File:Cladonia macilenta - geograph.org.uk - 1103931.jpg

English: A lichen - Cladonia macilenta This lichen is growing at the base of a tree, which is a very typical habitat for this species.

The main body of a lichen is called the thallus. Many species of Cladonia have what is called a primary thallus (which appears first, and may or may not persist) and a secondary thallus (which develops later). In this and similar species, the primary thallus is a covering of little scales (called squamules), seen here covering the surface of the wood.

Here, the squamules form a dense mat, and are a bluish shade of green (this photo was lit by a low winter sun, which affects the colours somewhat) and they have notched edges. The secondary thallus consists of numerous erect stalks (called podetia). In this image, the podetia are each about a centimetre high. The fruits, growing at the tips of the stalks, are red, but are quite inconspicuous in this photo.

This species and the similar Cladonia coniocraea (which has brown fruits, and which is often found growing with C. macilenta) are quite common on rotting wood and the bases of tree trunks. C. macilenta is a species that favours trees with very acidic bark (e.g. that of pine or birch); this is also an area with a very wet climate, and the leaching effects of rainwater can serve to make bark more acidic.

Lichens also vary greatly in their ability to tolerate air pollution (particularly sulphur dioxide levels); C. macilenta is fairly tolerant of sulphur dioxide pollution; C. coniocraea is even more so.