Julie Fallon, of Ardgowan School, asks :-

Why is mould formed?

Matthew Cromey, a plant pathologist at Crop Research, Lincoln, responded.

Moulds are growths of minute fungi. They are made up of tiny threads, known as hyphae, which are too small to see individually with the naked eye. What we see, then, is an accumulation of these hyphae, which form an interwoven mat.

Moulds have two requirements to allow them to grow. The first is a source of nutrients. The mould fungi, unlike plants, cannot harness the sun's energy to make nutrients for growth. They need therefore to grow on a source of nutrients, such as bread, or other plant or animal material.

Sometimes, when conditions are ideal, they can even find enough nutrients to grow on painted surfaces, such as walls. Different types of mould fungi are specialised for each habitat. For instance the bead mould (called Rhizopus) has long black or white hyphae which grow out a centimetre or more from the surface of the bread. The moulds that grow on damp walls are usually brown or black, and grow very slowly over the surface of the wall. Another type of mould, the sooty mould, grows on plant leaves, using aphid honeydew as its nutrient source.

The second requirement for the growth of moulds is moisture. Moulds like damp places, such as bathrooms, to grow. If conditions are too dry, the rate of growth of the mould will usually slow down, or even stop.