Toadstools and their relatives, the so-called ‘higher’ fungi, exist as a network of underground microscopic threads, called mycelium, which feeds on and breaks down dead organic matter.
The toadstool itself develops as a cluster of mycelial cells called a primordium, analagous to a plant bud where the cells are initially small and densely packed. The last phase of growth usually occurs after rain and is extremely rapid. The cells of the primordium expand greatly as they fill with water and, apparently overnight, a toadstool appears.
The hydraulic forces involved are remarkable. Although apparently soft and fragile, toadstools can force their way through tarmac and lift paving stones. Many species can do this, but among the most frequent are Coprinus species (ink caps) and various species of puffballs. The damage can occasionally be extensive and expensive.
Gardeners laying tarmac should take care to remove all potential food sources for such fungi, such as dead roots. No chemical treatment is possible and the toadstools are likely to reappear at intervals until the food source is exhausted.