The reason why these plants can live in such very different regions is that they do not take their food through roots out of the ground, but suck in water and gases through the thin membrane which covers their cell, and each cell does its own work. So it matters very little to them where they lie, so long as they have moisture and sunlight to help them in their work. Wherever they are, if they have these, they can take in carbonic acid from the air and work up the carbon with other gases which they imbibe with the water, and so make living material. In this way they grow, and as a cell grows larger the covering is stretched and part of the digested food goes to build up more covering membrane, and by and by the cell divides into two and each membrane closes up, so that there are two single-celled plants where there was only one before. This will sometimes go on so fast that a small pond may be covered in a few hours with a green film formed of new cells.
Now we have seen, when studying mushrooms, that the one difference between these green plants and the single-celled Fungi is that while the green cells make their own food, colourless cells can only take it in ready-made, and therefore prey upon all kinds of living matter. This is just what happens in the lichens; and botanists have discovered that these curious growths are really the result of a _partnership_ between single-celled green plants and single-celled fungi. The grey part is a fungus; but when it is examined under the microscope we find it is not a fungus only; a number of green cells can be seen scattered through it, which, when carefully studied, prove to be some species of the green single-celled plants.
Here are two drawings of sections cut through two different lichens, and enormously magnified so that the cells are clearly seen. 1, Fig. 30 is part of a hairy lichen (1, Fig. 28), and 2 is part of a leafy lichen (2, Fig. 28). The hairy lichen as you see has a row of green cells all round the tiny branch, with fungus cells on all sides of them. The leafy lichen, which only presents one surface to the sun and air while the other side is against the tree, has only one layer of green cells near the surface, but protected by the fungus above.