Joe David Tayse age 17, of Whitleyville, Tenn,,for his question:

What are those round, grey‑green plants that grow on logs?

Did you ever guess that two plants could be friends, or business partners? We don’t usually think of plants in this way, but it can happen, and does, It is a give‑and take between two or more plants and is called symbiosis, The little lichens are examples of symbiosis, All the 24,000 different kinds of lichen go in for friendly partnership,

Those flat grey‑green rosettes found growing on old logs are lichens, Each is a combination of two plants, a fungus and an alga. The fungus has no green chlorophyll with which to make plant food from light and air, But it sops up and holds plenty of moisture in its spongy fibers, The alga has chlorophyll bodies, but it has no way of holding onto the plentiful supply of moisture it needs.

So, these simple plants solve their problems by going into business together. The fungus provides soft, spongy fibers and grows the plant shape, The alga is a one‑celled plant, usually very tiny. It clusters in great numbers all over the fungus. It thrives among the moist fibers and uses its green chorophyll to make plant food for itself and its partner, This kind of partnership is also called mutualism,

At one time it was thought that the fungus held the little algae as captives for the algae can live alone in a moist place. The fungus, however, cannot manage by itself. But the idea of mutual agreement between the two little strugglers is now considered more likely.

The lichens are very humble and very hardy members of the plant world. They grow on the ground, on tree trunks, on logs and on old walls, Vari‑colored algae tint them grey, blue, green yellow, rose or blackish. They can grow in the hot desert or the freezing polar regions. They grow higher up the mountainside than any other plant life, And they can grow on the faces of bare rocks where there is no soil.

Soft mossy lichens grow beneath the snows of the far north. They provide the reindeer and northern animals with winter food, But the of the lichens is soil making, These humble partner‑plants are necessary to prepare the earth for more complicated plants.

Study a cluster of blotchy lichens on the face of a bare rock. The fungus is dissolving the hard stone to get its chemicals, The rock face chips and powders, Dust and debris sticks to the moist blotches, In time the hard rock crumbles into a mass of powdered dirt.

Bacteria join in the job of making the dirt richer, Soon a patch of earth is ready to support a moss plant, It grows and adds its richness to the dirt, At last the soil is rich enough to support the larger, more complex plants, But the little lichens are among the first to set up their partnerships on the bare rocks and in the drained lakes, They must live and die for the fancy foliage that is to follow