LICHEN. On places like tree trunks, rocks, old boards, and also on the ground grow strange splotches of various-colored plant life called lichens. They are of great scientific interest because they are not single plants; instead, each lichen is formed of a fungus and an alga living together so intimately as to seem a single plant. The lichens are one of the best illustrations of symbiosis, the intimate living together of two different kinds of organisms. The fungus makes the bulk of the body with its interwoven threads, and in the meshes of the threads live the algae. The special fungi that take part in this arrangement are almost never found growing separately, but t