The “Let’s Talk About Parks” series is designed to encourage exploration and discovery of Pittsburgh’s urban parks.

A hike through any woodland will lead you past many familiar forms of plant life — trees, flowers, grasses, bushes, weeds and perhaps even mushrooms. But did you know there is another growing thing you may have missed?

Lichens spread across rocks, walls, tree trunks and branches and even bare soil. It grows gradually, like moss, but instead of a soft green carpet, it forms crusty shapes that may look like scales, hardened leaves, blotches or leafy bumps.

Scientists do not technically classify lichens as plants because they are made up of two living parts — fungus and alga — these two parts function together in what is called a symbiotic relationship. This means they depend on each other to survive. Lichens do not have roots that absorb water and nutrients like other plants; they get everything they need from the air. The fungus portion provides structure and gathers water, while the alga produces food from sunlight through a process called photosynthesis. Working together fungus and alga make it possible for lichens to thrive and grow even in harsh weather conditions.

Lichen plants are so hardy that they can even survive in space. In 2005 the European Space Agency sent two species of lichen to space in sealed capsules. Once in orbit, the capsules were opened, exposing the lichens to cosmic radiation and extreme temperatures. When the lichens were brought back to Earth two weeks later, they were healthy and undamaged.

Lichens have an important function as they help to break down the rocks on which they grow. As the rock slowly crumbles, valuable minerals are returned to the soil. They also can tell us if there are unhealthy changes in our environment as they are sensitive to air pollution.

Many colors and kinds of lichens can be found around the world. The next time you’re in a Pittsburgh park, look up at the trees and try to spot the pale greenish circular patches of lichen. Along Frick Park’s Tranquil Trail, look for blue-green lichen growing on rock outcroppings and varied-colored lichen growing on fallen trees.

To learn more, visit www.pittsburghparks.org/letstalkaboutparks.

First Published: March 10, 2015, 4:00 a.m.