IT HAS long been claimed that lichens will grow only in places where the air is not polluted and that is one reason they are rarely found in towns and cities.

I refer to them now because I've noticed their recent appearance on rocks and trees where there was previously no sign of this curious growth. Indeed, I spotted a couple of very handsome yellow specimens on the stone window ledge of my study.

This would suggest that our countryside air has become less polluted than it used to be, and that must surely be beneficial to everyone and everything.

The pronunciation of lichens usually causes some discussion - is it 'likens' or 'litchens'? Both appear to be widely used, but the term covers more than 1,000 species of these moss-like growths.

Lichens are a mixture of a fungus and a plant from the alga group, and they grow in a wide variety of places, often where the atmosphere is slightly damp.

They are frequently found on tree trunks, gravestones, rocks, walls and roofs, where they appear as patches of a greenish-grey, yellow or brown moss-like growth, although a variety of colours is possible.

Some types grow near the seashore, and one kind forms the plant known as reindeer moss, which grows in the highlands of Scotland and Wales. As the name suggests, this is eaten by reindeer.

Yet another is a pale greenish colour and hangs from tree branches, rather like miniature seaweed.

Apparently, fungus is essential to the survival of lichens because it acts as a kind of protective shield to ensure the algal calls do not dry out. Lichens do not have roots, but need moisture. They absorb it, along with other nutrients, through their upper surfaces. Their reproductive systems are also highly complex, in some cases depending on parts of a lichen being broken off to start a new growth, while another method is to spread themselves through the dispersal of spores.

It is the extreme sensitivity of lichens which makes them so vulnerable to even slight changes in the pollution of the atmosphere, hence their value as an indicator of the purity of our air.

It seems we should welcome lichens to our houses and gardens and to the countryside in general. That is why I am pleased a pair of them have decided to colonise my study window sill.

This autumn, there has been a lot of comment about the profusion of berries on our trees, both in the wild and in gardens and parkland.

Our weather lore says this is a sign of a hard winter to follow, the berries being nature's way of providing food for birds and small animals.

I know of no scientific basis to support this belief, the reason for the profusion of fruit probably being nothing more than a combination of beneficial factors earlier in the year.

It is true, however, that most of our berry-bearing shrubs and trees have been dripping with ripe fruit this year and, during a recent walk around a local arboretum, I spotted an extremely handsome tree in its autumn colours. The tips of it branches were weighted down with clusters of bright red berries.

I must admit I could not immediately identify the tree, but quickly established that it was a whitebeam. Its leaves looked similar to those of an alder, although slightly more pointed at their tips, and the tree was about 20ft high with a smooth, greyish trunk.

In its autumn shades, therefore, and with such a heavy crop of brilliant crimson berries, it was a most handsome addition to the landscape.

It is the tree's natural beauty which makes it so desirable in parks and gardens and, because it does not grow very tall, it is often used as a decorative addition to streets and open spaces in towns.

In the spring, the undersides of the leaves are silvery-white and covered with a thick, downy growth which produces a wonderful sight when a breeze ruffles them. In May, it produces thick, creamy-white and highly scented blossom and then, in the autumn, we get those lovely colours and rich red berries.

For all its beauty, the whitebeam used to be considered a very useful tree. Its wood is hard and in former times, before iron came along, it was used for making cog wheels in early machinery.

The berries are tasty enough to be eaten direct from the tree once they are ripe, and they were once used in jelly eaten with venison. Another treat was to mix them with wine and honey to make a very palatable drink.

The popularity of this distinctive tree has also made it useful as a boundary marker, but its presence in parks and gardens has led to it hybridising with others of its family, producing similar trees with slightly different appearances. For example, one specimen produces yellow fruit instead of red.

The name comes from the Anglo-Saxon "beam", which means tree, and the prefix "white" arises from those distinctive white-backed leaves. A handsome tree by any standards.

Following my notes about yew trees and the Fortingall yew in particular (D&S Oct 1 and Oct 15), I have received an interesting letter from a reader in Barnard Castle.

He reminds me of many other famous trees in Britain, including the apple tree at Woolsthorpe Manor in Lincolnshire which managed to drop one of its apples on to the head of Sir Isaac Newton and so lead him into his famous understanding of gravity.

There is a solitary hawthorn on a mountainside in Scotland which is known as the Wishing Tree because people stick coins into its bark while making wishes.

This reminds me of childhood visits to the Wishing Stone in Arncliffe Wood, Glaisdale. Sixty years ago or more, a hawthorn grew from the middle of a large rock and it was believed that, if you placed a coin in the tree and walked three times around it upon the rock's high surface, good fortune would follow. Sadly, that old hawthorn has now withered and died, although the Wishing Stone remains.

Among other famous trees is the infamous Wyndham's Oak near Silton Church in Dorset. This is one of the few remaining Hanging Oaks - it was used to hang supporters of the Duke of Monmouth after his attempt to usurp the throne in 1685. There is also a magnificent yew tunnel in Wales. This has been formed by six yews whose branches have bent to the ground and taken root.

My correspondent has provided another point of interest. He mentions the term "skew-whiff" and ponders its derivation. The term appears in some dictionaries, where it indicates something which is not placed squarely or straight, and it was once widely used in local dialect.

"Skew" appears in my North Riding dialect dictionaries, where it can mean to twist or throw with a twisting action, and it can also mean twisted or askew.

It is interesting to note that this name was given to a bridge near Guisborough. It was called Skew Bridge and crossed a railway line once used to carry minerals. The line ran from Guisborough and crossed the A171 at Slapewath near Charltons before heading across country towards Brotton.

It closed some years ago and has now been dismantled, but Skew Bridge still crosses the road, so-named because it crosses at an angle. Some local people used to call this bridge Fancy Bridge and it was thought to be the first skew bridge ever built.

"Whiff", in the North Riding dialect, meant to puff or blow, and it included references to smoke, either when someone was whiffing a pipe, or when smoke whiffed down the chimney.

One can see why skew-whiff could mean something like smoke on the move, or something not positioned as straight as it should be.

"Whiffle-waffle", on the other hand, means useless or meaningless chatter. That reminds me of when my son was very young. He was waiting at a bus stop as two women chattered. "Dad", he asked me later. "What do they find to talk about?" I might have answered whiffle-waffle, or even tittle-tattle.