure that at least some of the pollen reaches another flower of the same species. The air is also full of fungal spores at most times of the year.