1, Section of a hairy lichen, _Usnea barbata_. 2, Section of a leafy lichen, _Sticta fuliginosa_. 3, Early growth of a lichen. _gc_, Green cells. _f_, Fungus.]
The way the lichen has grown is this. A green cell (_gc_ 3, Fig. 30) falling on some damp spot has begun to grow and spread, working up food in the sunlight. To it comes the spore of the fungus _f_, first thrusting its tubes into the tree-bark, or wall, and then spreading round the green cells, which remain always in such a position that sunlight, air, and moisture can reach them. From this time the two classes of plants live as friends, the fungus using part of the food made by the green cells, and giving them in return the advantage of being spread out to the sunlight, while they are also protected in frosty or sultry weather when they would dry up on a bare surface. On the whole, however, the fungus probably gains the most, for it has been found, as we should expect, that the green cells can live and grow if separated out of the lichen, but the fungus cells die when their industrious companions are taken from them.
At any rate the partnership succeeds, as you will see if you go into the wood, or into an orchard where the apple-trees are neglected, for every inch of the branches is covered by lichens if not already taken up by mosses or toadstools.
There is hardly any part of the world except the tropics where lichens do not abound. In the Alps of Scandinavia close to the limits of perpetual snow, in the sandy wastes of Arctic America, and over the dreary Tundras of Arctic Siberia, where the ground is frozen hard during the greater part of the year, they flourish where nothing else can live.
The little green cells multiply by dividing, as we saw them doing in the green film from the water-butt. The fungus, however, has many different modes of seeding itself. One of these is by forming little pockets in the lichen, out of which, when they burst, small round bodies are thrown, which cover the lichen with a minute green powder. There is plenty of this powder on the leafy lichen which you have by you. You can see it with the magnifying-glass, without putting it under the microscope. As long as the lichen is dry these round bodies do not grow, but as soon as moisture reaches them they start away and become new plants.