Lichens
Lichens are abundant on the stems of coyote brush. Watch for the increasing growth of lichens as you ascend into the fog belt. Lichens are not usually the bright green color of moss. These grayish­green organisms are actually two organisms-a fungus and an alga-living together. They are not parasitizing the coyote brush. The fungus provides the means of attachment and the alga produces food for the two from carbon dioxide and sunlight, like a plant. By living together, the two organisms as a lichen are suited to harsh environments such as exposed rocks and barren soil in which neither organism could survive alone. The leafy lichen with a black underside is Parmelia arnoldii. The lichen with yellowish, flattened branches is Ramalina farinacea, and the tufted lichen is Usnea.

Insect galls
Plant galls are abnormal growths (tumors) of plant cells formed as a response to a parasite. Galls occur on a wide variety of woody plants. Insects are the primary cause of galls. Gall-forming insects include aphids, gall gnats, and cynipid wasps (gall wasps). The gall wasps are the most important insects that induce plant galls. When an insect lays its eggs in the bud, the plant cells grow abnormally producing this tumor.

Along your Sweeney Ridge walk, a look at the terminal buds of coyote brush (left figure) often reveals galls. A midge (fly) larva is developing in the gall. Cylindrical galls on coyote brush stems are due to the presence of moth larvae. And irregular galls on the stem are due to a fungal infection.

Look for red galls on willow leaves (right figure) along the Sneath Lane trail. These galls are caused by a sawfly (wasp).

Paintbrush
As you're walking, you'll notice that Indian Paintbrush grows in thickets of other plants such as coyote brush. That's because paintbrush is a root parasite. It is called a hemiparasite because it can grow independently but as soon as the roots make contact with a host, growth rate improves greatly and the plant becomes much more vigorous. Paintbrush roots, called haustoria, invade the roots of a host plant.